AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 89.467475 (1992)
Human Marriage Systems and Sexual Dimorphism in Stature STEVEN J.C. GAULIN AND JAMES S. BOSTER Department of Anthropology, University o f Pittsburgh, Pittsburgh, Pennsylvania 15260 (S.J.C.G.); Department of Anthropology, University of California at Irvine, Irvine, California 92717 (J.S.B.)
KEY WORDS
Sexual selection, Monogamy, Polygyny, Sex differences, Stratification
Contemporary populations of Homo sapiens are sexually diABSTRACT morphic on a variety of traits. In terms of stature, men are reliably between 4% and 10% taller than women in well-sampled human populations. Are cross-cultural differences in the magnitude of sexual dimorphism consistent with expectations from sexual selection theory? Prior studies have provided conflicting answers to this question in part because they failed to agree on how the force of sexual selection should or could be operationalized. Here we offer a simple and unbiased method for operationalizing sexual selection and retest two separate predictions from earlier work (Alexander e t al., 1979) about its expected impact on stature dimorphism in a sample of 155 societies. Neither prediction matches the observed cross-cultural distribution of dimorphism. However, this is not the consequence of a random distribution of dimorphism across societies. Instead, the data exhibit a robust and unexpected pattern. o 1992 WiIey-Liss, Inc. The relationship between biology and culture is a central issue in anthropology. Here we examine the extent to which cultural practices (in particular, marriage practices) predict the pattern of sexual dimorphism in stature across a large sample of human societies. There are a priori reasons, arising out of sexual selection theory (Bateman, 1948; Williams, 1966; Trivers, 1972; Wade, 19791, to expect a n association between the form of the mating system and the magnitude of sexual dimorphism. For example, in effectively polygynous mating systems, some males monopolize breeding opportunities. This precipitates strong selection on males for traits that contribute to mating success. However, under such mating systems, females do not undergo similar selection for traits that enhance mating opportunities. Because the sexes thus experience divergent selection pressures, sexual dimorphism oftenevolves. Incontrast,undereffective monogamy, competition for mates is not disproportionately elevated in either sex. Selection 0 1992 WILEY-LISS, INC
pressures on females and males thus do not diverge, and sexual monomorphism is the typical result. The comparative method has generally confirmed these predicted correlations between the type of mating system and magnitude of sexual dimorphism. although exceptions are known (Ralls, 1976). For human populations, data on actual mating systems are lacking. Extensive accounts of human marriage practices are available. Only random paternity testing could reveal the extent to which actual human mating systems diverge from the stated marriage system. Lacking actual mating data, theorists examining the causes of sexual dimorphism in human populations have habitually used marriage system as a gauge of the intensity of sexual selection (Alexander et al., 1979; Gray and Wolfe, 1980; Wolfe and Gray, 1982a). In a n early paper, Alexander et al. (1979) analyzed cross-cultural variation in human
Received July 16, 1991; accepted May 27, 1992.
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sexual dimorphism in terms of sexual selection theory. Their initial result was that societies practicing monogamous marriage and societies practicing polygynous marriage did not differ in terms of their levels of sexual dimorphism. Examining the ratio of male to female stature, they found a mean of 1.078 in a sample of 46 polygynous societies and a very similar mean of 1.075 in a sample of 47 monogamous societies. Alexander et al. (1979) offered a possible explanation for this unexpected result. They argued that monogamy is not a unitary phenomenon among humans; some monogamous marriage systems are closely parallel to the condition in monogamous nonhuman mammals o r birds, but others are evolutionarily unique. Their logic can be traced back to Orians (1969), who noted that, in endotherms, a male’s reproductive success would be proportionate to the number of mates he had, and that males would therefore strive to be maximally polygynous. Orians recognized the implicit paradox: why are there any nonpolygynous endotherms? Perhaps polygyny imposes fitness costs on females. He reasoned that, where these costs are high, selection could favor female tactics that prevent polygyny; conversely, where females suffer little or no such costs, selection on males could be the dominant force shaping the mating system, and polygyny would spread. However, what factors affect the cost of polygyny to females? Orians was modeling a system where individual males control access to pools of reproductively useful resource (This would adequately model territoriality among animals or private property among humans.) In such a system, females pay a high cost for polygyny whenever there is little variance among males in the value of their resource pools. In this low-variance case, a monogamously mated female would have a typical resource pool available for use in reproduction, whereas a polygynously mated female would have no more than one-half a typical resource pool and would therefore suffer reduced fitness. In contrast, where there is high variance among male resource pools, females might cluster with the “wealthy” males without paying any significant cost for the resultant polygyny. Thus Orians ar-
gued that the distribution of resources among breeding males is a major determinant of mating systems. Orians’ model can explain much of the variation in human marriage patterns. Most stratified societies-those with uneven distributions of wealth-are indeed polygynous (Gaulin and Boster, 1990; Table 1). With reference to monogamous marriage, Alexander et al. (1979) argued that human societies are of two distinct types. They suggested that societies exhibit ecologically imposed monogamy (EIM) wherever ”individual men are typically unable to gain by attempting to provide for the offspring of more than one wife” (Alexander et al., 1979; 418419). In other words, where reproductively relevant resources are evenly distributed among males, a man who marries polygynously inflicts prohibitively high reproductive costs on each of his wives, because he forces them to share a resource base equivalent t o what monogamously married women have to themselves. In contrast, the second type of monogamy represents a n evolutionary novelty that has arisen in the last 10,000 years of human evolution. Despite very uneven wealth distributions, some highly stratified societies are nonetheless monogamous. Because there seems to be no resource-related barrier to polygyny in these societies, Alexander et al. (1979) argued that they exhibit socially imposed monogamy (SIM), a uniquely human phenomenon. Recognizing three human marriage systems (polygyny, EIM, and SIM) Alexander et al. (1979) reexamined the cross-cultural variance in stature dimorphism. They found that polygynous and SIM societies were both characterized by high levels of dimorphism (1.078) but that EIM societies were marked by significantly lower dimorphism (1.068). These conclusions have been the subject of debate. Some of the objections, for example Wolfe and Gray’s (1982a) claim that most of the cross-cultural variance in stature dimorphism is allometric, have been shown to be spurious (Gaulin and Boster, 1985). However, other objections are more serious. Several critics (Gray and Wolfe, 1980; Wolfe and Gray, 1982a; Gaulin and Boster, 1985) have argued that Alexander et al.
MARRIAGE SYSTEMS AND SEXUAL DIMORPHISM
(1979) had no independent method for operationalizing the distinction between EIM and SIM categories and that their findings on the distribution of dimorphism were thus subject to possible experimenter bias. In addition, we (Gaulin and Boster, 1985) showed that much of the cross-cultural variation in stature dimorphism arises from sampling error within societies; that is, the larger the sample of males and females measured in a given society, the more closely the societal dimorphism estimate converged on the species-wide average. Thus, while the theoretical rationale for distinguishing between SIM and EIM societies is clear and persuasive, it has been difficult in practice to demonstrate a relationship between the type of marriage system and the magnitude of stature dimorphism. Recently, in attacking a quite different problem, we have attempted to describe the dynamics of intrasexual competition within SIM societies (Gaulin and Boster, 1990). To test our ideas, we developed a n operational definition of S I N societies that allowed their unbiased identification on the basis of prior codings from the Ethnographic Atlas (Murdock, 1986). The prospect of objectively distinguishing between SIM and EIM societies led us to reevaluate the claims of Alexander et al. (1979). There are, of course, alternative accounts of cross-cultural differences in sexual dimorphism. For example, there could be regional variation in sexual dimorphism. although this finding would itself require explanation. Nevertheless, to provide a broad framework for discussion of stature dimorphism, we formally evaluate the regional hypothesis and outline several other possible explanations. MATERIALS AND METHODS By the original definition of Alexander et al. (1979), SIM societies are characterized by monogamous marriage in contexts where, due to a n uneven distribution of resources among breeding males, Orians’ (1969) model would predict polygyny. In other words, SIM societies practice monogamous marriage despite the fact that women ( a t least women with wealthy husbands) would pay a relatively low cost for polygyny.
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To gauge the magnitude of the cost that polygyny inflicts on women in any given society, we needed to assess whether resources are evenly or unevenly distributed. To provide this assessment, we chose the extent of social stratification, because stratification almost always entails unequal resource distribution and because many ethnographers have attended to this variable. Both marriage practices and social stratification have been previously coded for the societies of the Ethnographic Atlas (Miirduck, 1 9 8 6 ~and we grouped these existing codes to create dichotomous variables. We regarded a society as stratified if it was coded as having “elite,” “dual,” or “complex” stratification (Murdock, 1986:col. 65; codes 3 , 4, or 51, and as nonstratified otherwise. With respect to marital form, we regarded a society a s polygynous if polygyny was coded a s at least “occasional,” (Murdock, 1986:col. 9; codes 2 , 3 , 4 , 5 , or 61, and as monogamous otherwise. [For the present data set, this technique includes three polyandrous societies in the “monogamous” group. This is a reasonable coding decision because we are trying to assess whether high levels of malemale competition are associated with high levels of stature dimorphism. According to current theory (see, e.g., Trivers, 19721, male-male competition is expected to be high under polygyny but low under monogamy and polyandry.1 These dichotoniising rules yielded a 2 x 2 classification scheme for societies. First, consider societies practicing monogamous marriage. In nonstratified, monogamous societies, there is no resource-related polygyny potential, because resources are relatively evenly distributed among breeding males; thus these cases represent EIM. According to Orians’ model, stratified societies are expected to be polygynous; it follows that, when they are monogamous, the monogamy is socially imposed (SIM). J u s t as it distinguishes two types of monogamy, this classification scheme suggests that there are also two types of polygyny, one that occurs in stratified societies and one that occurs in nonstratified societies. The type of polygyny described by Orians (19691, where reproductive females cluster because certain males offer more or better
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resources, is called resource-defense polygyny (RDP) by ethologists (see, e.g., Alcock, 1989). This same term could be argued to be appropriate in the anthropological realm, because in stratified human societies the distribution of wives tends to parallel the distribution of wealth (Betzig, 1986; Borgerhoff Mulder, 1987, 1990). Similarly, the other type of polygyny may have instructive analogues. Among nonhuman animals, polygyny sometimes occurs in the absence of resource defense and hence in the absence of any resuurce-related inducements for females to cluster with particular males. In these cases, social dominance relationships determine which males are polygynous. Similarly, ethnographic data from nonstratified polygynous societies suggest that a man’s social dominance can be the primary determinant of his ability to marry polygynously (Chagnon, 1977; Hart and Pilling, 1960). Thus we suggest the term dominance-based polygyny (DBP) to refer to those cases where polygyny occurs in the absence of significant resource differentials among males, that is, in nonstratified societies. We selected stature dimorphism a s our measure of sexual dimorphism for both pragmatic and theoretical reasons. Stature is the most widely measured parameter on which the sexes differ; this permits the compilation of a large cross-cultural database. Moreover. in every human society for which data exist, men are taller than women, suggesting that some form of disruptive selection has moved males and females apart on this dimension. Sexual selection is a common form of disruptive selection and is thus a likely cause. The data on stature dimorphism derive from a cross-cultural sample of 289 societies originally compiled by J. Patrick Gray and previously analysed by Wolfe and Gray (1982a) and Gaulin and Boster (1985). Because dimorphism estimates are known to be sensitive to the number of males and females measured (cf. Gaulin and Boster, 1985:Fig. 8), we deleted the 22 societies for which these sample-size data were missing. Of the 267 remaining societies, 170 were unambiguously identical with societies coded in the Ethnographic Atlas, but, for 15 of
these 170 societies, information on either marriage, stratification, or both was missing (Murdock, 1986). Our sample universe thus included 155 societies for which marriage and stratification patterns could be confidently associated with stature dimorphism measurements. Sexual dimorphism may be more tightly associated with geographic region t h a n it is with the type of mating system. To allow a consideration of this hypothesis, we also extracted the geographic region codes (Miirdock, 1986:col. 90) for each of the 155 societies in our sample universe. Some of the cross-cultural variance in stature dimorphism is due to widely differing sample sizes within societies (Gaulin and Boster, 1985). Because estimates of stature dimorphism are ratios (male height/ female height), small samples for either sex lower the reliability of dimorphism estimates. To take such reliability issues into account, we defined a sample-size variable for each society as the smaller of two values, either the number of males o r the number of females measured in that society. We replicated each of our analyses three times using the following case-inclusion rules: 1)all societies regardless of sample size ( n = 155 societies), 2) all societies with sample sizes 3 1 0 (n = 141 societies), and 3) all societies with sample sizes 3 2 0 (n = 118 societies). We focus attention on those findings that remain stable with progressively more stringent case-inclusion rules. We used analysis of variance (ANOVA) to compare the magnitude of stature dimorphism in societies with different types of marriage and stratification patterns. Conservative two-tailed tests were used throughout, but, in reevaluating some of the hypotheses tested by Alexander et al. (19791, a halving of the resultant P value was justified because their a priori predictions were directional. We used the BrownForsythe method to calculate F and P values because it takes account of unequal cell variances. RESULTS Table 1 presents the mean values of stature dimorphism (male height/female height) for each of the four types of societies dis-
MARRIAGE SYSTEMS AND SEXUAL DIMORPHISM TABLE IA. Mean values for sexual dimorphism in stature by marriage system and stratification for human societies regardless o f internal samule size
Monogamous and polyandrous ~~~~~~~
Nonstratified (n)
Stratified ( n )
Column means ( n )
“EIM 1.068
Row Polygynous means~ ~ _ _ “DBP
(19)
1.076 (87)
“SIM
“RDP
1.079 113)
1.069 (36)
1.072 (49)
1.073 (32)
1074
in74 (155)
(123)
1.075
(106)
TABLE 1B Mean values for sexual dimorphism in stature by marriage system and stratification for human societies wath sample sizes 2 1 0
Monogamous and polyandrous Polygynous Nonstratified (n)
Stratified ( n )
Column means (n)
Row means
“EIM”
“DBP
1068 (17)
1075
“SIM
“RDP
1079 (13)
1069 (35)
(48)
1.073
1.073 (111)
1.073 (141)
(30)
(76)
074 (93) 072
TABLE 1C. Mean values for sexual dimorphism in stature by marriage system and stratification for human societies with sample sizes a20
Monogamous and polyandrous Nonstratified (nJ
Stratified (nJ
“EIM
“DBP
1.070
1.074
1.073
(13)
(59)
(72i
“SIM (12)
“RDP 1.068 (34)
1.071 (46)
1.074
1.072
1.073
(25)
(93)
(118)
1.079
Column means ( n )
Polygynous Row meanS
cussed above, for each of three case-inclusion rules. Within the table, the four cell means are similar; at the extremes, EIM and SIM societies differ in stature dimorphism by -1%. Nevertheless, it is valid to
47 1
ask whether these differences are meaningful by using ANOVA to compare the withincell and between-cell variances. In their initial comparison of monogamous and polygynous societies, Alexander et al. (1979) combined SIM and EIM societies a s a single group and contrasted this group with all polygynous societies. They found that this approach yielded no significant difference in stature dimorphism between polygynous and monogamous societies (Alexander et al., 1979:418). We replicated this design (Table 2, comparison 1)and also failed to find any significant difference in stature dimorphism based on marriage system. The testfails even if one the reported a step justified given that sexual selection theory predicts higher dimorphism values under polygyny. Furthermore, this result is stable: no signif. icant difference emerges even with progressively more stringent case-inclusion rules. As reported by Alexander et al. (1979), marriage system alone seems to have little impact o n the magnitude of stature dimorphism in human societies. Failing- to find any effect of - significant marriage system on dimorphism, Alexander et al. (1979) turned to their distinction between EIM and SIM societies. Thev found that SIM societies, which are stratified but monogamous, did not differ from polygynous societies in terms of stature dimorphism. However, both SIM and polygynous societies exhibited significantly higher levels of stature dimorphism than did EIM societies. We replicated this design by contrasting EIM societies with all others (Table 2, comparison 2). For the least stringent case-inclusion rule, EIM societies are indeed characterized by significantly lower levels of stature dimorphism ( P = 0.033, given that Alexander’s directional prediction allows the two-tailed P value to be halved), but this result is unstable and depends on the inclusion of societies where dimorphism is relatively poorly estimated. EIM societies do not show significantly less dimorphism if the analysis is restricted to societies where at least ten females and ten males were measured, and the alleged effect is even further weakened with more stringent case-inclusion criteria.
S.J.C. GAULIN AND J.S. BOSTER
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TABLE 2. One-way and two-wayanalyses of variance for the dimorphism data an Table 1A
One-way comparisons 1. (EIM + SIM) vs. (DBP + RDP) A. All societies B. Societies with n 3 10 C. Societies with n 3 20 2. EIM vs. (SIM + DBP + RDP) A. All societies B. Societies with n 3 10 C. Societies with n z 20 Two-way comparisons 3. EIM vs. SIM vs. DBP vs. RDP All societies Marriage f,ffect
Stratification effect Interaction effect Societies with n 10 Marriage effect Stratification effect Interaction effect Societies with n 2 20 Marriare effect Stratification effect Interaction effect
F
Effective d.f."
P value two-tailed)
0.19 0.01 0.49
1. 54 1, 48 1, 39
0.663 0.905 0.490
3.74 2.65 0.63
1, 26 1, 22 1, 16
0.065 0.118 0.439
0.1s 0.64 10.91
1, 44
1, 45 1, 45
0.698 0.430 0.002
0.29 0.63 10.07
1, 43 1, 45 1, 45
0.594 0.431 0.003
1.11 0.23 6.67
1. 36 1, 37 1, 37
0.632 0.014
Our analysis recognizes two types of polygyny as well as two types of monogamy. When the effects of all four marriage systems on dimorphism are considered in a two-way ANOVA, neither the marriage nor the stratification main effect is statistically significant. However, a n inspection of Table 1A-C shows that the diagonally opposite cells exhibit similar dimorphism values. A highly significant marriage-by-stratification interaction effect (Table 2. comparison 3) confirms this impression. The effects of marriage system on stature dimorphism are reversed for stratified and nonstratified societies: among nonstratified societies, polygyny is associated with higher levels of stature dimorphism than is monogamy, but, among stratified societies, polygyny is associated with lower levels of stature dimorphism than is monogamy. This unexpected result is stable in the face of progressively more stringent case-inclusion rules and is thus unlikely to be a n artifact of sample size. Perhaps the observed distribution of stature dimorphism is due not to sexual selection but to regional differences. The logic runs as follows. Marriage systems are not randomly distributed over the globe. For example, SIM is overrepresented in circumMediterranean and East Asian populations
n xnn ~ . ~
.
and is rare elsewhere. Each of the other three marriage systems also shows a unique regional bias, and a contingency table analysis of marriage system by geographic region is highly significant (likelihood ratio x2 = 68.23; P < .001). Patterned geographic variation in marriage systems could produce a spurious association between marriage system and dimorphism, but only if there were similarly patterned geographic variation in stature dimorphism. We tested this possibility by using ANOVA to examine the association between stature dimorphism and region. No significant relationships emerged, regardless of which case-inclusion method was used and despite the fact that, for some of our analyses, we grouped adjacent regions with similar dimorphism values to reduce cell number and enhance any explanatory power that region might have. This suggests that the observed association between dimorphism and marriage system is not a n artifact of regional differences in dimorphism. DISCUSSION AND CONCLUSIONS
Gaulin and Boster (1985) asked whether there was any pattern in the cross-cultural distribution of stature dimorphism that could be due to cross-cultural variation in
MARRIAGE SYSTEMS AND SEXUAL DIMORPHISM
the intensity of sexual selection. At that time, we argued that there was not. Our present, more detailed analyses largely support that conclusion, insofar a s they fail to reveal any consistent effects of sexual selection. There is, however, a robust, puzzling and previously unsuspected pattern in the distribution of stature dimorphism. Our observation that marriage system alone has no discernible effect on the level of stature dimorphism agrees with the findings of Alexander et al. (1979) and others (Gray and Wctlfe, 1980: Wolfe and Grav 1982a). The absence of such a n effect might be taken a s evidence that variation in the intensity of sexual selection is not the cause of cross-cultural differences in sexual dimorphism, but this conclusion is probably too simplistic. Following Orians (19691, Alexander et al. (1979)argue that a n even distribution of resources among breeding males is the typical cause of monogamy among endotherms. They note that only EIM societies exhibit such even resource distributions and that therefore only EIM societies should show the reduced dimorphism characteristic of other monogamous species. While their statistical tests support this more precise prediction, only our least stringent analysis offers support for the idea that EIM societies exhibit reduced stature dimorphism; the effect progressively disappears when the analysis is restricted to better estimated cases. The idea that ecologically monogamous societies are characterized by unusually low levels of stature dimorphism (Alexander et al., 1979:419) should probably be abandoned, at least until it can be supported by reliable data. Thus neither simple nor more subtle predictions from sexual selection theory match the cross-cultural pattern of sexual dimorphism. The remainder of our findings pose an important puzzle. The levels of stature dimorphism in the first rows of Table 1A-C could be argued to be consistent with the force of sexual selection. Ecologically imposed monogamy should be associated with low levels of dimorphism, and male-dominance competition (DBP) in the absence of resource-related inducements to polygynous mating should be associated with higher levels of physical dimorphism. However, it seems dif-
473
ficult to construct a n argument based on current sexual selection theory to explain the reversal of this pattern in the second rows of Table 1A-C. Why should societies practicing SIM be the most sexually dimorphic of human societies; why should they be more dimorphic than stratified polygynous (RDP) societies? If sexual selection cannot explain the pattern in the lower rows of Table 1A-C, its credibility as a n explanation for the upper rows is suspect. The anomalously high level of sexual dimorphism found in SIM societies could be reconciled with sexual selection theory if it resulted either from a recent history of marked polygyny in these highly stratified societies ( a not improbable situation; Betzig, 1986), from significant ongoing but covert polygyny, or from a combination of such historical and contemporary forces. However, this line of reasoning cannot explain why stratified polygynous (RDP) societies should have lower levels of dimorphism than stratified monogamous (SIM) societies. Some authors would not be surprised a t the apparent failure of sexual selection to account for cross-cultural differences in dimorphism. Lande (1980) argued that it could take much longer for populations to reach equilibria for sex-linked traits under disruptive selection than for nondimorphic traits under directional selection. Drawing on human data first published by Pearson and Lee (1903), Rogers and Mukherjee (1992)recently estimated the response to selection of cross-sex means and sex differences for several linear traits, including stature. They conclude that The additive genetic covariances between male and female length measurements are extremely high, suggesting that genes for such characters tend to affect males and females in the same way. The result is that the mean of the two sexes responds to selection many times faster than does sexual dimorphism (Rogers and Mukherjee, 1992).
Of course this conclusion does not imply that sexual selection cannot produce sexual dimorphism: it clearly has done so in many species, including primates (Gaulin and Sailer, 1984). However, if human marriage systems are relatively transient phenomena, they may seldom persist long enough to
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S.J.C. GAULlN AND J.S. BOSTER
measurably alter the magnitude of sexual dimorphism. If neither sexual selection nor regional differences can presently explain the crosscultural variance in stature dimorphism, are there viable alternative explanations? Several have been previously offered and should at least be mentioned here. First, some authors have argued that sexual dimorphism in Homo sapiens is primarily a consequence of the sexual division of labor: where male and female tasks differ most in clithe physical demands they make, sex~1i-11 morphism will be greatest (Frayer, 1980; Finkel, 1982). I n evaluating the plausibility of this explanation, it is important to be precise about the causal mechanism by which division of labor produces sexual dimorphism. Disruptive selection is one possible mechanism, different phenotypes being favored in the two sexes because they confront somewhat different ecological pressures. Alternatively, facultative (developmental) responses to divergent exercise regimes could produce adult sex differences. Nevertheless, the effects of disruptive selection will be slow (Lande, 1980; Rogers and Mukherjee, 1992) regardless of whether the disruptive pressures arise out of sexual selection or ecology. Thus it may not be surprising that Wolfe and Gray’s (1982b) attempt to test the division-of-labor hypothesis on a cross-cultural data set found no support. Second, substandard nutrition could cause individuals to fall short of their genetically set growth potential, and, importantly, males seem to be more sensitive to such developmental perturbations than females (Stini, 1969; Luce and Wand, 1977; Hamilton, 1982). Poor nutrition would thus depress male growth more than female growth and thereby reduce sexual dimorphism; by a similar logic, improved nutrition could increase sexual dimorphism. Thus, even if the difference between genetically programmed male and female growth curves were invariant across all human populations, there still might be population differences in the ratio of male to female stature due to differences in nutritional status. As a final resort, one might invoke genetic admixture (Valenzuela et al., 1978) to explain cross-cultural variation in stature di-
morphism. The problem with this argument is that admixture will tend to break down any cross-cultural differences. Thus admixture might be invoked to explain why the clear patterns predicted by sexual selection theory do not emerge, but it obviously fails as a n explanation for the robust stratification-by-marriage interaction effect. Likewise, social selection, the tendency for taller individuals to be more successful socially and to be preferred a s mates, could drive the evolution of sexual dimorphism (Bogin, i988), bul why would this process be stronger in DBP and SIM societies than it is in EIM and RDP societies? Without a n answer to this question, social selection also fails to explain the significant interaction effect. Empirical evaluations of these hypotheses are beyond the scope of this article, but the present data can offer a preliminary indication of whether a test of these hypotheses would be justified. The four groups of societies examined here are ranked a s follows: EIM and RDP societies are the least dimorphic, followed by DBP societies, with SIM societies being the most dimorphic. If a model appealing to nutrition, division-of-labor, genetic admixture, or social selection would predict such a ranking, then that model would warrant a fine-grained statistical test. Of course, the observed cross-cultural distribution of sexual dimorphism might well be due to a combination of these (and other) factors. At present it is probably wise to refralri from offering posthoc explanations for the observed cross-cultural distribution of sexual dimorphism. Although the data do not seem to conform to the expectations of sexual selection theory, any actual congruence between theory and data could be obscured by weak proxy measures of the relevant factors. To test whether cross-cultural differences in the intensity of sexual selection have produced societal differences in sexual dimorphism, we first need to assess how the intensity (and possibly the form) of malemale competition varies across societies. As Bateman (1948) showed in his pioneering work with fruitflies, variances in male and female reproductive success are the key parameters defining the force of sexual selection. Cross-cultural studies must use
MARRIAGE SYSTEMS AND SEXUAL DIMORPHISM
marriage practices to approximate these parameters because actual measurements of variance in reproductive success are not currently available for any human society. Thus it is premature to claim that sexual selection could never explain contemporary patterns of stature dimorphism in Homo sapiens. Considering the theoretical importance of sexual selection and the cross-cultural ubiquity of sex differences, this is fertile ground for future empirical work. ACKNOWLEDGMENTS
We thank Maria Freilino and Leslie Clark for assistance in compiling the data analyzed in this article. Alan Rogers, Rich Scaglion, Christine Milberg, Laurie Corwin, Linda Wolfe, and a n anonymous reviewer gave helpful criticism (although this article should not be construed a s necessarily representing their viewpoints). We thank Doug White for his tireless efforts a s founding editor of the electronic journal World Cultures. The University of Pittsburgh and the University of California a t Irvine provided some computing resources. LITERATURE CITED Alcock J (1989) Animal Behavior: An Evolutionary Approach, 4th ed. Sunderland, MA: Sinauer. Alexander RD, Hoogland JL, Howard RD, Noonan KM, and Sherman PW (1979) Sexual dimorphism in pinnipeds ungulates primates and humans. In N Chagnon and W Irons (eds.): Evolutionary Biology and Human Social Behavior: An Anthropological Perspective. North Scituate, MA: Duxbury, pp. 402435. Rateman AJ (1948) Intra-sexual selection in Drosophila. Heredity 2349-368. Betzig L (1986) Despotism and Differential Reproduction: A Darwinian View of History. New York: Aldine. Bogin B (1988)Patterns of Human Growth. Cambridge: Cambridge University Press. Borgerhoff Mulder M (1987) On cultural and reproductive success: Kipsigis evidence. Am. Anthropol. 89: 617-634. Borgerhoff Mulder M (1990) Kipsigis women’s preference for wealthy men: Evidence for female choice in mammals? Behav. Ecol. Sociobiol.27,255-264. Chagnon N (1977) Yanomamo: The Fierce People, 2nd ed. New York: Holt, Rinehart and Winston. Finkel DJ (1982) Sexual dimorphism and settlement pattern in Middle Eastern skeletal populations. In RL Hall (ed.): Sexual Dimorphism in Homo supiens: A Question of Size. New York: Praeger, pp. 165-185.
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Frayer DW (1980)Sexual dimorphism and cultural evolution in the late Pleistocene and Holocene of Europe. J. Hum. Evol. 9t399415. Gaulin SJC, and Boster JS (1985)Cross-cultural differences in sexual dimorphism: Is there any variance to be explained. Ethol. Sociobiol. 6:219-225. Gaulin SJC, and Boster JS (1990) Dowry as female competition. Am. Anthropol. 92:994-1005. Gaulin SJC, and Sailer LD (1984)Sexual dimorphism in weight among the primates: The relative impact of allometry and sexual selection. Internat. J. Primatol. 5: 5 15-535. Gray JP, and Wolfe LD (1980) Height and sexual dimorphism of stature among human societies Am .I Ph:;:. Xlithrupoi. 5d:441-456. Hamilton ME (1982) Sexual dimorphism in skeletal samples. In RL Hall (ed.): Sexual Dimorphism in Homo sapiens: A Question of Size. New York: Praeger, pp. 107-163. Hart CWM, and Pilling AR (1960) The Tiwi of North Australia. New York: Holt, Rinehart and Winston. Lande R (1980) Sexual dimorphism, sexual selection, and adaptation in polygenic characters. Evolution 34:292-305. Luce SR and Wand B (1977) Sex differences in health and illness. Can. Psychol. Rev. 18:79-91. Murdock GP (1986)Ethnographic Atlas. World Cultures Vol. 2, No. 4. Orians G (1969) On the evolution of mating systems in birds and mammals. Am. Nat. 103:589-603. Pearson K, and Lee A (1903) On the laws of inheritance in man. I. Inheritance of physical characters. Biometrika 2:357462. Ralls K (1976) Mammals in which females are larger than males. Q. Rev. Biol. 51:245-276. Rogers AR, and Mukherjee A (1992) Quantitative genetics of sexual dimorphism in human body size. Evolution 46,226-234. Stini WA (1969) Nutritional stress and growth: Sex difference in adaptive response. Am. J. Phyq Anthropo!. 31 417-426. Trivers RL (1972)Parental investment and sexual selection. In B Campbell (ed.): Sexual Selection and the Descent ofMan: 1871-1971. Chicago: Aldine, pp. 136179. Valenzuela CY, Rothhammer F, and Chakraborty R (1978) Sex dimorphism in adult stature in four Chilean populations. Ann. Hum. Biol. 5:533-538. Wade MJ (1979) Sexual selection and variance in reproductive success. Am. Nat. 114:742-747. Williams GC (1966) Adaptation and natural selection. Princeton, NJ: Princeton University Press. Wolfe L, and Gray J P (1982a) A cross-cultural investigation into the sexual dimorphism of stature. In RL Hall (ed.): Sexual Dimorphism in Homo supiens: A Question of Size. New York: Praeger, pp. 197-230. Wolfe LD, and Gray J P (198213)Subsistence practices and human sexual dimorphism of stature. J. Hum. Evol. 11:575-580.
Human Marriage Systems and Sexual Dimorphism in Stature STEVEN J.C. GAULIN AND JAMES S. BOSTER Department of Anthropology, University o f Pittsburgh, Pittsburgh, Pennsylvania 15260 (S.J.C.G.); Department of Anthropology, University of California at Irvine, Irvine, California 92717 (J.S.B.)
KEY WORDS
Sexual selection, Monogamy, Polygyny, Sex differences, Stratification
Contemporary populations of Homo sapiens are sexually diABSTRACT morphic on a variety of traits. In terms of stature, men are reliably between 4% and 10% taller than women in well-sampled human populations. Are cross-cultural differences in the magnitude of sexual dimorphism consistent with expectations from sexual selection theory? Prior studies have provided conflicting answers to this question in part because they failed to agree on how the force of sexual selection should or could be operationalized. Here we offer a simple and unbiased method for operationalizing sexual selection and retest two separate predictions from earlier work (Alexander e t al., 1979) about its expected impact on stature dimorphism in a sample of 155 societies. Neither prediction matches the observed cross-cultural distribution of dimorphism. However, this is not the consequence of a random distribution of dimorphism across societies. Instead, the data exhibit a robust and unexpected pattern. o 1992 WiIey-Liss, Inc. The relationship between biology and culture is a central issue in anthropology. Here we examine the extent to which cultural practices (in particular, marriage practices) predict the pattern of sexual dimorphism in stature across a large sample of human societies. There are a priori reasons, arising out of sexual selection theory (Bateman, 1948; Williams, 1966; Trivers, 1972; Wade, 19791, to expect a n association between the form of the mating system and the magnitude of sexual dimorphism. For example, in effectively polygynous mating systems, some males monopolize breeding opportunities. This precipitates strong selection on males for traits that contribute to mating success. However, under such mating systems, females do not undergo similar selection for traits that enhance mating opportunities. Because the sexes thus experience divergent selection pressures, sexual dimorphism oftenevolves. Incontrast,undereffective monogamy, competition for mates is not disproportionately elevated in either sex. Selection 0 1992 WILEY-LISS, INC
pressures on females and males thus do not diverge, and sexual monomorphism is the typical result. The comparative method has generally confirmed these predicted correlations between the type of mating system and magnitude of sexual dimorphism. although exceptions are known (Ralls, 1976). For human populations, data on actual mating systems are lacking. Extensive accounts of human marriage practices are available. Only random paternity testing could reveal the extent to which actual human mating systems diverge from the stated marriage system. Lacking actual mating data, theorists examining the causes of sexual dimorphism in human populations have habitually used marriage system as a gauge of the intensity of sexual selection (Alexander et al., 1979; Gray and Wolfe, 1980; Wolfe and Gray, 1982a). In a n early paper, Alexander et al. (1979) analyzed cross-cultural variation in human
Received July 16, 1991; accepted May 27, 1992.
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S.J.C. GAULIN AND J.S. BOSTER
sexual dimorphism in terms of sexual selection theory. Their initial result was that societies practicing monogamous marriage and societies practicing polygynous marriage did not differ in terms of their levels of sexual dimorphism. Examining the ratio of male to female stature, they found a mean of 1.078 in a sample of 46 polygynous societies and a very similar mean of 1.075 in a sample of 47 monogamous societies. Alexander et al. (1979) offered a possible explanation for this unexpected result. They argued that monogamy is not a unitary phenomenon among humans; some monogamous marriage systems are closely parallel to the condition in monogamous nonhuman mammals o r birds, but others are evolutionarily unique. Their logic can be traced back to Orians (1969), who noted that, in endotherms, a male’s reproductive success would be proportionate to the number of mates he had, and that males would therefore strive to be maximally polygynous. Orians recognized the implicit paradox: why are there any nonpolygynous endotherms? Perhaps polygyny imposes fitness costs on females. He reasoned that, where these costs are high, selection could favor female tactics that prevent polygyny; conversely, where females suffer little or no such costs, selection on males could be the dominant force shaping the mating system, and polygyny would spread. However, what factors affect the cost of polygyny to females? Orians was modeling a system where individual males control access to pools of reproductively useful resource (This would adequately model territoriality among animals or private property among humans.) In such a system, females pay a high cost for polygyny whenever there is little variance among males in the value of their resource pools. In this low-variance case, a monogamously mated female would have a typical resource pool available for use in reproduction, whereas a polygynously mated female would have no more than one-half a typical resource pool and would therefore suffer reduced fitness. In contrast, where there is high variance among male resource pools, females might cluster with the “wealthy” males without paying any significant cost for the resultant polygyny. Thus Orians ar-
gued that the distribution of resources among breeding males is a major determinant of mating systems. Orians’ model can explain much of the variation in human marriage patterns. Most stratified societies-those with uneven distributions of wealth-are indeed polygynous (Gaulin and Boster, 1990; Table 1). With reference to monogamous marriage, Alexander et al. (1979) argued that human societies are of two distinct types. They suggested that societies exhibit ecologically imposed monogamy (EIM) wherever ”individual men are typically unable to gain by attempting to provide for the offspring of more than one wife” (Alexander et al., 1979; 418419). In other words, where reproductively relevant resources are evenly distributed among males, a man who marries polygynously inflicts prohibitively high reproductive costs on each of his wives, because he forces them to share a resource base equivalent t o what monogamously married women have to themselves. In contrast, the second type of monogamy represents a n evolutionary novelty that has arisen in the last 10,000 years of human evolution. Despite very uneven wealth distributions, some highly stratified societies are nonetheless monogamous. Because there seems to be no resource-related barrier to polygyny in these societies, Alexander et al. (1979) argued that they exhibit socially imposed monogamy (SIM), a uniquely human phenomenon. Recognizing three human marriage systems (polygyny, EIM, and SIM) Alexander et al. (1979) reexamined the cross-cultural variance in stature dimorphism. They found that polygynous and SIM societies were both characterized by high levels of dimorphism (1.078) but that EIM societies were marked by significantly lower dimorphism (1.068). These conclusions have been the subject of debate. Some of the objections, for example Wolfe and Gray’s (1982a) claim that most of the cross-cultural variance in stature dimorphism is allometric, have been shown to be spurious (Gaulin and Boster, 1985). However, other objections are more serious. Several critics (Gray and Wolfe, 1980; Wolfe and Gray, 1982a; Gaulin and Boster, 1985) have argued that Alexander et al.
MARRIAGE SYSTEMS AND SEXUAL DIMORPHISM
(1979) had no independent method for operationalizing the distinction between EIM and SIM categories and that their findings on the distribution of dimorphism were thus subject to possible experimenter bias. In addition, we (Gaulin and Boster, 1985) showed that much of the cross-cultural variation in stature dimorphism arises from sampling error within societies; that is, the larger the sample of males and females measured in a given society, the more closely the societal dimorphism estimate converged on the species-wide average. Thus, while the theoretical rationale for distinguishing between SIM and EIM societies is clear and persuasive, it has been difficult in practice to demonstrate a relationship between the type of marriage system and the magnitude of stature dimorphism. Recently, in attacking a quite different problem, we have attempted to describe the dynamics of intrasexual competition within SIM societies (Gaulin and Boster, 1990). To test our ideas, we developed a n operational definition of S I N societies that allowed their unbiased identification on the basis of prior codings from the Ethnographic Atlas (Murdock, 1986). The prospect of objectively distinguishing between SIM and EIM societies led us to reevaluate the claims of Alexander et al. (1979). There are, of course, alternative accounts of cross-cultural differences in sexual dimorphism. For example, there could be regional variation in sexual dimorphism. although this finding would itself require explanation. Nevertheless, to provide a broad framework for discussion of stature dimorphism, we formally evaluate the regional hypothesis and outline several other possible explanations. MATERIALS AND METHODS By the original definition of Alexander et al. (1979), SIM societies are characterized by monogamous marriage in contexts where, due to a n uneven distribution of resources among breeding males, Orians’ (1969) model would predict polygyny. In other words, SIM societies practice monogamous marriage despite the fact that women ( a t least women with wealthy husbands) would pay a relatively low cost for polygyny.
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To gauge the magnitude of the cost that polygyny inflicts on women in any given society, we needed to assess whether resources are evenly or unevenly distributed. To provide this assessment, we chose the extent of social stratification, because stratification almost always entails unequal resource distribution and because many ethnographers have attended to this variable. Both marriage practices and social stratification have been previously coded for the societies of the Ethnographic Atlas (Miirduck, 1 9 8 6 ~and we grouped these existing codes to create dichotomous variables. We regarded a society as stratified if it was coded as having “elite,” “dual,” or “complex” stratification (Murdock, 1986:col. 65; codes 3 , 4, or 51, and as nonstratified otherwise. With respect to marital form, we regarded a society a s polygynous if polygyny was coded a s at least “occasional,” (Murdock, 1986:col. 9; codes 2 , 3 , 4 , 5 , or 61, and as monogamous otherwise. [For the present data set, this technique includes three polyandrous societies in the “monogamous” group. This is a reasonable coding decision because we are trying to assess whether high levels of malemale competition are associated with high levels of stature dimorphism. According to current theory (see, e.g., Trivers, 19721, male-male competition is expected to be high under polygyny but low under monogamy and polyandry.1 These dichotoniising rules yielded a 2 x 2 classification scheme for societies. First, consider societies practicing monogamous marriage. In nonstratified, monogamous societies, there is no resource-related polygyny potential, because resources are relatively evenly distributed among breeding males; thus these cases represent EIM. According to Orians’ model, stratified societies are expected to be polygynous; it follows that, when they are monogamous, the monogamy is socially imposed (SIM). J u s t as it distinguishes two types of monogamy, this classification scheme suggests that there are also two types of polygyny, one that occurs in stratified societies and one that occurs in nonstratified societies. The type of polygyny described by Orians (19691, where reproductive females cluster because certain males offer more or better
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S.J.C. GAULIN AND J.S. BOSTER
resources, is called resource-defense polygyny (RDP) by ethologists (see, e.g., Alcock, 1989). This same term could be argued to be appropriate in the anthropological realm, because in stratified human societies the distribution of wives tends to parallel the distribution of wealth (Betzig, 1986; Borgerhoff Mulder, 1987, 1990). Similarly, the other type of polygyny may have instructive analogues. Among nonhuman animals, polygyny sometimes occurs in the absence of resource defense and hence in the absence of any resuurce-related inducements for females to cluster with particular males. In these cases, social dominance relationships determine which males are polygynous. Similarly, ethnographic data from nonstratified polygynous societies suggest that a man’s social dominance can be the primary determinant of his ability to marry polygynously (Chagnon, 1977; Hart and Pilling, 1960). Thus we suggest the term dominance-based polygyny (DBP) to refer to those cases where polygyny occurs in the absence of significant resource differentials among males, that is, in nonstratified societies. We selected stature dimorphism a s our measure of sexual dimorphism for both pragmatic and theoretical reasons. Stature is the most widely measured parameter on which the sexes differ; this permits the compilation of a large cross-cultural database. Moreover. in every human society for which data exist, men are taller than women, suggesting that some form of disruptive selection has moved males and females apart on this dimension. Sexual selection is a common form of disruptive selection and is thus a likely cause. The data on stature dimorphism derive from a cross-cultural sample of 289 societies originally compiled by J. Patrick Gray and previously analysed by Wolfe and Gray (1982a) and Gaulin and Boster (1985). Because dimorphism estimates are known to be sensitive to the number of males and females measured (cf. Gaulin and Boster, 1985:Fig. 8), we deleted the 22 societies for which these sample-size data were missing. Of the 267 remaining societies, 170 were unambiguously identical with societies coded in the Ethnographic Atlas, but, for 15 of
these 170 societies, information on either marriage, stratification, or both was missing (Murdock, 1986). Our sample universe thus included 155 societies for which marriage and stratification patterns could be confidently associated with stature dimorphism measurements. Sexual dimorphism may be more tightly associated with geographic region t h a n it is with the type of mating system. To allow a consideration of this hypothesis, we also extracted the geographic region codes (Miirdock, 1986:col. 90) for each of the 155 societies in our sample universe. Some of the cross-cultural variance in stature dimorphism is due to widely differing sample sizes within societies (Gaulin and Boster, 1985). Because estimates of stature dimorphism are ratios (male height/ female height), small samples for either sex lower the reliability of dimorphism estimates. To take such reliability issues into account, we defined a sample-size variable for each society as the smaller of two values, either the number of males o r the number of females measured in that society. We replicated each of our analyses three times using the following case-inclusion rules: 1)all societies regardless of sample size ( n = 155 societies), 2) all societies with sample sizes 3 1 0 (n = 141 societies), and 3) all societies with sample sizes 3 2 0 (n = 118 societies). We focus attention on those findings that remain stable with progressively more stringent case-inclusion rules. We used analysis of variance (ANOVA) to compare the magnitude of stature dimorphism in societies with different types of marriage and stratification patterns. Conservative two-tailed tests were used throughout, but, in reevaluating some of the hypotheses tested by Alexander et al. (19791, a halving of the resultant P value was justified because their a priori predictions were directional. We used the BrownForsythe method to calculate F and P values because it takes account of unequal cell variances. RESULTS Table 1 presents the mean values of stature dimorphism (male height/female height) for each of the four types of societies dis-
MARRIAGE SYSTEMS AND SEXUAL DIMORPHISM TABLE IA. Mean values for sexual dimorphism in stature by marriage system and stratification for human societies regardless o f internal samule size
Monogamous and polyandrous ~~~~~~~
Nonstratified (n)
Stratified ( n )
Column means ( n )
“EIM 1.068
Row Polygynous means~ ~ _ _ “DBP
(19)
1.076 (87)
“SIM
“RDP
1.079 113)
1.069 (36)
1.072 (49)
1.073 (32)
1074
in74 (155)
(123)
1.075
(106)
TABLE 1B Mean values for sexual dimorphism in stature by marriage system and stratification for human societies wath sample sizes 2 1 0
Monogamous and polyandrous Polygynous Nonstratified (n)
Stratified ( n )
Column means (n)
Row means
“EIM”
“DBP
1068 (17)
1075
“SIM
“RDP
1079 (13)
1069 (35)
(48)
1.073
1.073 (111)
1.073 (141)
(30)
(76)
074 (93) 072
TABLE 1C. Mean values for sexual dimorphism in stature by marriage system and stratification for human societies with sample sizes a20
Monogamous and polyandrous Nonstratified (nJ
Stratified (nJ
“EIM
“DBP
1.070
1.074
1.073
(13)
(59)
(72i
“SIM (12)
“RDP 1.068 (34)
1.071 (46)
1.074
1.072
1.073
(25)
(93)
(118)
1.079
Column means ( n )
Polygynous Row meanS
cussed above, for each of three case-inclusion rules. Within the table, the four cell means are similar; at the extremes, EIM and SIM societies differ in stature dimorphism by -1%. Nevertheless, it is valid to
47 1
ask whether these differences are meaningful by using ANOVA to compare the withincell and between-cell variances. In their initial comparison of monogamous and polygynous societies, Alexander et al. (1979) combined SIM and EIM societies a s a single group and contrasted this group with all polygynous societies. They found that this approach yielded no significant difference in stature dimorphism between polygynous and monogamous societies (Alexander et al., 1979:418). We replicated this design (Table 2, comparison 1)and also failed to find any significant difference in stature dimorphism based on marriage system. The testfails even if one the reported a step justified given that sexual selection theory predicts higher dimorphism values under polygyny. Furthermore, this result is stable: no signif. icant difference emerges even with progressively more stringent case-inclusion rules. As reported by Alexander et al. (1979), marriage system alone seems to have little impact o n the magnitude of stature dimorphism in human societies. Failing- to find any effect of - significant marriage system on dimorphism, Alexander et al. (1979) turned to their distinction between EIM and SIM societies. Thev found that SIM societies, which are stratified but monogamous, did not differ from polygynous societies in terms of stature dimorphism. However, both SIM and polygynous societies exhibited significantly higher levels of stature dimorphism than did EIM societies. We replicated this design by contrasting EIM societies with all others (Table 2, comparison 2). For the least stringent case-inclusion rule, EIM societies are indeed characterized by significantly lower levels of stature dimorphism ( P = 0.033, given that Alexander’s directional prediction allows the two-tailed P value to be halved), but this result is unstable and depends on the inclusion of societies where dimorphism is relatively poorly estimated. EIM societies do not show significantly less dimorphism if the analysis is restricted to societies where at least ten females and ten males were measured, and the alleged effect is even further weakened with more stringent case-inclusion criteria.
S.J.C. GAULIN AND J.S. BOSTER
472
TABLE 2. One-way and two-wayanalyses of variance for the dimorphism data an Table 1A
One-way comparisons 1. (EIM + SIM) vs. (DBP + RDP) A. All societies B. Societies with n 3 10 C. Societies with n 3 20 2. EIM vs. (SIM + DBP + RDP) A. All societies B. Societies with n 3 10 C. Societies with n z 20 Two-way comparisons 3. EIM vs. SIM vs. DBP vs. RDP All societies Marriage f,ffect
Stratification effect Interaction effect Societies with n 10 Marriage effect Stratification effect Interaction effect Societies with n 2 20 Marriare effect Stratification effect Interaction effect
F
Effective d.f."
P value two-tailed)
0.19 0.01 0.49
1. 54 1, 48 1, 39
0.663 0.905 0.490
3.74 2.65 0.63
1, 26 1, 22 1, 16
0.065 0.118 0.439
0.1s 0.64 10.91
1, 44
1, 45 1, 45
0.698 0.430 0.002
0.29 0.63 10.07
1, 43 1, 45 1, 45
0.594 0.431 0.003
1.11 0.23 6.67
1. 36 1, 37 1, 37
0.632 0.014
Our analysis recognizes two types of polygyny as well as two types of monogamy. When the effects of all four marriage systems on dimorphism are considered in a two-way ANOVA, neither the marriage nor the stratification main effect is statistically significant. However, a n inspection of Table 1A-C shows that the diagonally opposite cells exhibit similar dimorphism values. A highly significant marriage-by-stratification interaction effect (Table 2. comparison 3) confirms this impression. The effects of marriage system on stature dimorphism are reversed for stratified and nonstratified societies: among nonstratified societies, polygyny is associated with higher levels of stature dimorphism than is monogamy, but, among stratified societies, polygyny is associated with lower levels of stature dimorphism than is monogamy. This unexpected result is stable in the face of progressively more stringent case-inclusion rules and is thus unlikely to be a n artifact of sample size. Perhaps the observed distribution of stature dimorphism is due not to sexual selection but to regional differences. The logic runs as follows. Marriage systems are not randomly distributed over the globe. For example, SIM is overrepresented in circumMediterranean and East Asian populations
n xnn ~ . ~
.
and is rare elsewhere. Each of the other three marriage systems also shows a unique regional bias, and a contingency table analysis of marriage system by geographic region is highly significant (likelihood ratio x2 = 68.23; P < .001). Patterned geographic variation in marriage systems could produce a spurious association between marriage system and dimorphism, but only if there were similarly patterned geographic variation in stature dimorphism. We tested this possibility by using ANOVA to examine the association between stature dimorphism and region. No significant relationships emerged, regardless of which case-inclusion method was used and despite the fact that, for some of our analyses, we grouped adjacent regions with similar dimorphism values to reduce cell number and enhance any explanatory power that region might have. This suggests that the observed association between dimorphism and marriage system is not a n artifact of regional differences in dimorphism. DISCUSSION AND CONCLUSIONS
Gaulin and Boster (1985) asked whether there was any pattern in the cross-cultural distribution of stature dimorphism that could be due to cross-cultural variation in
MARRIAGE SYSTEMS AND SEXUAL DIMORPHISM
the intensity of sexual selection. At that time, we argued that there was not. Our present, more detailed analyses largely support that conclusion, insofar a s they fail to reveal any consistent effects of sexual selection. There is, however, a robust, puzzling and previously unsuspected pattern in the distribution of stature dimorphism. Our observation that marriage system alone has no discernible effect on the level of stature dimorphism agrees with the findings of Alexander et al. (1979) and others (Gray and Wctlfe, 1980: Wolfe and Grav 1982a). The absence of such a n effect might be taken a s evidence that variation in the intensity of sexual selection is not the cause of cross-cultural differences in sexual dimorphism, but this conclusion is probably too simplistic. Following Orians (19691, Alexander et al. (1979)argue that a n even distribution of resources among breeding males is the typical cause of monogamy among endotherms. They note that only EIM societies exhibit such even resource distributions and that therefore only EIM societies should show the reduced dimorphism characteristic of other monogamous species. While their statistical tests support this more precise prediction, only our least stringent analysis offers support for the idea that EIM societies exhibit reduced stature dimorphism; the effect progressively disappears when the analysis is restricted to better estimated cases. The idea that ecologically monogamous societies are characterized by unusually low levels of stature dimorphism (Alexander et al., 1979:419) should probably be abandoned, at least until it can be supported by reliable data. Thus neither simple nor more subtle predictions from sexual selection theory match the cross-cultural pattern of sexual dimorphism. The remainder of our findings pose an important puzzle. The levels of stature dimorphism in the first rows of Table 1A-C could be argued to be consistent with the force of sexual selection. Ecologically imposed monogamy should be associated with low levels of dimorphism, and male-dominance competition (DBP) in the absence of resource-related inducements to polygynous mating should be associated with higher levels of physical dimorphism. However, it seems dif-
473
ficult to construct a n argument based on current sexual selection theory to explain the reversal of this pattern in the second rows of Table 1A-C. Why should societies practicing SIM be the most sexually dimorphic of human societies; why should they be more dimorphic than stratified polygynous (RDP) societies? If sexual selection cannot explain the pattern in the lower rows of Table 1A-C, its credibility as a n explanation for the upper rows is suspect. The anomalously high level of sexual dimorphism found in SIM societies could be reconciled with sexual selection theory if it resulted either from a recent history of marked polygyny in these highly stratified societies ( a not improbable situation; Betzig, 1986), from significant ongoing but covert polygyny, or from a combination of such historical and contemporary forces. However, this line of reasoning cannot explain why stratified polygynous (RDP) societies should have lower levels of dimorphism than stratified monogamous (SIM) societies. Some authors would not be surprised a t the apparent failure of sexual selection to account for cross-cultural differences in dimorphism. Lande (1980) argued that it could take much longer for populations to reach equilibria for sex-linked traits under disruptive selection than for nondimorphic traits under directional selection. Drawing on human data first published by Pearson and Lee (1903), Rogers and Mukherjee (1992)recently estimated the response to selection of cross-sex means and sex differences for several linear traits, including stature. They conclude that The additive genetic covariances between male and female length measurements are extremely high, suggesting that genes for such characters tend to affect males and females in the same way. The result is that the mean of the two sexes responds to selection many times faster than does sexual dimorphism (Rogers and Mukherjee, 1992).
Of course this conclusion does not imply that sexual selection cannot produce sexual dimorphism: it clearly has done so in many species, including primates (Gaulin and Sailer, 1984). However, if human marriage systems are relatively transient phenomena, they may seldom persist long enough to
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S.J.C. GAULlN AND J.S. BOSTER
measurably alter the magnitude of sexual dimorphism. If neither sexual selection nor regional differences can presently explain the crosscultural variance in stature dimorphism, are there viable alternative explanations? Several have been previously offered and should at least be mentioned here. First, some authors have argued that sexual dimorphism in Homo sapiens is primarily a consequence of the sexual division of labor: where male and female tasks differ most in clithe physical demands they make, sex~1i-11 morphism will be greatest (Frayer, 1980; Finkel, 1982). I n evaluating the plausibility of this explanation, it is important to be precise about the causal mechanism by which division of labor produces sexual dimorphism. Disruptive selection is one possible mechanism, different phenotypes being favored in the two sexes because they confront somewhat different ecological pressures. Alternatively, facultative (developmental) responses to divergent exercise regimes could produce adult sex differences. Nevertheless, the effects of disruptive selection will be slow (Lande, 1980; Rogers and Mukherjee, 1992) regardless of whether the disruptive pressures arise out of sexual selection or ecology. Thus it may not be surprising that Wolfe and Gray’s (1982b) attempt to test the division-of-labor hypothesis on a cross-cultural data set found no support. Second, substandard nutrition could cause individuals to fall short of their genetically set growth potential, and, importantly, males seem to be more sensitive to such developmental perturbations than females (Stini, 1969; Luce and Wand, 1977; Hamilton, 1982). Poor nutrition would thus depress male growth more than female growth and thereby reduce sexual dimorphism; by a similar logic, improved nutrition could increase sexual dimorphism. Thus, even if the difference between genetically programmed male and female growth curves were invariant across all human populations, there still might be population differences in the ratio of male to female stature due to differences in nutritional status. As a final resort, one might invoke genetic admixture (Valenzuela et al., 1978) to explain cross-cultural variation in stature di-
morphism. The problem with this argument is that admixture will tend to break down any cross-cultural differences. Thus admixture might be invoked to explain why the clear patterns predicted by sexual selection theory do not emerge, but it obviously fails as a n explanation for the robust stratification-by-marriage interaction effect. Likewise, social selection, the tendency for taller individuals to be more successful socially and to be preferred a s mates, could drive the evolution of sexual dimorphism (Bogin, i988), bul why would this process be stronger in DBP and SIM societies than it is in EIM and RDP societies? Without a n answer to this question, social selection also fails to explain the significant interaction effect. Empirical evaluations of these hypotheses are beyond the scope of this article, but the present data can offer a preliminary indication of whether a test of these hypotheses would be justified. The four groups of societies examined here are ranked a s follows: EIM and RDP societies are the least dimorphic, followed by DBP societies, with SIM societies being the most dimorphic. If a model appealing to nutrition, division-of-labor, genetic admixture, or social selection would predict such a ranking, then that model would warrant a fine-grained statistical test. Of course, the observed cross-cultural distribution of sexual dimorphism might well be due to a combination of these (and other) factors. At present it is probably wise to refralri from offering posthoc explanations for the observed cross-cultural distribution of sexual dimorphism. Although the data do not seem to conform to the expectations of sexual selection theory, any actual congruence between theory and data could be obscured by weak proxy measures of the relevant factors. To test whether cross-cultural differences in the intensity of sexual selection have produced societal differences in sexual dimorphism, we first need to assess how the intensity (and possibly the form) of malemale competition varies across societies. As Bateman (1948) showed in his pioneering work with fruitflies, variances in male and female reproductive success are the key parameters defining the force of sexual selection. Cross-cultural studies must use
MARRIAGE SYSTEMS AND SEXUAL DIMORPHISM
marriage practices to approximate these parameters because actual measurements of variance in reproductive success are not currently available for any human society. Thus it is premature to claim that sexual selection could never explain contemporary patterns of stature dimorphism in Homo sapiens. Considering the theoretical importance of sexual selection and the cross-cultural ubiquity of sex differences, this is fertile ground for future empirical work. ACKNOWLEDGMENTS
We thank Maria Freilino and Leslie Clark for assistance in compiling the data analyzed in this article. Alan Rogers, Rich Scaglion, Christine Milberg, Laurie Corwin, Linda Wolfe, and a n anonymous reviewer gave helpful criticism (although this article should not be construed a s necessarily representing their viewpoints). We thank Doug White for his tireless efforts a s founding editor of the electronic journal World Cultures. The University of Pittsburgh and the University of California a t Irvine provided some computing resources. LITERATURE CITED Alcock J (1989) Animal Behavior: An Evolutionary Approach, 4th ed. Sunderland, MA: Sinauer. Alexander RD, Hoogland JL, Howard RD, Noonan KM, and Sherman PW (1979) Sexual dimorphism in pinnipeds ungulates primates and humans. In N Chagnon and W Irons (eds.): Evolutionary Biology and Human Social Behavior: An Anthropological Perspective. North Scituate, MA: Duxbury, pp. 402435. Rateman AJ (1948) Intra-sexual selection in Drosophila. Heredity 2349-368. Betzig L (1986) Despotism and Differential Reproduction: A Darwinian View of History. New York: Aldine. Bogin B (1988)Patterns of Human Growth. Cambridge: Cambridge University Press. Borgerhoff Mulder M (1987) On cultural and reproductive success: Kipsigis evidence. Am. Anthropol. 89: 617-634. Borgerhoff Mulder M (1990) Kipsigis women’s preference for wealthy men: Evidence for female choice in mammals? Behav. Ecol. Sociobiol.27,255-264. Chagnon N (1977) Yanomamo: The Fierce People, 2nd ed. New York: Holt, Rinehart and Winston. Finkel DJ (1982) Sexual dimorphism and settlement pattern in Middle Eastern skeletal populations. In RL Hall (ed.): Sexual Dimorphism in Homo supiens: A Question of Size. New York: Praeger, pp. 165-185.
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