KILLING THE COMPETITION: Female/Female and Male/Male Homicide Martin Daly McMaster University

Margo Wilson McMaster University

Sex- and age-specific rates of killing unrelated persons of one's own sex were computed for Canada (1974--1983), England/Wales (1977-1986), Chicago (1965--1981), and Detroit (1972) from census information and data archives of all homicides known to police. Patterns in relation to sex and age were virtually identical among the four samples, although the rates varied enormously (from 3.7 per million citizens per annum in England/Wales to 216.3 in Detroit). Men's marital status was related to the probability of committing a same-sex, nonrelative homicide, but age effects remained conspicuous when married and unmarried men were distinguished. These findings and the treatment of age and sex effects by criminologists are discussed in the light of contemporary evolutionary psychological models of sex differences and life-span development. Same-sex homicides in which killer and victim are unrelated can be interpreted as an assay of competitive conflict. In every human society for which relevant information exists, men kill one another vastly more often than do women. Lethal interpersonal competition is especially prevalent among young men, which accords with many other aspects of life-span development in suggesting that sexual selection has maximized male competitive prowess and inclination in young adulthood. KEYWORDS: Adolescence; Age-crime relationship; Evolutionary psychology; Homicide; Intrasexual competition; Life-span developmental psychology; Risk-taking; Sex differences; Sexual selection Received May 26, 1989; accepted June 2, 1989. Address all correspondenceto Martin Daly or Margo Wilson, Department of Psychology, McMaster University, Hamilton, Ontario, Canada L8S 4K1. Copyright 9 1990 by Walter de Gruyter, Inc. New York Human Nature, Vol. 1, No. 1, pp. 81-107. 81

1045-6767/90/$1.00+.10

82

Human Nature, Vol. 1, No. 1, 1990

Homicidal violence manifests a large, cross-culturally universal sex difference. This fact is seldom mentioned by authors reviewing sex differences in h u m a n psychology and behavior, despite their considerable interest in "aggression" (e.g., Deaux 1985; Maccoby and Jacklin 1974). Criminologists and epidemiologists, by contrast, have often commented on the sex difference in killing (e.g., Curtis 1974; Holinger 1987; Wilson and Herrnstein 1985; Wolfgang 1978), but evolution-minded readers will be disappointed by their treatment, since it makes no reference to fundamental differences in the agendas of w o m e n and men. Analysis and understanding of the role of gender in violence have been impeded by the fact that almost all discussants have embraced the false dichotomy of "social" vs. "biological" explanations. Reviewers of biological factors typically discuss hormonal effects, Y-chromosomes, sexually differentiated body builds, and the supposed generality of greater male aggressivity among n o n h u m a n animals (e.g., Burrowes et al. 1988; Mednick et al. 1982; Wilson and Herrnstein 1985). Others d e n y that the relevance of these factors has been demonstrated and propose, as an alternative "social" explanation, that sex differences are caused by people's differential treatment of girls vs. boys and w o m e n vs. men. Occasionally explicit (e.g., Adler 1975; Eagly and Steffen 1986; Hagan 1986; Tieger 1980), but more often implied, is the proposition that w o m e n and men would behave identically if treated identically. The common practice of invoking "sex roles in our society" w h e n discussing sex differences carries the additional implication, whether intended or unwitting, that the differences under consideration are absent or reversed in other societies. In the case of violence, neither of these propositions has any evidentiary basis or coherent theoretical rationale (Daly and Wilson 1988a; Klama 1988). Given this dichotomous way of thinking, most social scientists seem to feel that some sort of middle ground is the only tenable position. "We believe that both biological and sociocultural factors contribute," wrote Maccoby and Jacklin (1980:977) in a typical statement of this compromise stance. These writers correctly reject naive assumptions about the arbitrariness and reversibility of sex differences, which are embraced by the advocates of purely "social" explanations, but share with them a narrow view of biology's potential contribution as residing solely in its mechanistic subdisciplines (genetics, endocrinology, neurology). Untouched by the revolution in evolutionary functional analysis of social phenomena inspired by Hamilton (1964) and Williams (1966), m a n y social scientists persist in equating "biological" with "invariant" and assume that biology is mute about aspects of sex differences manifesting developmentally, experientially, and circumstantially contingent variations. The irony is that developmentally, experientially, and circumstantially

Killing the Competition

83

contingent variation is precisely what evolutionary biological theories of social phenomena are about. What sort of contingent social responsiveness would be favored by selection? What social, demographic, and ecological variables influence social development, how, and why? Mechanistic details about the roles of androgens and amygdalas in aggressive action constitute partial accounts of h o w such action changes with puberty or rank or particular social situations, but such "proximate explanations" do not address the question of w h y the creature in question has evolved to aggress in this but not that life stage or situation. Exemplary theoretical work specifically concerning the strategic logic of aggression and potentially lethal violence includes that of Hamilton (1979), Maynard Smith (1974), Mock (1987), O'Connor (1978), Parker (1974), Popp and DeVore (1979), and Rohwer (1982). SAME-SEX NONRELATIVE HOMICIDES AS A N

ASSAY OF COMPETITIVE CONFLICT Whereas research on any sublethal form of violent conflict is likely to suffer from reporting biases, killings are typically discovered and investigated. Homicide archives thus provide exceptional materials for the analysis of human conflicts. We have used such archives to assess various evolutionary psychological hypotheses about h o w the intensity of interpersonal conflict varies in relation to kinship, demography, and other factors (Daly and Wilson 1982, 1988a, 1988b; Wilson and Daly 1985). In this paper, our focus will be on those homicides in which victim and killer were of the same sex and unrelated. Our principal rationale for an analysis that excludes cases in which victim and killer are of the opposite sex is that we wish to compare men's vs. women's use of dangerous violence. Since homicides are often "victim-precipitated," one cannot assume that the killer was the initial aggressor or the party responsible for the escalation to lethal danger. This makes opposite-sex cases ambiguous with regard to men's vs. women's use of violence. Many, perhaps most, cases in which w o m e n kill their husbands, for example, have strong elements of self-defense (Browne 1987; Daly and Wilson 1988a; Jones 1980; Totman 1978). In a same-sex case, by contrast, though the principal or initial aggressor's identity may be ambiguous, his or her sex is not. "Competition" refers to any conflict of interests in which one party's possession or use of a mutually desired resource precludes the other party's possession or use of the same. Robbery homicides are unequivocal instances, as are many "sexual triangle" cases. More subtle examples are the "face" and "status" disputes among acquaintances that

84

Human Nature, Vol. 1, No. 1, 1990

constitute a very large proportion of U.S. homicides; the "social resources" that are contested in these conflicts are limited means to the end of more tangible resources (Wilson and Daly 1985). However, although many conflicts can be interpreted as competitive, not all can: If a woman spurns one suitor for another, for example, then she and the rejected man have a conflict of interests, but they are not competitors, whereas the rival suitors are. In general, competitive confict is predominantly a same-sex affair because same-sex individuals are usually more similar in the resources they desire than are opposite-sex individuals. In particular, opposite-sex individuals are often the "resource" being competed for. The present analyses are furthermore limited to cases in which victim and killer were unrelated, because conflicts precipitating family homicides are often rather different from the competitive conflicts that are our focus. A large proportion of the cases perpetrated by women, for example, are infanticides, the variable risk of which can be interpreted as reflecting evolved motivational mechanisms for maternal manipulation of lifetime reproductive effort (Daly and Wilson 1988a); if such a case can be considered "competitive," then the victim's competitor is not its killer/ mother but its (existing or future) siblings. Marital relatives present a different complication: male--male homicides might outnumber femalefemale cases simply by virtue of a difference in the numbers of such relationships (e.g., if stepfathers outnumber stepmothers) or in spatial proximity (e.g., if married couples have more frequent contact with the wife's relatives than the husband's). More generally, genetic and marital relationships entail an element of overlapping interests in the welfare of joint kin as vehicles of fitness, so the exclusion of relatives permits a sharper focus on more straightforwardly competitive conflicts. Competitive violence among women vs. men thus seems best compared in terms of nonrelative cases.

SAME-SEX NONRELATIVE HOMICIDES A M O N G MEN VS. WOMEN Table 1 presents our tabulations of the gross numbers of same-sex nonrelative homicides from four case-by-case data sets, as well as results from all published studies of which we are aware in which the requisite information was reported. The largest data sets in Table 1 are all from Western industrial nations, but other sorts of societies are also represented, including tribal horticulturalists (Bhil, Maria, Mayans, Munda, Oraon, Tiv) and foragers (!Kung San). The four data files from which we have tabulated the cases (Chicago,

Killing the Competition

85

Table 1. N u m b e r s of Same-Sex Nonrelative H o m i c i d e s in Various Studies ~

Homicides Location/society

Male

Female

Chicago, 1965-1981 Detroit, 1972 Miami, 1980 Canada, 1974-1983 England/Wales, 1977-1986 Scotland, 1953-1974 Iceland, 1946-1970

7439 316 358 2387 2195 143 7

195 11 0 59 95 5 0

Reference This study This study Wilbanks (1984) This study This study Gillies (1976) Hansen and Bjarnason

(1974) A Mayan village, 1938-1965 Bison-Horn Maria (India), 1920-1941 Munda (India) Oraon (India) Bhil (India), 1971-1975 Tiv (Nigeria), 1931-1949 BaSoga (Uganda), 1952-1954 Gisu (Uganda), 1948-1954 BaLuyia (Kenya), 1949-1954 Banyoro (Uganda), 1936-1955 JoLuo (Kenya), ca. 1949 Alur (Uganda), 1945-1954 !Kung San (Botswana), 1920-1955

15 36

0 1~

Nash (1967) Elwin (1950)

34 26 50 74 38 44 65 9 22 33 12

0 0 Ia 1 0 2 3a 1~ 2a 1~ 0

Saran (1974) Saran (1974) Varma (1978) Bohannan (1960b) Fallers and Fallers (1960) LaFontaine (1960) Bohannan (1960a) Beattie (1960) Wilson (1960) SouthaU (1960) Lee (1979)

a Victim and killer were unrelated co-wives of a polygynous man in the lone female-female cases in the Maria, Bhil, Banyoro, and Alur samples, as well as in one of three Baluyia cases and one of two JoLuo cases. We exempted co-wife cases from the rule excluding marital as well as genetic relatives because unrelated co-wives represent a female analogue of male-male rivalries.

1965-1981; Detroit, 1972; C a n a d a , 1974-1983; E n g l a n d / W a l e s , 19771986) consist of all h o m i c i d e s k n o w n to police, a n d a case is eligible for inclusion if police h a v e identified the killer to their o w n satisfaction, r e g a r d l e s s of p r o s e c u t i o n or conviction. Reliance o n judicial criteria in o r d e r to consider a killer's i d e n t i t y c o n f i r m e d w o u l d lead to the exclusion of large n u m b e r s of solved cases d e e m e d self-defensive or otherwise justifiable or excusable, as well as a n y cases in w h i c h the killer h a s d i e d before adjudication; t h o s e c o n v i c t e d or i n c a r c e r a t e d therefore constitute seriously biased s a m p l e s of identified killers (Daly a n d Wilson 1988a). Most r e p o r t e d studies of h o m i c i d e s d o n o t classify cases in a m a n n e r p e r m i t t i n g inclusion in the table. O f t e n , the set of cases h a s b e e n selected o n s o m e potentially biasing criterion, s u c h as arrest or c o n v i c t i o n

86

Human Nature, Vol. 1, No. 1, 1990

after trial, a n d e v e n w h e r e the data a p p r o p r i a t e l y include all k n o w n cases in s o m e specified time a n d place, the requisite cross-tabulation of killer's sex b y victim's sex b y relationship is s e l d o m r e p o r t e d . A possible objection to this w a y of c o m p a r i n g the sexes is that the exclusion of relatives from Table 1 c o u l d exaggerate the sex difference because m e n ' s " r o u t i n e activities" entail m o r e interaction w i t h nonrelatives in the "public d o m a i n " t h a n d o t h o s e of w o m e n . At the extreme, if w o m e n a n d m e n killed same-sex p e r s o n s equally often a n d i n d e p e n dently of relationship, our selection criterion m i g h t still p r o d u c e a prep o n d e r a n c e of male cases because m e n ' s interactions are m o r e often with nonrelatives. But in fact, a d d i n g relatives o t h e r t h a n o w n children into the analysis hardly changes the results. Chicago's 7439 male homicides vs. 195 female cases (Table 1) b e c o m e 7796 vs. 214 w h e n such relatives are included, the male cases constituting 97.4% of all same-sex cases in either comparison; C a n a d a ' s 2387 vs. 59 (97.6% male) c h a n g e to 2839 vs. 87 (97.0% male); Detroit's 316 vs. 11 (96.6% male) b e c o m e 358 vs. 15 (96.0% male); a n d in England/Wales, 2195 vs. 95 (95.9% male) b e c o m e 2637 vs. 132 (95.2% male). O u r point is not to d e n y that sex differences in killing may partly reflect sex differences in e n c o u n t e r frequencies with unrelated p e r s o n s of the same sex a n d h e n c e in opportunities for interactions involving conflict. But if the sexes i n d e e d differ in the distributions of p e r s o n s with w h o m they interact, t h e n that fact, rather than constituting an explanation of sex-typical social strategies, is part of w h a t n e e d s explaining. It seems likely that o n e ' s competitive inclinations w o u l d affect b o t h the time one s p e n d s in certain social situations and one's belligerence w h e n in them. M a n y writers have invoked the c o n c e p t of culture in o r d e r to explain the huge, universal sex difference in violence. Curtis (1974:160), for example, writes The great majority of killers are males in countries throughout the world . . . . Although local insitutions and customs vary, the preponderance of men in these crimes is related to socially and culturally defined roles such as aggressor, provider and leader; to the greater social pressures exerted by these positions; and to the greater frequency of external contacts. But "culturally d e f i n e d roles" like " g e n o t y p e s " - - - h a v e e x p l a n a t o r y potential only w h e n the p h e n o m e n a u n d e r consideration v a r y b e t w e e n g r o u p s (and e v e n t h e n a case can be m a d e that i n v o k i n g culture is seldom m o r e t h a n a p s e u d o - e x p l a n a t o r y relabeling of the p h e n o m e n a to be explained; see Tooby and C o s m i d e s 1989). Overall h o m i c i d e rates vary t r e m e n d o u s l y and can be conceived of as cultural (Daly a n d Wilson 1989), but the fact of a sex difference t r a n s c e n d s cultural variation.

Killing the Competition

87

W h e t h e r sex differences can be traced to " n o r m s " a n d " s e x role socialization" is not the issue; invoking such c o n c e p t s begs the q u e s t i o n of w h y the sex differences instilled b y t h e m s h o u l d exhibit such consistency. Despite the universality of a large sex difference in rates of homicide, the differences b e t w e e n h u m a n g r o u p s are so great that w o m e n in the most violent p o p u l a t i o n s exhibit h i g h e r per capita rates of killing t h a n do m e n in the least violent. N o t i n g this fact with respect to " c r i m e " in general, Steffensmeier and Allan (1988:76) argue, sex differences in crime are not simply a reflection of biology or genetic makeup. This conclusion is clear from the strong covariance of male and of female rates within the comparison groups, coupled with extreme cross-group variability in male and female arrest rates, such that the female rate of one group may exceed the male rate of the comparison groups. (Urban females, for example, have higher rates for some crimes than rural males.) The fallacy in this reasoning can be seen b y substituting " c r i m e . " S u p p o s e m e n e v e r y w h e r e g r e w taller t h a n w o m e n lent nutritional status, but that w e l l - n o u r i s h e d w o m e n w e r e u n d e r n o u r i s h e d men. W o u l d this lead us to c o n c l u d e that ences in height are " n o t simply a reflection of b i o l o g y , " and, w o u l d we m e a n b y such a claim?

h e i g h t for of equivataller t h a n sex differif so, w h a t

T H E L O G I C OF SEX DIFFERENCES I N INTRASEXUAL C O M P E T I T I O N M e n b e c o m e embroiled in d a n g e r o u s c o m p e t i t i v e interactions far m o r e often t h a n d o w o m e n . The data in Table I indicate that a l t h o u g h this sex difference is variable in m a g n i t u d e , it is universally large. T h e r e is n o evidence e v e n suggesting that it is c o n t r a v e n e d a n y w h e r e , n o t w i t h standing the perennial appeal of such fantasies as the ancient G r e e k legend of the A m a z o n s and Margaret M e a d ' s (1935) just-so stories of sex-role reversal in N e w G u i n e a (see Daly a n d Wilson 1988a). This species-typical sex difference is o n e that w e share with o t h e r effectively p o l y g y n o u s m a m m a l s , a n d its link w i t h effective p o l y g y n y is well u n d e r s t o o d (Trivers 1972; Williams 1966). By "effective p o l y g y n y , " w e refer to a b r e e d i n g s y s t e m in w h i c h the variance in fitness a m o n g males exceeds that a m o n g females (Clutton-Brock 1988; Daly a n d Wilson 1983). Diverse threads of morphological, physiological, d e v e l o p m e n t a l , and psychological evidence are consistent in indicating that h o m i n i d evolution has b e e n characterized by effective p o l y g y n y (Alexander 1979;

88

Human Nature, Vol. 1, No. 1, 1990

Alexander et al. 1979; Daly and Wilson 1983, 1988a; Symons 1979). Effective polygyny is a circumstance conducive to the evolution of a male psychology more combative and risk-prone that that of females. The reason w h y departures from m o n o g a m y are associated with the evolution of sex differences in risky competitiveness is that high variance means relatively great rewards for success and penalties for failure: males in effectively polygynous species have a higher maximal reproductive success than females, but they also have a higher probability of dying without issue. Greater variance in rewards favors greater acceptance of risk in their pursuit. Women compete, too, and may even kill one another in the process, but their lesser fitness variance has generally meant that they have little to gain, and at least something to lose, by dangerous tactics. (Note, too, that differential taste for risky confrontational competition need not imply a domain-general sex difference in tolerating danger; females might well be found to tolerate greater risk than males in defense of children, for example.) What makes the comparative evidence relevant to the h u m a n case is the body of theory and data linking cross-species diversity in sexual differentiation of behavior and morphology to ecology, demography, and mating system. The point is not some typological claim about the significance of maleness (or androgens or Y-chromosomes) across the animal kingdom; in fact, the familiar sex difference in belligerence is reversed in precisely those species in which the fitness variance differential is reversed, namely in effectively polyandrous species, such as spotted sandpipers (Maxson and Oring 1980), Wilson's phalaropes (Colwell and Oring 1988), and ja~anas (Stephens 1982). In comparative perspective, Homo sapiens is an effectively polygynous primate, and more specifically, one whose degree of effective p o l y g y n y - - m o r e than the gibbon's, for example, but much less than the gorilla's---is intelligibly related to the magnitude of sex differences in maturation, senescence, and body size. This is not to say that the precise magnitudes of sex differences in violent competitive behavior are predictable from available theories of sexual selection and comparative considerations, however. Although the direction of the differences in Table 1 is readily predictable, their magnitude might still be deemed surprisingly large. We are, after all, a biparental species, and only slightly polygynous and dirnorphic as compared to many other mammals. What are now required are theories of the contingent control of competitive inclinations sufficiently precise to predict the variable magnitude of sex differences in behavior. There is a substantial literature on adaptive decision-making under variable conditions of risk, where "risk" is defined as variance in the magnitude of payoffs for a given course of action (Real and Caraco 1986).

Killing the Competition

89

Rather than simply maximizing the expected (mean) return in some desired commodity, such as food, animals should be---and demonstrably are---sensitive to variance as well. This follows from the fact that the fitness values of desired commodities, and hence their evolved psychological utilities, do not generally increase linearly with increases in the amounts acquired. An overwintering songbird, for example, might achieve a slight gain in expected fitness by the addition of n calories to its energy budget, whereas a loss of n calories would be fatal; such an animal will be "risk-averse," avoiding high-variance feeding situations even at some cost in expected intake. "Risk" in the sense of an acceptance of some probability of death or injury is a distinct but related concept that could be incorporated into such analyses. Dangerous acts are adaptive choices if positive fitness consequences are large enough and probable enough to offset the possible negative consequences (Maynard Smith and Price 1973; Popp and DeVore 1979). Daly and Wilson (1988a), for example, show that selection can favor competitive tactics that entail increasing danger to oneself as the fitness payoffs for success vs. failure in the competition become more disparate. Disdain of danger to oneself is especially to be expected where available risk-averse alternatives are likely to produce a fitness of zero: if opting out of dangerous competition maximizes longevity but never permits the accrual of sufficient resources to reproduce, then selection will favor opting in (Rubin and Paul 1979). Further theoretical modeling and research are needed to determine whether even small sex differences in fitness variance, such as those likely to have been characteristic of our foraging ancestors, could be sufficient to account for large sex differences in risk-proneness and mortality. Alternatively, an adequate explanation of the large h u m a n sex difference in violent competition may have to focus on peculiarities of this species. Perhaps the h u m a n propensity to coalitional aggression and deterrent vengeance has raised the premium on masculine competitive preoccupations and skills beyond what is d e m a n d e d of less socially complex animal species (Alexander 1971; Chagnon 1988). Perhaps a hunting specialization has incidentally elevated men's violent capabilities so that the cost-benefit structure of male-male vs. female-female conflicts is more different than it would be if the sexes foraged similarly. Perhaps contemporary weapon technology s o m e h o w amplifies sex differences in lethal outcomes, which were less extreme in the environments in which the h u m a n social psyche evolved (though the cross-cultural evidence provides no particular reason to believe this). Transforming such speculations into testable theories is a challenge that will require (at the least) strategic modeling (Tooby and DeVore 1987) and explicit postulation of empirically demonstrable psychological mechanisms. We also n e e d to

90

Human Nature, Vol. 1, No. 1, 1990

develop theories of facultative variation in conflictual inclinations in response to demographic and ecological factors. The intensities of malemale and female-female competition vary inversely with one another in relation to operational sex-ratio variation and attendant competition for mates (Emlen and Oring 1977), for example; competition also varies in relation to cohort size (Cohen and Land 1987; Easterlin 1980) and, between societies, in relation to modes of subsistence and the monopolizability of resources (Flinn and Low 1986). Elucidation of the relationships between these factors and the strategic organization of competitive psychology will require further theory development and research. THE KILLER'S AGE

In 1983, two prominent American criminologists, Travis Hirschi and Michael Gottfredson, provoked a storm of controversy by arguing (among other things) that (1) "the age distribution of crime is invariant across social and cultural conditions," [and] (2) "the age distribution of crime cannot be accounted for by any variable or combination of variables currently available to criminology" (1983:554). This age distribution entails onset in adolescence followed by a quick rise to maximal rates or probabilities of offending, which then decline steadily throughout adulthood. Much of the ensuing controversy is of tangential relevance to our present topic, concerning, for example, the merits of longitudinal vs. cross-sectional research designs and the validity of the concept of "career criminals." But other aspects of the debate have been enlightening. Several critics have disputed Hirschi and Gottfredson's claims, attacking proposition (1) by documenting some variability in age distributions and proposition (2) by asserting the explanatory adequacy of existing criminological theories. Ironically, these attacks have underscored the considerable element of truth in Hirschi and Gottfredson's polemic. Tittle (1988), for example, maintains that traditional criminological theories of "labeling" and "social control" can account for the age effect perfectly well; to a skeptical reader, Tittle's argument shows only that "labeling" and "social control" are not predictive theories at all and could be talked about whatever the age pattern. [See Gove (1985) who derives from labeling theory the contrary prediction that criminal activity should increase with age.] Other critics have focused on the overstatement in

Killing the Competition

91

Hirschi and Gottfredson's claim of "invariance." Steffensmeier et al. (1989), for example, show that robbery and arson peak a little later than auto theft and burglary, and that white collar crimes peak much later still; in so doing, they grant the remarkable consistency of offense-specific age patterns and the universality of a unimodal inverted-U age distribution. The general impression left by these exchanges is that Hirschi and Gottfredson's age distribution is extremely robust, especially with regard to crimes involving an element of confrontation and risk of injury. Figure 1 presents age- and sex-specific rates of committing the crime with which this paper is concerned, namely killing unrelated persons of one's o w n sex. Though the rates of such homicides differ greatly among the four samples, sex differences and the shapes of age distributions are remarkably similar. The median ages of males w h o killed unrelated males in Canada, Chicago, Detroit, and England/Wales were 26, 24, 27, and 25, respectively; the corresponding median ages for females were 26, 24, 24, and 35. The data archives from which Figure 1 is derived are victim-based. If a man killed three unrelated men in a single incident, for example, he contributed three homicides to the numbers perpetrated by his age category. One might argue that an analysis such as this should count killers rather than victims; we would reply that the number of persons killed constitutes the more straightforward measure of the lethality perpetrated by a particular demographic class. A single killer is counted for each body, although multiple-offender cases occur in all four archives. Use of an offender-based file would have little effect on the patterns in Figure 1 in any case; it would shift them slightly toward youth because a larger proportion of the cases perpetrated by y o u n g m e n involve multiple offenders than of those perpetrated by older men. The individual "credited" with the killing was the first offender listed by the police. The first offender is usually the party most clearly culpable and charged with the most serious offence if such variability among offenders exists. However, Block (1987) notes that there is also a potential age bias in the Chicago file: "older offenders tend to be listed first" (1987:20). The impact of any such bias on the age patterns in Figure 1 cannot be large since the tendency Block notes is slight, multipleoffender cases constitute a minority of the cases portrayed, and the offenders in such cases are almost always very close in age to one another. THE EVOLUTIONARY PSYCHOLOGY OF LIFE-STAGE-SPECIFIC COMPETITIVE RISK-PRONENESS Not only are young men the principal perpetrators of potentially lethal violence; they are its principal victims, too (e.g., Block 1987; Holinger

92

H u m a n Nature, Vol. 1, No. 1, 1990 A

80-

~ 50~.4o.

~.~. Mole

A~ C~ore)

30-

~.20.

.e- Male

e-

.o

0 "T

-~ Female

5,

0

..f-._.._~, ~

~

~

; ~

: ~ _

.

Age 6,ears) Figure 1. Age- and sex-specific rates of killing nonrelatives of one's own sex: (A) Canada, 1974-1983; (B) England/Wales, 1977-1986; (C) Chicago, 1965-1981; (D) Detroit, 1972.

Killing the Competition

93

9oo.

~ L

C

~o~'~176 i~ 7oo-

~. ~

2oo. 100. O~

1400-

D

1200-

10009e- Mole

800-

-.- F e m a l e

600-

.g "I-

200-

O:

A ~ (yo~,,,)

94

Human Nature, Vol. 1, No. 1, 1990

1987; Daly and Wilson 1988a). This is especially so where rates of lethal violence are high, because cases in which victim and killer are unrelated young men constitute the most volatile component of variable homicide rates (Block 1975; Daly and Wilson 1989). Many homicides arise out of social conflicts among acquaintances in which either party might have killed the other, with the result that victim and killer populations are similar demographically (Wilson and Daly 1985). Accident victimization risk tends to parallel homicide victimization and perpetration (Hewlett 1988; Holinger 1987; Wilson and Daly 1985). Maleness, youth, and poor economic circumstances and prospects are all predictive of relatively dangerous or reckless behavioral inclinations. We have already discussed the utilitarian logic by which poverty and poor prospects favor risk-proneness: as Dylan (1965) put it, " w h e n you got nothing, you got nothing to lose." But the relationship between youth and risk-proneness requires further explanation. Why should it be young m e n - - a t the peak of their physical capabilities and in the very stage of life at which mortality from disease and defect is l o w e s t - - w h o are maximally risk- prone and vulnerable to violent death? All else being equal, might we not expect increasing age to be associated with increasing disdain for one's own safety? And as for gender, w h y should not young w o m e n respond as desperately as young men to their no less desperate circumstances? It is a fact so familiar as to seem ordinary that the very demographic class that is most formidable in violence (young adult males) is also most vulnerable thereto, but it is a remarkable fact nonetheless and one in need of explanation. We hypothesize that the requisite explanation lies in the social demands and agendas that confronted ancestral men and w o m e n in particular life stages (see also Rubin and Paul 1979). Young men are both especially formidable and especially risk-prone because they constitute the demographic class on which there was the most intense selection for confrontational competitive capabilities among our ancestors. In the foraging societies in which the h u m a n psyche evolved, the young man who would acquire a wife had to display prowess in hunting and warfare and a capacity to defend his interests, to w o m e n and to any men who might hinder or facilitate his ambitions. Moreover, although young adulthood is an especially competitive stage for any male mammal, human social complexity can make the consequences of differential performance in that stage enduring and hence even more crucial. In most effectively polygynous animals, a male's continuing success is dependent on his continuing competitive prowess; a senior stag is as aggressive and dangerous as the y o u n g bucks or he is finished (CluttonBrock et al. 1982). But in people, ubiquitous practices of long-term paternal and nepotistic investment, solidary kin groups, between-group

Killing the Competition

95

hostilities, and coalitional reciprocity create an arena within which individual reputations have lasting effects. The hypothesis that competitive success or failure in early adulthood has been an especially strong determinant of total lifetime fitness in men as compared to other male mammals warrants empirical investigation; early competitive performance may have major long-term consequences in other creatures, too. Our general argument about sex- and life-stagespecific competitive inclinations having been shaped by sexual selection does not require that people be special in these matters. People are clearly special, however, in that individuals acquire reputations which affect their social fortunes. Demonstrations of competence in the face of danger reap reputational (and perhaps direct material) rewards whose utility persists over the rest of the lifespan. Successful risk-taking is admired (e.g., Fishbain et al. 1987; Moore 1972; Wilson and Daly 1985), and much dangerous and violent behavior by young men functions as social display facilitated by the presence of an audience to impress (Daly and Wilson 1988a; Jackson and Gray 1976; Rothe 1987; Wilson and Daly 1985). Several other lines of evidence about life-span development support the idea that young men constitute a demographic class specialized by a history of selection for maximal competitive effort. Changes in muscle strength which are apparently unrelated to exercise, for example, show characteristic sex-specific patterns: male strength increases abruptly at puberty and only slightly in the following decade (McComas et al. 1973); strength subsequently declines gradually in adults of both sexes, but especially in men (Murray et al. 1985). Aerobic capacity is likewise maximal and maximally sexually differentiated in the teens and twenties (e.g., Shephard 1986). Sex differences in various sorts of motor performance favoring males tend generally to increase around puberty, but this is much more striking with respect to measures of strength and quick energetic bursts than in measures of balance and precision (Thomas and French 1985). Not only are young men the most physically formidable of human demographic classes, they also appear to be psychologically specialized to embrace danger and confrontational competition. In various activities, young men have been found to be especially motivated by competition and especially undeterred by danger to self (reviews by Gore 1985; Holinger 1987; Jonah 1986; Kandel 1980; Tonkin 1987; Wilson and Daly 1985). "Age is by far the most powerful predictor of the cessation of those forms of deviant behavior that involve substantial risk and/or physically demanding behavior" begins Walter Gove (1985:115), in a provocative chapter in which he asserts that his own discipline of sociology has no handle on age or sex differences in "deviance" and argues the need for

96

Human Nature, Vol. 1, No. 1, 1990

"a biopsychosocial perspective." Gove reviews m u c h intriguing but sketchy evidence of life-span developmental changes in motives, tastes, and attitudes in spheres such as competitiveness, self-absorption, moralizing, need for approval, emotionality, and stimulation-seeking, all of which seem to reflect a declining appetite and/or aptitude for risk and competition over the course of adulthood. Unfortunately, although Gove offers a rich characterization of these life-span changes, he offers no functional (strategic) interpretation of them, and he gratuitously implies that life-span development progresses toward superior attributes rather than constituting a series of life-stage-appropriate phenotypes. [See Alexander (1987) for a critique and reevaluation of similarly gerontocentric accounts of "moral development."] It is interesting to inquire how age- and gender-related variations in effective risk-proneness are instantiated psychologically. Wilson and Herrnstein (1985) argue from diverse evidence that men who engage in predatory violence and other risky criminal activity have different "timehorizons" than law-abiding men, weighing the near future relatively heavily in their decision making and discounting the long term. What these authors fail to note is that facultative adjustment of one's personal time-horizons could be an adaptive response to predictive information about one's prospects for longevity and eventual success. Many other psychological processes of potential relevance can be envisioned. One could become more risk-prone as a result of intensified desire for the fruits of success or intensified fear of the stigma of nonparticipation, by finding the adrenalin rush of danger pleasurable in itself, by underestimating objective dangers, by overestimating one's competence, or by ceasing to care whether one lives or dies, among other possibilities. Several of these processes appear to be relevant to the risk-proneness of young men. As drivers, for example, they both underestimate objective risks and overestimate their own skills, as compared to older drivers (Brown and Groeger 1988; Finn and Bragg 1986; Matthews and Moran 1986). Apparent disdain for their own lives might be inferred from the fact that men's suicide rates maximally surpass women's in young adulthood (e.g., Holinger 1987). And G o r e (1985) provides some evidence that the pleasure derived from skilled encounters with danger diminishes with age. IS AGE SIMPLY A CORRELATE OF CIRCUMSTANCE? An alternative to the hypothesis that m e n have evolved an adaptive life-span schedule of risk-proneness is that age patterns such as those evident in Figure 1 are entirely the result of changes in relevant circum-

Killing the Competition

97

stances which happen to be correlated with age. Mated status, for example, would be expected to inspire a reduction in dangerous behavior, because access to mates is a principal issue inspiring competition and married men have more to lose than their single counterparts. One might therefore hypothesize that the decline in violence with age is entirely accounted for by increases in the proportion married. This proves not to be the case. Figure 2 presents such a breakdown for the two of our four homicide data sets for which the requisite information on marital status was available. Marital status is indeed related to the probability of committing a same-sex, nonrelative homicide, but age effects remain conspicuous w h e n married and unmarried m e n are distinguished. We must caution that the evidence in Figure 2 of an association between marital status and same-sex, nonrelative homicides warrants no condusion about the direction of causality. It seems plausible that the state of marriage makes men more pacific and risk-averse; however, the data are equally consistent with the interpretation that marriage is not itself effectual but is just more likely to be entered into by men who are less homicidally inclined. Evidence against this latter possibility and in favor of the pacifying effect of marriage per se comes from a further breakdown of the "unmarried" category in the Canadian data: although numbers are small and rate estimates noisy, both divorced and w i d o w e d men exhibit homicide rates similar to those of single men and m u c h higher than those of married men at all ages, suggesting that the riskproneness of single men is reinstated if marriage ends. An alternative interpretation is that those who divorce might constitute a violenceprone sample of ever-married men, but it seems less likely that the same would apply to those widowed. Other life events may account for additional age-associated variability. Restricted access to material resources, like restricted access to the opposite sex, can inspire a utilitarian escalation of dangerous competitive tactics and can be especially a problem in y o u t h (Cohen and Machalek 1988). Unemployed men behave more dangerously than those with jobs, and unemployment rates are maximal in y o u n g adulthood (Wilson and Daly 1985). It also seems likely that becoming a father would be associated with reduced risk-proneness, but the requisite data to assess any such influence of fatherhood on the patterns in Figures 1 and 2 are not available. Hirschi and Gottfredson argue that "Change in crime with age apparently cannot be e x p l a i n e d . . , by change in the social situation of people over the course of life" (1986:67). The evidence they introduce in support of this claim is selected with respect to life stage, and it is of dubious probative value. They deny the relevance of increasing e m p l o y m e n t as

98

H u m a n Nature, Vol. 1, No. 1, 1990

A

90. 80.

70. b

Q.

~ Unmarried I [] Married

60.

g 9-

50.

E

b

40.

~

3o.

"~

20-

Q..

1015-24

25-34

35-44 45-54 Age (,years)

55-64

>64

1400-

1200 -

s

1"1Unmorried I [] Married

~ 1000-

lJ a.

~

800-

E b O.

600.

m ID

.o

400200.

15-24.

25-34

35-4.4- 4.5-54. Age (years)

55-64

>64.

Figure 2. Age-specific rates of killing unrelated m a l e s by married vs. unmarried men: (A) Canada, 1974--1983; (B) Detroit, 1972.

Killing the Competition

99

a partial determinant of age-related declines in crime, for example, by citing some evidence that working teenagers are not less likely to be arrested than nonworking teenagers. But this comparison is probably more a matter of out-of-school vs. in-school than of employed vs. unemployed; in Detroit, homicide perpetration is unrelated to employment status among teenagers, but it is much more prevalent among the unemployed than the employed at all subsequent ages (Wilson and Daly 1985). Similarly, Hirschi and Gottfredson (1986) deny the relevance of acquiring a girlfriend, on the basis of evidence that delinquent boys are likelier to have girlfriends than their nondelinquent counterparts. Again, their point works only because they restrict the comparison to teenagers, among w h o m early maturers are likely to be both earlier delinquent and earlier sexually active than later maturers. Figure 2 shows that although homicide rates by married and single men are similar for the youngest group, they are very different beyond 25 years of age. Finally, Hirschi and Gottfredson (1986) deny the relevance of becoming a father, but on this issue they present no evidence at all. We agree that age effects would be unlikely to disappear altogether in an analysis that controlled for correlated circumstances, but no such analysis has yet been conducted, and what we do k n o w provides no reason to doubt that changing circumstances account for much age-associated variation. CONCLUDING REMARKS

This question of whether age (or sex) effects would evaporate if associated circumstances were factored out appears to be the substantive point of contention behind much of the confusing debate about social vs. biological explanations. If 15-year-olds and 50-year-olds were found to differ solely as a result of differences in the frequencies at which they encounter specific situations, and not in their responses thereto, then the idea of an evolved schedule of life-stage-appropriate psychology would be superfluous. But then so would any notion of postpubertal development. Presumably no one defends such an extreme version of situational determinism: even the most enthusiastic sociological critic of psychological reductionism recognizes that cumulative experience changes people. What is less often recognized is that some sort of complex functional theory of life-span development is necessary to account for the particular ways in which experiences matter. The prevalent view of life-span development in the social sciences is the obsolete model of "general process learning theory." After reviewing some interesting evidence that the affect associated with "adrenaline

100

Human Nature, Vol. 1, No. 1, 1990

highs" shifts from positive to negative over the life span, for example, Gove adds the gratuitous claim that this change is understandable as resulting from conditioning, since "adrenaline highs are paired with substantial risk and thus anxiety" (1985:134). Not only does this argument provide no more reason to suppose that "adrenaline highs" will become negative than that "anxiety" will become positive, but the lifespan changes that Gove reviews clearly do not proceed as a function of the number of "pairings," as general process learning theory requires. Similarly, Baldwin argues that the rapid postadolescent decline in crime can be understood as reflecting a predictable decline in "the sensory rewards of thrill and adventure seeking" (1984:1326) " d u e to habituation" (1984:1327). As Symons has responded to an analogous proposal about waning sexual interest in familiar partners, "an actual neural process, habituation, becomes a metaphor for boredom. The problem with this metaphor is that it also explains w h y koalas become bored eating eucalyptus leaves" (1987:137). Appeals to domain-general concepts such as "learning," "habituation," and "reinforcement" are commonly mistaken for explanations of developmental patterns, but they are at best redescriptions of the phenomena begging explanation (Cosmides and Tooby 1987; Symons 1987; Tooby and Cosmides 1989). Because of its domain-general pretensions, learning theory offers no insight into the reasons w h y familiarity breeds attachment in one domain and loss of interest in another. The domain-specific ways in which classes of experience affect development are themselves targets of selection over evolutionary time, leading to strategically organized ontogenies, buffered against manipulation by individuals with antagonistic interests. Note that an evolutionary psychological view of people does not imply that they are irrational automata, but rather that their behavior is the result of decision processes with a cost-benefit structure that instantiates age-, sex-, and circumstance-specific valuations of various classes of material and social goods. Evolutionary models of the life-span agendas of men and w o m e n indicate how behavioral control mechanisms must be strategically organized to have achieved adaptive ends in ancestral environments, and this way of thinking can direct the attention of psychologists to relevant variables and processes. Knowing what the mind has been "designed" by selection to achieve affords numerous hints about its probable organization (Cosmides 1989; Cosmides and Tooby 1989). Despite considerable research on risk perception, subjective probability, and the heuristics of decision-making (e.g., Kahneman et al. 1982; Lopes 1987; Nisbett and Ross 1980; Slovic 1987), differences in response as a function of age or sex have hitherto been treated largely as "noise" rather than as meaningful phenomena. Moreover, the distinction be-

Killing the Competition

101

tween outcomes impacting directly on the decision-maker and those more hypothetical or impacting on someone else has been trivialized. Researchers in this area often maintain that h u m a n decision processes, by relying on irrational "rules of t h u m b " such as over-valuing recent information and ignoring negative instances, constitute flawed but adequate approximations to formal logical inference. We cannot judge h o w well or poorly our heuristics serve our purposes, however, until we specify those purposes (Cosmides 1989). Decision theorists have yet to address the functional significance of domain-specific decision processes in any depth, presumably because they lack a fundamental theory of self-interest from which to derive predictions about the utilities and subjective costs of alternative consequences of one's decisions. One m a y readily presume, for example, that death will be a major cost, and an appreciable risk thereof a major deterrent, without apprehending that an evolved psyche might effectively treat the "genetic death" of nonreproduction as an equally negative outcome: a sexually selected appetite for confrontational competition can be adaptive even where it entails a high risk of mortality. We wish to thank the Harry Frank Guggenheim Foundation, the Social Sciences and Humanities Research Council of Canada, and the Natural Sciences and Engineering Research Council of Canada for financial support. We thank Carolyn Rebecca Block and Richard Block (Chicago); James Bannon, Robert Hislop, and Marie Wilt Swanson (Detroit);)oanne Lacroix, Craig McKie, and Bryan Reingold (Canada); and Rosemary Gartner, Mary Tuck, L. Davidoff, Kathleen Shaw, and Michael O'Brien (England/Wales) for their help in creating and making available the homicide data files; and Jim Karr for his role in the number crunching. Martin Daly and Margo Wilsoncollaboratein research on the evolutionarypsychologyof human conflict, and in field research in mammalian behavioral ecology. They are the authors of Sex, Evolution and Behavior (Wadsworth: second edition, 1983) and Homicide (Aldine de Gruyter: 1988). REFERENCES Adler, F. 1975 Sisters in Crime. New York: McGraw-Hill. Alexander, R.D. 1971 The Search for an Evolutionary Philosophy of Man. Proceedings of the Royal Society of Victoria 84:99-120. 1979 Darwinism and Human Affairs. Seattle: University of Washington Press. 1987 The Biology of Moral Systems. Hawthorne, New York: Aldine de Gruyter. Alexander, R.D., J.L. Hoogland, R.D. Howard, K.M. Noonan, and P.W. Sherman 1979 Sexual Dimorphisms and Breeding Systems in Pinnipeds, Ungulates,

102

H u m a n Nature, Vol. 1, No. 1, 1990

Primates and Humans. In Evolutionary Biology and Human Social Behavior, N.A. Chagnon and W. Irons, eds. Pp. 402-435. North Scituate, Massachusetts: Duxbury. Baldwin, J.D. 1984 Thrill and Adventure Seeking and the Age Distribution of Crime: Comment on Hirschi and Gottfredson. American Journal of Sociology 90:13261330. Beattie, J.H.M. 1960 Homicide and Suicide in Bunyoro. In African Homicide and Suicide, P. Bohannan, ed. Pp. 130-153. Princeton: Princeton University Press. Block, C.R. 1987 Homicide in Chicago. Chicago: Loyola University. Block, R. 1975 Homicide in Chicago: A Nine-Year Study (1965-1973). Journal of Criminal Law and Criminology 66:496-510. Bohannan, P. 1960a. Homicide and Suicide in North Kavirondo. In African Homicide and Suicide. P. Bohannan, ed. Pp. 154-178. Princeton: Princeton University Press. 1960b Homicide among the Tiv of Central Nigeria. In African Homicide and Suicide. P. Bohannan, ed. Pp. 30-64. Princeton: Princeton University Press. Brown, I.D., and J.A. Groeger 1988 Risk Perception and Decision Taking During the Transition between Novice and Experienced Driver Status. Ergonomics 31:585-597. Browne, A. 1987 When Battered Women Kill. New York: Free Press. Burrowes, K.L., R.E. Hales, and E. Arrington 1988 Research on the Biologic Aspects of Violence. Psychiatric Clinics of North America 11:499-509. Chagnon, N.A. 1988 Life Histories, Blood Revenge, and Warfare in a Tribal Population. Science 239:985-992. Clutton-Brock, T.H., ed. 1988 Reproductive Success. Chicago: University of Chicago Press. Clutton-Brock, T.H., F.E. Guinness, and S.D. Albon 1982 Red Deer. Chicago: University of Chicago Press. Cohen, L.E., and K.C. Land 1987 Age Structure and Crime: Symmetry versus Asymmetry and the Projection of Crime Rates Through the 1990s. American Sociological Review 52:170183. Cohen, L.E., and R. Machalek 1988 A General Theory of Expropriative Crime: An Evolutionary Ecological Approach. American Journal of Sociology 94:465-501. Colwell, M.A., and L. W. Oring 1988 Wing Fluttering Display by Incubating Male Wilson's Phalaropes. Canadian Journal of Zoology 66:2315-2317.

Killing the Competition

103

Cosmides, L. 1989 The Logic of Social Exchange: Has Natural Selection Shaped How Humans Reason? Studies with the Wason Selection Task. Cognition 31:187-276. Cosmides, L., and J. Tooby 1987 From Evolution to Behavior: Evolutionary Psychology as the Missing Link. In The Latest on the Best. J. Dupre, ed. Pp. 277-306. Cambridge, Massachusetts: MIT Press. 1989 Evolutionary Psychology and the Generation of Culture, Part II. Case Study: A Computational Theory of Social Exchange. Ethology and Sociobiology 10:51-97. Curtis, L.A. 1974 Criminal Violence. Lexington, Massachusetts: Lexington Books. Daly, M., and M. Wilson 1982 Homicide and Kinship. American Anthropologist 84:372-378. 1983 Sex, Evolution and Behavior. Belmont, California: Wadsworth. 1988a Homicide. Hawthorne, New York: Aldine de Gruyter. 1988b Evolutionary Social Psychology and Family Homicide. Science 242:519524. 1989 Homicide and Cultural Evolution. Ethology and Sociobiology 10:99-110. Deaux, K. 1985 Sex and Gender. Annual Review of Psychology 36:49-81. Dylan, B. 1965 "Like a Rolling Stone." Warner Brothers Music. Eagley, A.H., and V.J. Steffen 1986 Gender and Aggressive Behavior: A Meta-analytic Review of the Social Psychological Literature. Psychological Bulletin 100:309-330. Easterlin, R.A. 1980 Birth and Fortune. New York: Basic Books. Elwin, V. 1950 Maria Murder and Suicide, second edition. Bombay: Oxford University Press. Emlen, S.T., and L.W. Oring 1977 Ecology, Sexual Selection, and the Evolution of Mating Systems. Science 197:215-223. Fallers, L.A., and M.C. Fallers 1960 Homicide and Suicide in Busoga. In African Homicide and Suicide, P. Bohannan, ed. Pp. 65-93. Princeton: Princeton University Press. Finn, P., and B.W.E. Bragg 1986 Perception of the Risk of an Accident by Young and Older Drivers. Accident Analysis and Prevention 18:289-298. Fishbain, D.A., J.R. Fletcher, T.E. Aldrich, and J.H. Davis 1987 Relationship between Russian Roulette Deaths and Risk-Taking Behavior: A Controlled Study. American Journal of Psychiatry 144:563-567. Flinn, M.V., and B.S. Low 1986 Resource Distribution, Social Competition, and Mating Patterns in Hu-

104

H u m a n Nature, Vol. 1, No. 1, 1990

man Societies. In Ecological Aspects of Social Evolution, D.I. Rubenstein and R.W. Wrangham, eds. Pp. 217-243. Princeton: Princeton University Press. Gillies, H. 1976 Homicide in the West of Scotland. British Journal of Psychiatry 128:105127. Gore W.R. 1985 The Effect of Age and Gender on Deviant Behavior: A Biopsychological Perspective. In Gender and the Life Course, A.S. Rossi, ed. Pp. 115-144. New York: Aldine. Hagan, J. 1986 The Unexplained Crimes of Class and Gender. In Critique and Explanation, T.F. Hartnagel and R.A. Silverman, eds. Pp. 71-90. New Brunswick, New Jersey: Transaction Books. Hamilton, W.D. 1964 The Genetical Evolution of Social Behaviour. Journal of Theoretical Biology 7:1-52. 1979 Wingless and Fighting Males in Fig Wasps and Other Insects. In Sexual Selection and Reproductive Competition in Insects, M.S. Blum and N.A. Blum, eds. Pp. 167-220. London: Academic Press. Hansen, J.P.H., and O. Bjarnason 1974 Homicide in Iceland, 1946-1970. Forensic Science 4:107-117. Hewlett, B.S. 1988 Sexual Selection and Paternal Investment among Aka Pygmies. In Human Reproductive Behaviour, L. Betzig, M. Borgerhoff Mulder, and P. Turke, eds. Pp. 263-276. Cambridge: Cambridge University Press. Hirschi, T., and M. Gottfredson 1983 Age and the Explanation of Crime. American Journal of Sociology 89:552584. 1986 The Distinction between Crime and Criminality. In Critique and Explanation, T.F. Hartnagel and R.A. Silverman, eds. Pp. 55-70. New Brunswick, New Jersey: Transaction Books. Holinger, P.C. 1987 Violent Deaths in the United States. New York: Guilford Press. Jackson, T.T., and M. Gray 1976 Field Study of Risk-Taking Behavior of Automobile Drivers. Perceptual and Motor Skills 43:471-474. Jonah, B.A. 1986 Accident Risk and Risk-Taking Behaviour among Young Drivers. Accident Analysis and Prevention 18:255-271. Jones, A. 1980 Women Who Kill. New York: Holt, Rinehart & Winston. Kahneman, D., P. Slovic, and A. Tversky, eds. 1982 Judgment under Uncertainty. New York: Cambridge University Press. Kandel, D.B. 1980 Drug and Drinking Behavior among Youth. Annual Review of Sociology 6:235-285.

Killing the Competition

105

Klama, J. 1988 Aggression: The Myth of the Beast Within. New York: Wiley. LaFontaine, J.S. 1960 Homicide and Suicide among the Gisu. In African Homicide and Suicide, P. Bohannan, ed. Pp. 94-129. Princeton: Princeton University Press. Lee, R.B. 1979 The !Kung San: Men, Women, and Work in a Foraging Society. Cambridge: Cambridge University Press. Lopes, L.L. 1987 Between Hope and Fear: The Psychology of Risk. Advances in Experimental Social Psychology 20:255-295. Maccoby, E.E., and C.N. Jacklin 1974 The Psychology of Sex Differences. Palo Alto: Stanford University Press. 1980 Sex Differences in Aggression: A Rejoinder and Reprise. Child Development 51:964-980. Matthews, M.L., and A.R. Moran 1986 Age Differences in Male Drivers' Perception of Accident Risk: The Role of Perceived Driving Ability. Accident Analysis and Prevention 18:299-313. Maxson, S.J., and L.W. Oring 1980 Breeding Season Time and Energy Budgets of the Polyandrous Spotted Sandpiper. Behaviour 74:200-263. Maynard Smith, J. 1974 The Theory of Games and the Evolution of Animal Conflict. Journal of Theoretical Biology 47:209-222. Maynard Smith, J., and G.R. Price 1973 The Logic of Animal Conflict. Nature (London) 246:15-18. McComas, A.J., R.E.P. Sica, and F. Petito 1973 Muscle Strength in Boys of Different Ages. Journal of Neurology, Neurosurgery and Psychiatry 36:171-173. Mead, M. 1935. Sex and Temperament in Three Primitive Societies. New York: Morrow. Mednick, S.A., V. Pollock, J. Volavka, and W.F. Gabrielli 1982 Biology and Violence. In Criminal Violence, M.E. Wolfgang and N.A. Weiner, eds. Pp. 21-80. Beverly Hills: Sage. Mock, D.W. 1987 Siblicide, Parent-Offspring Conflict, and Unequal Parental Investment by Egrets and Herons. Behavioral Ecology and Sociobiology 20:247-256. Moore R.J. 1972 Canadian Adolescents and the Challenge to Demonstrate Competence at Personal Physical Risk. Adolescence 7:245-264. Murray, M.P., E.H. Duthie, S.R. Gambert, S.B. Sepic, and L.A. Mollinger 1985 Age-related Differences in Knee Muscle Strength in Normal Women. Journal of Gerontology 40:275-280. Nash, J. 1967 Death as a Way of Life: The Increasing Resort to Homicide in a Maya Indian Community. American Anthropologist 69:455-470.

106

H u m a n Nature, Vol. 1, No. 1, 1990

Nisbett, R., and L. Ross 1980 Human Inference: Strategies and Shortcomings of Social Judgment. Englewood Cliffs, New Jersey: Prentice-Hall. O'Connor, R.J. 1978 Brood Reduction in Birds: Selection for Fratricide, Infanticide, and Suicide? Animal Behaviour 26:79-96. Parker, G.A. 1974 Assessment Strategy and the Evolution of Animal Conflicts. Journal of

Theoretical Biology 47:223-243. Popp, J.L., and I. DeVore 1979 Aggressive Competition and Social Dominance Theory, In The Great Apes, D.A. Hamburg and E.R. McCown, eds. Pp. 317-338. Menlo Park, California: Benjamin/Cummings. Real, L., and T. Caraco 1986 Risk and Foraging in Stochastic Environments. Annual Review of Ecology and Systematics 17:371-390. Rohwer, S. 1982 The Evolution of Reliable and Unreliable Badges of Fighting Ability. American Zoologist 22:531-546. Rothe, J.P. 1987 Rethinking Young Drivers. North Vancouver: Insurance Corporation of British Columbia. Rubin, P.H., and C.W. Paul 1979 An Evolutionary Model of Taste for Risk. Economic Inquiry 17:585-596. Saran, A.B. 1974 Murder and Suicide among the Munda and the Oraon. Delhi: National Publishing House. Shephard, R.J. 1986 Fitness of a Nation. Basel: Karger. Slovic, P. 1987 Perception of Risk. Science 236:280-285. Southall, A.W. 1960 Homicide and Suicide among the Alur. In African Homicide and Suicide, P. Bohannan, ed. Pp. 214-229. Princeton: Princeton University Press. Steffensmeier, D.J., and E.A. Allan 1988 Sex Disparities in Arrests by Residence, Race, and Age: An Assessment of the Gender Convergence/Crime Hypothesis. Justice Quarterly 5:53-80. Steffensmeier D.J., E.A. Allan, M.D. Harer, and C. Streifel 1989 Age and the Distribution of Crime. American Journal of Sociology 94:803831. Stephens, M.L. 1982 Mate Takeover and Possible Infanticide by a Female Northern Ja~ana (Jacana spinosa). Animal Behaviour 30:1253-1254. Symons, D. 1979 The Evolution of Human Sexuality. New York: Oxford University Press. 1987. If We're all Darwinians, What's the Fuss About? In Sociobiology and

Killing the Competition

107

Psychology, C. Crawford, M. Smith, and D. Krebs, eds. Pp. 121-146. Hillsdale NJ: Erlbaum. Thomas, J.R., and K.E. French 1985 Gender Differences across Age in Motor Performance: A Meta-analysis. Psychological Bulletin 98:260-282. Tieger, T. 1980 On the Biological Basis of Sex Differences in Aggression. Child Development 51:943-963. Tittle, C.R. 1988 Two Empirical Regularities (Maybe) in Search of an Explanation: Commentary on the Age/Crime Debate. Criminology 26:75-85. Tonkin, R.S. 1987 Adolescent Risk-Taking Behavior. Journal of Adolescent Health Care 8:213220. Tooby, J., and L. Cosmides 1989 Evolutionary Psychology and the Generation of Culture, Part I: Theoretical Considerations. Ethology and Sociobiology 10:29-49. Tooby, J., and I. DeVore 1987 The Reconstruction of Hominid Behavioral Evolution through Strategic Modeling. In The Evolution of Human Behavior: Primate Models, W.G. Kinzey, ed. Pp. 183-237. Albany: State University of New York Press. Totman, J. 1978 The Murderess. San Francisco: R & E Research Associates. Trivers, R.L. 1972 Parental Investment and Sexual Selection. In Sexual Selection and the Descent of Man, 1871-1971, B. Campbell, ed. Pp. 136-179. Chicago: Aldine. Varma, S.C. 1978 The Bhil Kills. Delhi: Kunj Publishing House. Wilbanks, W. 1984 Murder in Miami. Lanham, Maryland: University Press of America. Williams, G.C. 1966. Adaptation and Natural Selection. Princeton: Princeton University Press. Wilson, G.M. 1960 Homicide and Suicide among the Joluo of Kenya. In African Homicide and Suicide, P. Bohannan, ed. Pp. 179-213. Princeton: Princeton University Press. Wilson, J.Q., and R.J. Herrnstein 1985 Crime and Human Nature. New York: Simon & Schuster. Wilson, M., and M. Daly 1985 Competitiveness, Risk-Taking and Violence: The Young Male Syndrome. Ethology and Sociobiology 6:59-73. Wolfgang, M.E. 1978 Family Violence and Criminal Behavior. In Violence and Responsibility, R.L. Sadoff, ed. Pp. 87-104. New York: Spectrum.

male homicide.

Sex- and age-specific rates of killing unrelated persons of one's own sex were computed for Canada (1974-1983), England/Wales (1977-1986), Chicago (19...
1MB Sizes 0 Downloads 0 Views