Medical Hypotheses
--I 0306-9877/90/0033-
Med~col Hypofheses (1990) 33, 161~ 162 0 Longman Group UK Ltd 1990
0161/%10.00
Is the 20th Century Increase in Human Height Entirely Nongenetic? L. E. ARNOLD Ohio State
University,
Department
of Psychiatry,
473
W.
12th Avenue,
Columbus,
OH 432 10, USA
Abstract - The twentieth-century increase in human height in affluent nations may not be entirely due only to such nurturant considerations as food supply and electric lights. A possible genetic role might be found in interactions of food supply, maternal pelvic size, and height-survival/vulnerability links, and in mate selection. Humans seem to be following the paleontological principle that most species tend to evolve larger sizes over time.
The mean height increase of people in North America and other affluent regions in the past century is generally attributed to improved nutrition and health, or even the effect of electric lights on hormones, rather than genes. This assumption is supported by the observation that the increase often seems to occur in one generation in newly affluent nations. However, this superficially plausible conclusion does not consider the fact that throughout the animal kingdom there is a general tendency for most species to become larger with time. Furthermore, it does not explain why there appears to be further height increase after the first generation, especially in North America, where food has been abundant for several generations. The latter phenomenon could be explained by an interaction of genetic evolution with nutrition and general health. If genes for larger size are somehow associated with viability factors that have in the past
been counterbalanced by greater birth mortality because of the limiting size of the birth canal, and if the mean birth canal size becomes more adequate as a result of improved nutrition, the improved perinatal survival of fetuses carrying genes for larger size could skew the overall survival rate in favor of genes for larger size. The wider use of Caesarean section could also contribute to better survival of the larger fetuses, especially if there are mutations for larger size. A deviation-amplifying loop could result. If more fetuses with genes for large size survive birth, then more future mothers, carrying genes for large size and exposed to adequate nutrition, may develop even larger birth canals, allowing survival of even larger fetuses in the third generation. An objection might be raised that if this were possible it should have happened earlier in evolution. This objection is answered by the arguments that full development of the maternal pelvis in the past was limited by the nutrition available
Date received 26 April 1989 Date accepted 2 May 1989
161
162
MEDICAL
and that mean genetic potential for pelvis size may have been limited by the same perinatal mortality cap as mean genetic height potential. Another possible factor might be a shift from selection of petite wives to stylishly tall ‘model’ type women as wives. Insofar as some women remain unmarried and do not reproduce, this could mildly select for genetic tallness. This mild selection process could not account for dramatic one-generation mean height increases, but could account for some of the small continuing increments in succeeding generations. (A similar selection could result from women’s preference for tall men, but there is no reason to suspect that such a preference is of recent origin). This hypothesis receives some indirect support from Jensen and Sinha’s (1990) report of a 0.2 correlation between height and IQ, which is attributable to leg length. They hypothesize that high - IQ, high income men are most successful in courting longstemmed beauties. Their offspring, inheriting high IQ from the father, would also inherit long legs from the mother. Jensen and Sinha’s hypothesis, if valid, would support the notion that tall women have recently had a mating advantage. There are several ways to test these hypotheses: 1.
Heights of individual middle-class and farm families could be plotted over several generations along with nutritional and economic histories. Care would need to be taken to correct for effects of the Great Depression, es-
2.
3.
HYPOTHESES
pecially in children born to mothers whose pelves were developing during a time of parental unemployment. (Inclusion of multigenerational farm families could help control for this.) Obstetrical histories would need to be included in the analyses. Offspring of Caesarean section could be compared in height to non-Caesarean matched controls. More importantly, their children could also be compared. The wife-selection hypothesis could be checked by comparing mean heights of married mothers to age-matched unmarried nonmothers. A heuristic refinement would be to include also age-matched groups of unmarried mothers and married non-mothers. If heights of women by age, marital status, and parity are available from times before the modern tall ‘model’ became popular, these could be checked for the hypothesized reverse skew.
References Jensen A, Sinha SN (1990) Physical Correlations of Human Intelligence. In: Biological Approaches to the Study of Human Intelligence, PH Venson (Ed.), quoted in Fortune, April 23, 1990, p. 332 - 336 McMahon TA, Bonner JT (1983) The Natural History of Size. In: On Size and Life. Scientific American Books Inc. New York. New York 1983
--I 0306-9877/90/0033-
Med~col Hypofheses (1990) 33, 161~ 162 0 Longman Group UK Ltd 1990
0161/%10.00
Is the 20th Century Increase in Human Height Entirely Nongenetic? L. E. ARNOLD Ohio State
University,
Department
of Psychiatry,
473
W.
12th Avenue,
Columbus,
OH 432 10, USA
Abstract - The twentieth-century increase in human height in affluent nations may not be entirely due only to such nurturant considerations as food supply and electric lights. A possible genetic role might be found in interactions of food supply, maternal pelvic size, and height-survival/vulnerability links, and in mate selection. Humans seem to be following the paleontological principle that most species tend to evolve larger sizes over time.
The mean height increase of people in North America and other affluent regions in the past century is generally attributed to improved nutrition and health, or even the effect of electric lights on hormones, rather than genes. This assumption is supported by the observation that the increase often seems to occur in one generation in newly affluent nations. However, this superficially plausible conclusion does not consider the fact that throughout the animal kingdom there is a general tendency for most species to become larger with time. Furthermore, it does not explain why there appears to be further height increase after the first generation, especially in North America, where food has been abundant for several generations. The latter phenomenon could be explained by an interaction of genetic evolution with nutrition and general health. If genes for larger size are somehow associated with viability factors that have in the past
been counterbalanced by greater birth mortality because of the limiting size of the birth canal, and if the mean birth canal size becomes more adequate as a result of improved nutrition, the improved perinatal survival of fetuses carrying genes for larger size could skew the overall survival rate in favor of genes for larger size. The wider use of Caesarean section could also contribute to better survival of the larger fetuses, especially if there are mutations for larger size. A deviation-amplifying loop could result. If more fetuses with genes for large size survive birth, then more future mothers, carrying genes for large size and exposed to adequate nutrition, may develop even larger birth canals, allowing survival of even larger fetuses in the third generation. An objection might be raised that if this were possible it should have happened earlier in evolution. This objection is answered by the arguments that full development of the maternal pelvis in the past was limited by the nutrition available
Date received 26 April 1989 Date accepted 2 May 1989
161
162
MEDICAL
and that mean genetic potential for pelvis size may have been limited by the same perinatal mortality cap as mean genetic height potential. Another possible factor might be a shift from selection of petite wives to stylishly tall ‘model’ type women as wives. Insofar as some women remain unmarried and do not reproduce, this could mildly select for genetic tallness. This mild selection process could not account for dramatic one-generation mean height increases, but could account for some of the small continuing increments in succeeding generations. (A similar selection could result from women’s preference for tall men, but there is no reason to suspect that such a preference is of recent origin). This hypothesis receives some indirect support from Jensen and Sinha’s (1990) report of a 0.2 correlation between height and IQ, which is attributable to leg length. They hypothesize that high - IQ, high income men are most successful in courting longstemmed beauties. Their offspring, inheriting high IQ from the father, would also inherit long legs from the mother. Jensen and Sinha’s hypothesis, if valid, would support the notion that tall women have recently had a mating advantage. There are several ways to test these hypotheses: 1.
Heights of individual middle-class and farm families could be plotted over several generations along with nutritional and economic histories. Care would need to be taken to correct for effects of the Great Depression, es-
2.
3.
HYPOTHESES
pecially in children born to mothers whose pelves were developing during a time of parental unemployment. (Inclusion of multigenerational farm families could help control for this.) Obstetrical histories would need to be included in the analyses. Offspring of Caesarean section could be compared in height to non-Caesarean matched controls. More importantly, their children could also be compared. The wife-selection hypothesis could be checked by comparing mean heights of married mothers to age-matched unmarried nonmothers. A heuristic refinement would be to include also age-matched groups of unmarried mothers and married non-mothers. If heights of women by age, marital status, and parity are available from times before the modern tall ‘model’ became popular, these could be checked for the hypothesized reverse skew.
References Jensen A, Sinha SN (1990) Physical Correlations of Human Intelligence. In: Biological Approaches to the Study of Human Intelligence, PH Venson (Ed.), quoted in Fortune, April 23, 1990, p. 332 - 336 McMahon TA, Bonner JT (1983) The Natural History of Size. In: On Size and Life. Scientific American Books Inc. New York. New York 1983
