Height and reproductive success i...
HEIGHT A N D REPRODUCTIVE SUCCESS IN A COHORT OF BRITISH MEN D a n i e l N e t t l e The Open University, Milton Keynes, UK Two recent studies have shown a relationship b e t w e e n male height and n u m b e r of offspring in contemporary d e v e l o p e d - w o r l d populations. O n e of them argues as a result that directional selection for male tallness is both positive and unconstrained. This p a p e r uses data from a large and so- cially representative national cohort of m e n w h o were b o r n in Britain in March 1958. Taller men were less l i k e l y to be childless than shorter ones. T h e y did not have a greater mean n u m b e r of children. If anything, the pat- tern was the reverse, since men from h i g h e r socioeconomic groups t e n d e d to be taller and also to have smaller families. However, clear evidence was f o u n d that men w h o were taller than average were more l i k e l y to find a long-term partner, and also more likely to have several different long-term partners. This confirms the finding that tall m e n are considered more at- tractive and suggests that, in a noncontracepting environment, they w o u l d have more children. There is also evidence of stabilizing selection, since extremely tall m e n had an excess of h e a l t h p r o b l e m s a n d an increased l i k e l i h o o d of childlessness. The conclusion is that m a l e tallness has b e e n selected for in recent h u m a n evolution b u t has b e e n constrained b y de- v e l o p m e n t a l factors and stabilizing selection on the extremely tall. KEY WORDS: Height H u m a n evolution Mate choice Reproductive suc- cess T w o r e c e n t s t u d i e s h a v e f o u n d t h a t m a l e t a l l n e s s is a s s o c i a t e d w i t h in- c r e a s e d r e p r o d u c t i v e success in c o n t e m p o r a r y d e v e l o p e d - w o r l d p o p u l a - Received January 15, 2002 accepted March 7, 2002. Address all correspondence to Daniel Nettle, Departments of Biological Sciences and Psy- chology, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK. E-maih D.Nettle@open.ac.uk Copyright 2002 by Walter de Gruyter, Inc., New York Human Nature, Vol. 13, No. 4, pp. 473--491. 1045-6767/01/$1.00+.10 473
474 Human Nature, Vol. 13, No. 4, 2002 tions. The first, by Pawlowski and colleagues (2000), used medical records of men aged 25--60 in Wroclaw, Poland. The authors found that, for two subpopulations of the sample, tallness was a highly significant though weak predictor of the number of children a man had for his age. In the sec- ond study (Mueller and Mazur 2001), the sample consisted of a cohort of military academy graduates for w h o m extensive physical, health, and life course information had been gathered. Mueller and Mazur found that male height was associated with increased numbers of children. The mechanism by which this occurred was not increased fecundity of the tall men's wives, but rather a higher probability that the taller men would take a second or subsequent wife. Moreover, there was no relationship between height and socioeconomic status within the Mueller and Mazur sample, so the mech- anism of selection on tall men was directly through their physical attrac- tiveness to potential mates rather than indirectly via their socioeconomic achievements. Thus it seems that tall men enjoy increased fitness by their intrinsic ability to attract more mates. Mueller and Mazur also examined the type of selection involved. Rather than there being a threshold of height above which no further advantage accrues by being taller, they suggest that fitness increases linearly with in- creasing height. They also find no evidence of stabilizing selection im- pacting negatively on the extremely tall. They thus conclude that selection for male tallness is unconstrained in the human population and will lead to the evolution of ever-increasing male height until some limit or con- straint is reached. These recent results stand in contrast to those of an older study (Vetta 1975), which used data from a large sample of Harvard alumni (Damon and Thomas 1967). Vetta's analysis suggested an inverse U-shaped relationship between male height and number of children, with a decline in offspring among extremely short and extremely tall men. Vetta's paper was silent on whether the peak of the 13 was at the mean height for the population, though it was evidently close to it, and did not go into any more detail about the health, marital, or socioeconomic histories of the men involved. Thus the mechanisms associating height and reproductive success, and possible confounding factors, could not be further scrutinized. The present study investigates similar effects in the UK's National Child Development Study (NCDS). This is an ongoing longitudinal study of all the children born in the UK in one week in 1958. The cohort members are now 42, and a considerable amount of information about their physical de- velopment and social and reproductive lives is now available. These data enable questions to be asked about the relationship between height and re- productive success. The questions to be pursued within this dataset are as follows. First, is there an increase in reproductive success with increasing tallness? The two recent studies found an effect by looking at populations of men who were
Evolution of Stature 475 quite homogenous in other ways. The NCDS, by design, covers the whole range of the UK population of a particular age set in an unbiased way, so if an effect is found, it will show that the reproductive advantage of tall- ness holds generally across the population. Any increase in numbers of children for tall men may be confounded by the fact that men of higher socioeconomic status tend to be taller, but that in modern societies the higher socioeconomic groups choose to have smaller families. Because of this effect, over a modern population the num- ber of children born to taller men may actually be less even if they are more successful at attracting partners. One way of countering this is to look at the number of long-term partners the men have. Mueller and Mazur (2001) found that tall men's reproductive advantage resided in their abil- ity to attract second and third wives. This substantiates a solid psycholog- ical finding that taller men are rated more attractive than shorter ones (Feingold 1982 Gillis and Avis 1980 Hensley 1994 Hensley and Cooper 1987 Jackson and Ervin 1992). If taller men attract more mates, then we can infer that they are the beneficiaries of a psychological mechanism of at- tractiveness which in ancestral environments would have meant increased reproductive success, even if today other factors intervene. The two recent studies cited above did not find the number of children confounded by socioeconomic status group differences. However, they re- stricted the domain of comparison to relatively homogenous subpopula- tions--just military officers in Mueller and Mazur (2001), just rural or just urban men in Pawlowski et al. (2000). Mueller and Mazur argue that, in modern societies, different socioeconomic strata constitute essentially sep- arate marriage markets. Success within each stratum may be correlated with number of children, but since each stratum has different norms, com- paring across the strata will find no correlation between number of chil- dren and factors such as tallness. The key factors segregating society into different strata are education and social class. The present dataset can be divided into separate strata to investigate the extent to which this picture is correct. Data on both occupational class and educational level are avail- able for all the men. The second question to be investigated here concerns the interaction between height and socioeconomic status (SES). Being tall is robustly associated with high socioeconomic status (Peck and Lundberg 1995 Sil- ventoinen et al. 1999). There are two possible, non-mutually exclusive di- rections of causality for this effect. On the one hand better maternal nutrition and better health care within the higher SES groups could lead to men growing taller. On the other hand tall men, through the strong associ- ation of height and attractiveness, could be advantaged in professional and social competition, thereby tending to rise to more rewarded professions. If they enjoy a reproductive advantage, this could be directly due to their attractiveness, or indirectly due to the socioeconomic status they have been