Big and tall parents have more sons: Further generalizations of the Trivers–Willard hypothesis

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Abstract

This paper proposes the generalized Trivers–Willard hypothesis (gTWH), which suggests that parents who possess any heritable trait which increases male reproductive success at a greater rate than female reproductive success in a given environment will have a higher-than-expected offspring sex ratio, and parents who possess any heritable trait which increases female reproductive success at a greater rate than male reproductive success in a given environment will have a lower-than-expected offspring sex ratio. Since body size (height and weight) is a highly heritable trait which increases male (but not female) reproductive success, the paper hypothesizes that bigger and taller parents have more sons. The analysis of both surviving children and recent pregnancies among respondents of the National Child Development Survey and the British Cohort Survey largely supports the hypothesis.

Introduction

In their classic paper, Trivers and Willard (1973) suggest that parents might under some circumstances be able to vary the sex ratio of their offspring in order to maximize their reproductive success. The Trivers–Willard hypothesis (TWH) proposes that, for all species for which male fitness variance exceeds female fitness variance, male offspring of parents in better material and nutritional condition are expected to have greater reproductive success than their female siblings, because their greater size allows them to outcompete their intrasexual rivals and monopolize available reproductive opportunities. The converse is true of offspring of parents in poorer material and nutritional condition, because the smaller males, who are not intrasexually competitive, are excluded from mating opportunities. Parental conditions affect the reproductive prospects of female offspring to a much lesser extent. Almost all females get to reproduce some offspring, even though no female can produce a large number due to their greater obligatory parental investment into each offspring (Trivers, 1972).

It therefore pays parents in good condition to bet on male rather than female offspring. Since females have much lower variance in reproductive success, parents in poor material and nutritional condition should prefer to produce females as a safe bet. Trivers and Willard (1973) thus hypothesize that parents in better condition should produce more male offspring than female offspring. Their facultative parental investment into male and female offspring should be similarly biased. These predictions have been supported by data from a large number of experiments with a wide array of species (Venezuelan opossum: Austad and Sunquist, 1986; Red deer: Clutton-Brock et al., 1986; Spider monkey: Symington, 1987).

Evolutionary psychologists have since applied the original formulation of the TWH to modern humans and derived further hypotheses. Sons’ expected reproductive success depends largely on the parents’ wealth, so that sons from wealthy families are expected to attain much greater reproductive success than sons from poor families. This is because sons from wealthy families typically inherit the wealth from their fathers, and can in turn invest the resources into their offspring. Women prefer to mate with men with greater resources, and thus wealthy men throughout human evolutionary history have been able to attract a large number of high-quality mates (Betzig, 1986).

In contrast, daughters’ expected reproductive success is largely orthogonal to parents’ wealth, because it mostly depends on their youth and physical attractiveness. Men in general prefer younger and physically more attractive women, not wealthy women, for their mates (Buss, 1989; Kanazawa, 2003). The TWH in both of its specifications (offspring sex ratio and biased parental investment) has been supported with data from a wide variety of human societies, including the contemporary United States (Betzig and Weber, 1995; Gaulin and Robbins, 1991; Kanazawa, 2001; Mueller, 1993). Cronk (1991) provides a comprehensive review of the empirical evidence in support of the hypothesis, and Trivers (2002, pp. 120–122) adds a brief update on the status of the TWH.

While the TWH is one of the most celebrated principles in evolutionary biology and the preponderance of empirical evidence supports it, it has nonetheless received some criticisms. Myers (1978) and Leimar (1996) provide analytical critiques of the TWH's predictions. A comprehensive review (Brown, 2001) and a meta-analysis (Brown and Silk, 2002) find no consistent evidence for the TWH in the non-human primate literature. For the human populations, Koziel and Ulijaszek (2001) provide only qualified support, and Freese and Powell (1999) and Keller et al. (2001) find no support for the TWH for the contemporary United States.

While the TWH in its original formulation has specifically to do with material and economic conditions of parents and their ability to vary the sex ratio of their offspring in response to such conditions, the basic insight behind it may be more general. The fundamental assumption underlying the TWH is that, if males are expected to attain greater reproductive success than females, for whatever reason, then parents may have more sons than daughters. If, in contrast, females are expected to attain greater reproductive success than males, for whatever reason, then parents may have more daughters than sons.

For example Kanazawa and Vandermassen (2005) synthesize the TWH with Baron-Cohen's extreme male brain theory of autism. Baron-Cohen, 1999, Baron-Cohen, 2002, Baron-Cohen, 2003; Baron-Cohen and Hammer, 1997; Baron-Cohen et al., 2004) proposes that there are “male (or Type S) brains,” which are good at systemizing (dealing with physical objects and entities in non-social manners) and were adaptive for our ancestral men, and “female (or Type E) brains,” which are good at empathizing (relating to people in social situations) and were adaptive for our ancestral women. Baron-Cohen further suggests that brain types are substantially heritable. Kanazawa and Vandermassen then derive logical implications of the convergence of Baron-Cohen's theory and the TWH, and predict that, if Type S brain increases male reproductive success in the ancestral environment and Type E brain increases female reproductive success in the ancestral environment, then individuals with strong Type S brains (such as engineers and mathematicians) should have more sons than daughters, and individuals with strong Type E brains (such as nurses and school teachers) should have more daughters than sons. Their analysis of the 1994 US General Social Surveys supports their predictions.

In an entirely different paper, Kanazawa (2004) ponders why so many battered women choose to remain in their abusive relationships. He first points out that violence and aggression were adaptive for men (but not for women) in the ancestral environment, where much of male intrasexual competition for status and thus reproductive access to women was physical; violent and aggressive men may therefore have often had greater reproductive success in the ancestral environment than less violent and aggressive men. Kanazawa then notes that men's tendency toward violence and aggression, particularly, their tendency toward domestic violence, is a function of their baseline levels of testosterone (Booth and Osgood, 1993; Dabbs and Morris, 1990; Soler et al., 2000), and that testosterone levels are highly heritable (h2=0.60) (Harris et al. 1998). These two sets of observations lead Kanazawa to predict that battered women have more sons than daughters. His analysis of both American and British samples confirms his prediction.

At the same time, there is some evidence for the logical converse of the TWH. Tallal et al. (1989) show that mothers (but not fathers) with a developmental language impairment have an exceedingly high sex ratio (0.7143: 25 boys vs. 10 girls). Women normally have greater language and communication skills than men, and thus language impairment is relatively more problematic and maladaptive for girls than for boys. It is an example of a heritable trait that would decrease the female reproductive success to a much greater extent than it decreases male reproductive success, and thus the logical converse of the TWH would predict that language-impaired parents should have more sons than daughters.

The conditions that trigger biased sex ratio may therefore not be limited to the parents’ material and economic conditions, but may extend to all factors that affect sex-specific reproductive success in a given environment, so long as such factors are heritable. I therefore propose the generalized Trivers–Willard hypothesis (gTWH):

gTWH: Parents who possess any heritable trait which increases the male reproductive success at a greater rate (or decreases the male reproductive success at a smaller rate) than female reproductive success in a given environment will have a higher-than-expected offspring sex ratio (more males). Parents who possess any heritable trait which increases the female reproductive success at a greater rate (or decreases the female reproductive success at a smaller rate) than male reproductive success in a given environment will have a lower-than-expected offspring sex ratio (more females).

Since parental wealth and status are two heritable (at least culturally, if not genetically) traits of parents which increase the sons’ reproductive success to a much greater degree than they increase the daughters’, the original formulation of the TWH (Trivers and Willard, 1973) is indeed a special case of the gTWH as stated above. Burley's (1986) experiment has previously demonstrated a similar effect of parental attractiveness on the offspring sex ratio among zebra finches.

One highly heritable phenotype which influences sex-specific reproductive success is the body size. In the ancestral environment, where male intrasexual competition was both fierce (in the absence of socially imposed monogamy) and largely if not entirely physical, big and tall men had particular advantages over smaller and shorter men. In contrast, large body size was not particularly adaptive for ancestral women. Probably for this reason, taller men to this day have greater reproductive success than shorter men (Nettle, 2002a; Pawlowski et al., 2000), but shorter women have greater reproductive success than taller women (Nettle, 2002b). And body size (height and weight) is substantially heritable (Chambers, et al., 2001; Silventoinen et al., 2001). The gTWH therefore suggests novel predictions about the effect of body size on the offspring sex ratio.

H1

Parents who are taller have a higher-than-expected number of sons. Conversely, parents who are shorter have a lower-than-expected number of sons (or a higher-than-expected number of daughters).

H2

Parents who are heavier have a higher-than-expected number of sons. Parents who are lighter have a lower-than-expected number of sons (or a higher-than-expected number of daughters).

Among human populations, the expected (mean) sex ratio at birth is 105:100 (0.5122), 105 boys for every 100 girls (Grant, 1998).

No one to my knowledge has proposed or empirically tested these hypotheses regarding the effect of body-height and body-weight on the offspring sex ratio among humans, which is very curious, given that body size was the key variable of interest in the original formulation of the TWH among non-human species (Trivers and Willard, 1973). Material and nutritional conditions of the mother were important in determining the offspring sex ratio among non-human species because these factors were (correctly) assumed to influence the offspring body size significantly; well-fed and well-cared-for offspring among non-human species grow larger than their poorly fed and less-well-cared-for counterparts. When modern evolutionary psychologists began applying the TWH to human populations, however, the attention quickly shifted to parental wealth and social class, away from body size. I will therefore test these curiously novel predictions with empirical data. While the original formulation of the TWH focuses on the environmental determinants of body size (material and nutritional conditions), the current hypotheses focus instead on their genetic (heritable) determinants and thus individual differences in body size.

Section snippets

Data

I use the 1999–2000 combined follow-up sample of the National Child Development Study (NCDS) and the 1970 British Cohort Study (BCS70). The NCDS originates in the “Perinatal Mortality Survey,” which examines social and obstetric factors associated with stillbirth and infant mortality. All babies born in Great Britain (England, Wales, and Scotland) during the week of March 03–09, 1958, were contacted for inclusion into the study. The initial sample in 1958 consists of more than 17,000 babies.

Discussion

The results presented in Table 1, Table 2 largely support the hypotheses, derived from the gTWH, that taller and bigger parents have higher-than-expected numbers of male offspring, while shorter and smaller parents have lower-than-expected numbers of female offspring. All coefficients, except for one, have the right sign, and half of them are statistically significant. Any supportive evidence for the hypotheses in modern society is remarkable, given that male intrasexual competition no longer

Acknowledgement

The author thanks two anonymous reviewers for their comments on an earlier draft.

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