Trivers–Willard at birth and one year: evidence from US natality data 1983–2001

Douglas Almond, Lena Edlund


Trivers & Willard (TW) hypothesized that evolution would favour deviations from the population sex ratio in response to parental condition: parents in good condition would have more sons and parents in poor condition would have more daughters. We analyse the universe of US linked births and infant deaths to white mothers 1983–2001, covering 48 million births and 310 000 deaths. We find that (i) married, better educated and younger mothers bore more sons and (ii) infant deaths were more male if the mother was unmarried and young. Our findings highlight the potential role of offspring sex ratio as an indicator of maternal status, and the role of infant mortality in shaping a TW pattern in the breeding population.



  • 1 At balanced breeding sex ratios.

  • 2 Greater variance in male reproductive success implies polygyny among successful males (and possibly polyandry (or abstinence) among less successful males, at balanced sex ratios). Polygyny of successful males, in turn, implies hypergamy, the systematic marrying up of females (and down of males who marry). While hypergamy may be another rationale for a TW effect, greater variance in male reproductive success may be the more fundamental asymmetry between the sexes (Trivers 1972).

  • 3 Male sex ratios have been associated with, for instance, dominant (Grant 1994) or aggressive (Kanazawa 2005) personality, stature (Kanazawa 2006) and nutritional status (Gibson 2003).

  • 4 Generalized daughter preference is more rare (e.g. Cronk 1989). However, for an endogamous population, male-biased sex ratios among the higher socio-economic strata are likely to prevail for TW reasons (Edlund 1999).

  • 5 Arguably, the case in the United States, although Gallup polls show a consistent bias towards sons in the United States among those with a preference (see, for instance, The New York Times, Sugar and Spice, and Sour Dads, November 16, 2003).

  • 6 Compare with the definition of parental investment in Trivers (1972).

  • 7 Essock-Vitae (1984) studied the number of children among Forbes list of the 400 wealthiest Americans. While she found that they had more children than the population at large, the women on the list actually had more children than the men.

  • 8 Teen mothers were less likely to be married and had less education than older mothers in the 1983–2001 NCHS data. The 1988 US Maternal and Infant Health Survey, which also records income, indicates that teen mothers had significantly lower household income and were more likely to receive welfare than older mothers (results available from authors).

  • 9 Darwin (1871, p. 281) noted as a ‘mysterious fact that…the excess of male to female births is less when they are illegitimate than legitimate’ and hypothesized that males suffer disproportionately from adverse conditions.

  • 10 If the excess mortality was not 100% male, then the mortality rate would have to be adjusted up accordingly.

  • 11 To see this, consider an initial offspring population of 50 sons and 50 daughters. If one son were to die, then this would correspond to mortality rate of 1%, and a reduction in the probability of a son of approximately 1% ((49/99-50/100)/0.5).

    • Received April 18, 2007.
    • Accepted July 12, 2007.
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