Low fertility in contemporary humans and the mate value of their children: sex-specific effects on social status indicators Ralf Kaptijn a, ⁎ , Fleur Thomese a , Theo G. van Tilburg a , Aart C. Liefbroer a,b , Dorly J.H. Deeg a a VU University Amsterdam, Amsterdam, The Netherlands b Netherlands Interdisciplinary Demographic Institute, The Hague, The Netherlands Initial receipt 8 January 2009; final revision received 31 July 2009 Abstract Evolutionary explanations of low fertility in modern affluent societies commonly state that low fertility is the outcome of high parental investments in the quality of their children. Although the empirical evidence that modern parents do face a quantity–quality trade-off is strong, two issues that are relevant from an evolutionary perspective have not received much attention. First, sex differences in the proximate aspects of quality have been largely ignored. Second, the relationship between the quantity of children and their reproductive success in contemporary low-fertility societies remains unclear. In this article, we study the quantity–quality trade-off as a trade-off between the number of children and the mate value and reproductive success of those children. We examine the trade-off in two steps. First, a lower number of children is expected to increase the mate value of these children. Second, greater mate value is expected to lead to greater reproductive success. Using sex-specific indicators of mate value, we test these hypotheses in a representative sample of the Dutch population aged 55–85 in 1992 (n=3229). This sample contains information on three successive generations in which the middle generation has completed fertility. We find support for the first hypothesis, but only partial support for the second hypothesis. A higher number of children is traded off against the mate value of the children, but not against their reproductive success. We conclude that the conditions under which the quantity of children is traded off against their reproductive success depend on the social environment. © 2010 Elsevier Inc. All rights reserved. Keywords: Life history theory; Fertility; Parental investment; Mate value; Social status; Reproductive success 1. Introduction Contemporary affluent societies typically face below replacement fertility. Life history theory can explain low fertility as an outcome of high investments in the quality of offspring at the costs of investments in the quantity of offspring. It is expected that, all else equal, the less offspring one gets the greater the fitness of that offspring (Hill & Kaplan, 1999; Stearns, 1992). Many evolutionary explana- tions of contemporary low fertility explicitly or implicitly assume that low fertility is, at least partly, the result of high parental investments per child (e.g., Beauchamp, 1994; Boone & Kessler, 1999; Harpending & Rogers, 1990; Hill & Reeve, 2005; Kaplan, 1996; Kaplan, Lancaster, Tucker & Anderson, 2002; MacDonald, 1999; Mace, 1998; Rogers, 1990; Turke, 1989). Such a trade-off between the number of children and their fitness has been observed in preindustrial societies. Higher parental fertility has detrimental effects on the children's survival chances (Penn & Smith, 2007; Strass- mann & Gillespie, 2002). Furthermore, mainly due to this higher child mortality among children of larger families, the number of grandchildren is maximized at intermediate levels of fertility, at least for women in poor socioeconomic conditions (Borgerhoff Mulder, 2000; Gillespie, Russell & Lummaa, 2008). In modern societies where child mortality is very low, it is unlikely that higher mortality among children of larger families can cause a trade-off between the number of children and their fitness. However, in modern societies higher parental fertility and thus lower parental investments per child have detrimental effects on the children's socioeconomic outcomes, which can be seen as proxies for their reproductive success. Sociological studies Evolution and Human Behavior 31 (2010) 59 – 68 ⁎ Corresponding author. Faculty of Social Sciences, VU University Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands. Tel.: +31 20 598 2947; fax: +31 20 598 6940. E-mail address: rwj.kaptijn@fsw.vu.nl (R. Kaptijn). 1090-5138/$ – see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.evolhumbehav.2009.07.007