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Sex Ratio and the Timing of Pubertal Maturation in Girls: a Life History Approach

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Sex Ratio and the Timing of Pubertal Maturation in Girls: a Life History Approach Sex Ratio and the Timing of Pubertal Maturation in Girls: A Life History Approach Sarah Feingold University of Minnesota Sex ratio, defined as the ratio of males to females in a given population, is an important factor influencing a variety of mating and parental behaviors in both animals and humans. In this review, I draw on the frame- work of Life History Theory, which predicts that mating and parenting decisions are influenced by environ- mental cues about resource scarcity, to propose that sex ratio may serve as an indicator of resource scarcity – particularly the likelihood of finding and keeping a long-term mate. I therefore suggest that skewed sex ra- tios should influence mating behaviors in the same way that other scarce resources do. Specifically, in fe- male-skewed environments where the availability of long-term mates for females is limited, women may ex- perience earlier pubertal timing, because (1) it lengthens their reproductive lifespan and (2) energy put into embodied capital would be better spent on intra-sexual competition for mates and reproductive effort. The sex ratio is demographically defined as the terns (e.g., rates of marriage, divorce, and out-of- number of men per 100 women in a given popula- wedlock births), and spending and career related tion (Guttentag & Secord, 1983). A male-skewed decisions. sex ratio occurs when there are more males than One unexplored and potentially negative con- females, whereas a female-skewed ratio occurs sequence of skewed sex ratios is a shift in the tim- when there are more females than males. Various ing of pubertal maturation. Pubertal maturation is Asian countries have recently become intensely a complex biological process that marks the shift male-skewed (Gu & Roy, 1995; Guilmoto, 2009; from the pre-reproductive to the reproductive por- Jha et al., 2006; Sahni et al., 2008). In the most ex- tion of one’s life. Most studies exploring pubertal treme instance, China will soon have an excess of maturation in girls have measured a single signal of over 40 million men, resulting in a sex ratio of pubertal timing: menarche, or the onset of first more than 120 men for every 100 women (Hes- menstruation. In the United States mean age at keth, 2009). Furthermore, variation in the sex ratio menarche is 12.9 years (SD = 1.2) in Caucasian is not unique to individual countries; skewed sex girls and 12.2 years (SD = 1.2) in African-American ratios often occur within regions of a single coun- girls (Herman-Giddens et al., 1997). Because most try. For example, inequalities within the United pubertal development occurs before menarche, the States penal system often lead to female-skewed sex onset of menarche serves as an indicator that a girl ratios in communities of color and lower socioeco- has reached an advanced level of pubertal maturity. nomic status (Sampson, 1995). This happens be- There is considerable variation in the timing of cause men of color, and men of lower socioeco- pubertal development. Some individuals mature nomic status, are far more likely to be imprisoned early, completing pubertal development in grade than Caucasian men or men of higher socioeco- school, whereas others do not complete their de- nomic status. These high rates of imprisonment, in velopment until the end of high school or even the turn, lead to an unequal distribution of men and beginning of college. Evidence suggests that there women in communities of color or lower socioeco- are a variety of factors that impact the timing of nomic status. These variations in sex ratio may pubertal maturation, including body mass, psycho- have significant societal consequences: Various re- social stress, genetic influences (such as mother’s search studies have shown a strong correlation be- pubertal timing), and nutritional history (reviewed tween sex ratio, mating/parental investment pat- in Ellis, 2004). Feingold / SEX RATIO AND PUBERTAL MATURATION 19 There is a less known, but well researched rates of teen out-of-wedlock births rose as sex ratio framework for understanding individual differences shifted from male- to female-skewed. Barber ex- in pubertal timing: Life History Theory (Charnov, plains that the sex ratio serves as an indicator of 1993; Roff, 1992; Stearns, 1992). I contend that, one’s marriage prospects, and interprets the rela- when examining sex ratio through a life-history tionship between sex ratio and out-of-wedlock framework, sex ratio in the local population births to mean that the decision to reproduce early emerges as an additional factor that may impact (whether conscious or unconscious) is shaped by the timing of pubertal maturation. Because there the existing sex ratio as well as by one’s marriage are negative consequences of early menarche, un- prospects. In addition, male-skewed sex ratios are derstanding this previously unexplored connection associated with lower rates of non-marital births, between sex ratio and pubertal timing is important, higher rates of paternal investment, higher mar- and could potentially inform future research in bi- riage rates, and fewer out-of-wedlock births to 15- ology and psychology. 19 year olds (Fossett & Kiecolt, 1999). These par- ticular patterns make sense, given the idea that an Sex Ratio unfavorably skewed sex ratio changes the behav- iors of the majority sex to match the desires of the The study of sex ratio and its relationship to be- minority sex (Kenrick, Li, & Butner, 2003). havior began with the study of animal behavior Although the vast majority of sex ratio re- and evolutionary biology (James, 1987) and has search has been correlational, there have been at mainly focused on the disparity between the num- least two notable experimental studies of sex ratio. ber of males and females of reproductive age in a Griskevicius et al. (in press) manipulated sex ratio given population. This is referred to as the opera- by showing participants either male-skewed or fe- tional sex ratio (Emlen & Oring, 1977). Mating male-skewed photo arrays, as well as news articles behaviors are dependent on the availability of pro- that described the local population as either fe- spective mates, as well as on the level of competi- male-skewed or male-skewed. These primes were tion for those mates. The operational sex ratio af- followed by questions about financial choices, sav- fects mating behaviors by changing the way one ing and borrowing habits, and decisions about how competes for a mate. In most mammals, including financial resources are spent. The authors found humans, females have inherently higher levels of that male-skewed sex ratios led men to discount required parental investment than males, due to the future, making them more likely to incur debt factors such as length of gestation and lactation in an effort to attract a mate and less likely to save (Trivers, 1972). Consistent with these different lev- money for the future. Male-skewed sex ratios also els of parental investment, a skewed sex ratio af- led men and women to expect men to spend more fects each sex differently. Male-skewed sex ratios on dating/mating related expenses such as dinner, give females the chance to be more selective about gifts, or an engagement ring (Griskevicius et al., in their mates and to require more from potential ma- press). This happens because male-skewed envi- tes (e.g., greater parental investment, nuptial gifts, ronments afford women the luxury of being choos- higher levels of fidelity). In contrast, female-skewed ier, while men need to exert more mating effort to sex ratios result in less selective females because attract and keep a mate. Durante et al. (in press) males don’t have to compete as intensely for access manipulated sex ratio in a similar way, using both to mates. picture arrays and news articles that gave the im- A large body of correlational research has pression of either a male-skewed or female-skewed shown that sex ratio is related to mating/parental local population. These sex ratio primes were fol- behaviors (Barber, 2001; James, 1987; Kruger, lowed by questions gauging the importance of ca- 2009; Licher et al., 1992; Pollet & Nettle, 2007). reer versus family. The authors found a significant Most sex ratio research has focused on the rela- relationship between female-skewed sex ratios and tionship between sex ratio and its effects on mar- women’s increased desire for a career. Durante et riage and family. Guttentag and Secord (1983) pos- al. suggest that sex ratio cues serve as indicators of tulate that, in female-skewed populations, men and the availability of potential romantic partners, as women are more likely to stay single, if married are well as how much effort one needs to put into at- more likely to divorce, and are more likely to have tracting a potential mate. Therefore, when the sex children out-of-wedlock. In support of Guttentag ratio is female-skewed and women have a lower and Secord's hypotheses, Barber (2001) found that likelihood of finding and maintaining a long-term 20 YALE REVIEW OF UNDERGRADUATE RESEARCH IN PSYCHOLOGY mate, they will place more importance on their ca- birth, and nurturing of offspring. These approach- reer. es, which are labeled “slow” (growth, embodied As discussed, variations in sex ratio influence capital, future reproduction) vs. “fast” (current re- a number of mating behaviors. However, I propose production), also result in trade-offs between the that researchers may gain an even deeper under- quantity and quality of offspring (Kaplan & standing of sex ratio and mating behaviors by ac- Gangestad, 2005). Individuals taking a fast ap- counting for Life History Theory and its predic- proach will reproduce early and quickly, which re- tions for an individual’s reproductive choices.
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