Does Nubility Indicate More Than High Reproductive Value? Nubile Primiparas’ Pregnancy Outcomes in Evolutionary Perspective. William D. Lassek, Steven J. C. Gaulin. Evolutionary Psychology, September 15, 2021. https://doi.org/10.1177/14747049211039506
Abstract: The idea that human males are most strongly attracted to traits that peak in women in the nubile age group raises the question of how well women in that age group contend with the potential hazards of a first pregnancy. Using data for 1.7 million first births from 1990 U.S. natality and mortality records, we compared outcomes for women with first births (primiparas) aged 16–20 years (when first births typically occur in forager and subsistence groups) with those aged 21–25 years. The younger primiparas had a much lower risk of potentially life-threatening complications of labor and delivery and, when evolutionarily novel risk factors were controlled, fetuses which were significantly more likely to survive despite lower birth weights. Thus, nubile primiparas were more likely to have a successful reproductive outcome defined in an evolutionarily relevant way (an infant of normal birth weight and gestation, surviving to one year, and delivered without a medically necessary cesarean delivery). This suggests that prior to the widespread availability of surgical deliveries, men who mated with women in the nubile age group would have reaped the benefit of having a reproductive partner more likely to have a successful first pregnancy.
Keywords: first pregnancy, female attractiveness, nubility, primiparas, pregnancy complications, mating preferences, reproductive timing
Nubile Mothers Have Better First-Pregnancy Outcomes
As noted in the Introduction section, recent evidence suggests that heterosexual men are attracted to attributes characteristic of physically and sexually mature women between 15 and 19 years of age, closely corresponding to the 16–20 years age group when first births typically occur in natural-fertility populations (Kramer & Lancaster, 2010; Lassek & Gaulin, 2019; Symons, 1979; Walker, 2019). Relevant to that preference, the current study is consistent with others in a variety of populations indicating that, when surgical deliveries are not available, this is also the age group with the best first-pregnancy outcomes.
In our large sample, primiparas were at much higher risk than multiparas for CPD, critical C-sections, and other complications of labor and delivery that increase the risk of maternal mortality, but primiparas aged 16–20 years had substantially lower risks than those 21–25, experiencing a 30% lower risk of any C-section, a 27% lower risk of a critical C-section, and much lower risks for serious complications of labor and delivery, including CPD, abnormal labor, and fetal distress (Table 4). These findings are consistent with many other studies that show a reduced risk of C-sections in primiparas aged 16–19 years versus older mothers, including studies in many non-Western countries (Conde-Agudelo et al., 2000; Ganchimeg et al., 2013, 2014).
The reduced risk of complications of labor and delivery in primiparas aged 16–20 years is of special importance in a species where the conjunction of bipedalism and very large brains has made vaginal births difficult. Only very recently have these conflicting selection pressures been relieved by the surgical innovation of C-section, an intervention still not available everywhere. Where and when such surgical births are unavailable, it is essential for a first-time mother to produce a child who can successfully pass through her birth canal so that mother and child can survive and continue to augment her fitness.
Our study does not present data on maternal survival, but the lower risk of labor-and-delivery complications for mothers in the nubile age group would likely decrease the risk of maternal deaths in childbirth. In a recent study of maternal mortality in 144 countries (Nove et al., 2014), in a third of countries mortality was lower for mothers aged 15–19 years than those aged 20–24 years; and this included most of the countries with the highest maternal mortality rates.
There may also be survival benefits for the infants of younger mothers. Our study is consistent with others showing comparable survival in the newborns of mothers aged 16–19 years with those aged 20–24 years when social and behavioral risk factors are controlled (Bradford & Giles, 1989; Conde-Agudelo et al., 2000; Gallais et al., 1996; Ganchimeg et al., 2013, 2014; Geist et al., 2006; Geronimus & Korenman, 1992; Phipps-Yonas, 1980; Scholl et al., 1984; Smith & Pell, 2001).
When evolutionarily novel risk factors were controlled, the fetuses and newborns of primiparas aged 16–20 years did as well or better as those of primiparas aged 21–25 years and were more likely to survive to 1 year. Although the 16–20-year old primiparas had significantly more preterm births with normal birth weights (which have good survival rates), their risks for preterm births with low birth weight, overall low birth weight, and neonatal mortality were significantly lower.
These results are also consistent with the finding that in 18th–19th century Germany, when infant and child mortality rates were much higher, the children of mothers aged 15–19 years were more likely to survive to reproductive age than those of older mothers (Knodel & Hermalin, 1984). Such high infant and child mortality rates were also likely in the environment of evolutionary adaptedness (EEA; Tooby and Cosmides, 1992), with almost half of children dying before reaching puberty (Volk & Atkinson, 2013).
Because until quite recently most women gave birth at home with assistance from their relatives, another potential advantage for younger mothers is their greater likelihood of having mothers, grandmothers, and aunts to help with pregnancy and childbirth, especially in groups with shorter life expectancies, where senior kin may have been less common.
Thus, it is not surprising that women's evolved life history seems to schedule first reproduction soon after the attainment of adult size and sexual maturity, as revealed by the demography of subsistence populations. Primiparas aged 16–20 years were likely to have a better newborn outcome than older primiparas and much better labor-and-delivery outcomes, which combine to yield a substantially greater chance for a successful pregnancy outcome (Figure 3).
Evolution of Male Preferences for Nubility
Because first pregnancies are most likely to be successful in women in the 16–20 years age group, we would expect positive selection on male preferences that targeted any reliable phenotypic correlates of this female life stage. Thus, this study, together with others showing the advantages of first births in this age range, when first births typically occur in subsistence populations, supports nubility as a key criterion of female attractiveness.
Men’s preferences for certain female traits associated with nubility—such as low WHRs, low BMIs (in well-nourished populations), and low waist–stature ratios (which may be the best predictor of attractiveness)—have recently been explained in terms of their correlation with female reproductive value (Andrews et al., 2017; Butovskaya et al., 2017; Fessler et al., 2005; Lassek & Gaulin, 2019; Marlowe, 1998; Prokop et al., 2020; Röder et al., 2013; Sugiyama, 2005; Symons, 1979, 1995), an inherently future-oriented parameter (Fisher, 1930). Complementing those findings, this study suggests an additional, more immediate, benefit to preferences for nubility.
If, as our findings suggest, nubile women have had more successful first-pregnancy outcomes than older women (thus enhancing their own reproductive success), men with preferences for traits correlated with nubility would have experienced a parallel fitness advantage. Conscious awareness would not have been required for this preference to evolve (Gaulin & McBurney, 2001; Kenrick, 1995); genetic variance in the preference and a reliable correlation between the preferred trait (e.g., any sign of nubility) and a positive fitness outcome (e.g., a successful first birth) would be sufficient.
The main cost to males who prefer nubile women is the lower frequency of ovulation in younger women (see Lassek and Gaulin, 2018a). However, this can be largely overcome by an increased frequency of coitus, which is usual for this age group (Weinstein et al., 1990). The fact that the typical age at first birth falls in the nubile period in traditional populations suggests that copulatory effort is normally sufficient to overcome the lower probability of conception with women in this age group.
Because of the lower fertility of nubile women, Symons (1979) suggested that males seeking short-term mating with minimal commitment might prefer older women who would have a greater chance of conceiving, as fertility is maximal in the late 20s. However, Symons (1995) changed his view because of studies in modern hunter-gatherers showing most potentially fertile women over 20 are either pregnant or nursing, with small windows of time when conception is possible. In this context, nubile women are likely to have a greater chance of conceiving despite their reduced frequency of ovulation.
Why do Nubile Primiparas Have Better Obstetric Outcomes?
Three factors help to explain the lower risk of critical surgical deliveries in the 16–20-year-old mothers: (a) Younger primiparas had smaller newborns than older primiparas, with more neonates weighing 2.5–3.4 kg, in the lower two-thirds of the normal range (2.5–3.9 kg), and fewer weighing 3.5 kg or more (Table 3). As shown in Figure 1, smaller neonates are less likely to have CPD or require a critical C-section. (b) For newborns of the same birth weight, nubile primiparas had a much lower risk of a critical C-section (Figure 1), and all but one of its associated complications, than those over 20. (c) The fetuses of younger primiparas were less likely to experience complications during labor and delivery, including fetal distress, cord prolapse, placenta previa, and abnormal position (Table 4). In total, younger mothers seem to have an enhanced ability to move their fetus through the birth canal and their fetuses also seem to be more tolerant of the stresses of labor and delivery.
Negative Consequences of Teen Pregnancy Today
It is important to stress that our analysis also documents the disadvantages of teen pregnancy in WEIRD (as defined by Henrich et al., 2010) populations, such as the United States in 1990, especially in younger teens. Where both surgical births and birth control are widely available and where teen pregnancy is usually associated with social and behavioral risk factors, teen pregnancies are very likely to have many negative long-term consequences for the mother and infant (Black et al., 2012). Pregnancies in teens younger than 16 years had much poorer infant outcomes and were more likely to have an operative delivery (forceps or vacuum extraction). Thus, efforts to prevent teen pregnancies are highly desirable and are not in any way contradicted by any of our conclusions concerning past selection on female life history.
Despite the disadvantages of teen pregnancy in contemporary WEIRD societies, we suggest that our findings are relevant to understanding human evolution, in particular women’s life history and men’s mating preferences. Given the obvious evolutionary importance of a successful vaginal delivery, early first pregnancies and a male preference for nubility were probably advantageous in the premodern era. They would have produced the best odds of a successful reproductive outcome—a benefit to mother, father, and infant.
Limitations
The use of data from a modern North American population to gauge probable reproductive outcomes in evolutionarily relevant populations is not ideal; but because of the extremely recent nature of shifts in human reproductive ecology, the relevant underlying biology may be largely unchanged. By controlling for evolutionarily novel risk factors which make “teen pregnancy” disadvantageous in contemporary populations, the results should have some validity for natural-fertility populations where first pregnancies in 16–20 year-olds are normative. Our findings of a much lower risk of critically necessary surgical births and a slightly lower risk of neonatal deaths for mothers in the 16–20 years age group (with controls for social and behavioral risk factors) are consistent with findings from a large variety of non-Western countries (Conde-Agudelo et al., 2000; Ganchimeg et al., 2013, 2014) and from 18th–19th century Germany (Knodel & Hermalin, 1984).
It might be argued that modern American obstetric practices are quite different from those in traditional societies where experienced midwives play a crucial role, but the uniformity of findings in the US and in three different samples of 23, 29, and 18 non-Western countries points to common underlying factors in reproductive biology and suggests that similar biological factors are likely to have been operating in the EEA.
Because all of the many available studies uniformly show a decrease in obstetric complications requiring a C-section in the 16–19 years age group, this is more likely to be species-typical. Limiting the analysis to C-sections associated with complications that would be likely to cause significant harm to the mother or fetus without surgical intervention may give some indication of the risk in natural-fertility populations without access to surgical deliveries.
Traditional populations are also likely to have much higher infant and child mortality: Infant mortality has been estimated at 27% in the EEA (Volk & Atkinson, 2013), compared with 0.8% in the 1990 data set. However, our analysis controlling for contemporary maternal risk factors suggests that the infants of nubile mothers in the past would likely have done as well or better than those of older mothers, as also indicated by findings from 18th to19th century Germany, where the overall infant mortality rate was 23% (Knodel & Hermalin, 1984).
When comparable infant outcomes are combined with the much lower risk of death in childbirth from complications of labor and delivery, it seems likely that nubile women would have been the most successful primiparas, thus suggesting an adaptive explanation for the timing of first births in a wide range of forager and subsistence populations (Kramer & Lancaster, 2010; Lassek & Gaulin, 2019; Symons, 1979; Walker, 2019).
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