The co‐varying factors of prematurity and low birth weight are the primary causes of neonatal mortality in developed countries (Mathews and MacDorman, 2006). These birth outcomes reflect human life history trade‐offs, and variation may be partially shaped by mechanisms evolved to evaluate local environmental conditions historically related to the offspring's prospects for survival (Haig, 1993). Mothers manipulate offspring's size, body composition, and metabolism based on a selective investment of energy stores (Laskey and Prentice, 1997). Eventual gestational age and birth weight will be a compromise between maternal and fetal strategies; each pursued by aggressive hormonal regulation (Haig, 1993). In good‐quality environments, mothers will have more resources to invest and outcomes will tend toward the theoretical optimum for offspring fitness, but in more adverse environments maternal and offspring interests diverge (Wells, 2003). Maternal fitness is generally maximized at the expense of the fitness of each individual offspring (Smith and Fretwell, 1974; Trivers, 1974). Under adverse circumstances, maternal survival will be favored at the expense of investment in offspring (Hirschfield and Tinkle, 1975). In marginal environments, reduced somatic investment will lead to low birth weight infants, reducing maternal demands and preserving resources for future offspring (Haig, 1993). In the most severe environments, maternal reproductive investment will be constricted through an inability to conceive, miscarriage early in pregnancy, or stillbirth (for a review, see Haig, 1993).

In humans, paternal investments of resource provisioning, training in life skills, and defense from threats contributes to the prospects of offspring survival and reproduction (Geary, 2005). Among the foraging Ache, children who grew up without an investing father present suffered higher mortality rates (Hill and Hurtado, 1996). Thus, expectations for paternal investment may be an important factor in shaping maternal investment in a gestating fetus.

The relative proportions of men and women in a population influence the average level of paternal investment. In female biased populations, where men are scarce, males have higher returns from mating effort and lower incentives for long‐term commitment and investment. Women marry later and are less likely to be married (Lichter et al., 1995). Where males are relatively scarce, there are higher divorce rates (Trent and South, 1989), more out‐of‐wedlock births and more single mother households (Barber, 2004), higher rates of teenage pregnancies (Barber, 2000), and lower expectations for paternal care of offspring (Guttentag and Secord, 1983).

Because women in populations with male scarcity face lower prospects for paternal investment, which is historically associated with higher infant and child mortality, they may reduce somatic investment in gestating offspring to conserve maternal resources for investment in potential future offspring. Truncated maternal investment may result in shorter pregnancies and lighter offspring, increasing the rates of premature gestation births and low‐birth weight births. Thus, modern populations with male scarcity or female bias may have shorter gestational times and lower birth weights on average, even when controlling for relevant socio‐economic factors. The proportion of families with children that are single mother households reflects the ambient level of paternal investment, and this factor likely mediates the effects of the sex ratio.