Ecological variation in wealth-fertility relationships in Mongolia: the 'central theoretical problem of sociobiology' not a problem after all?

Abstract

The negative wealth-fertility relationship brought about by market integration remains a puzzle to classic evolutionary models. Evolutionary ecologists have argued that this phenomenon results from both stronger trade-offs between reproductive and socioeconomic success in the highest social classes and the comparison of groups rather than individuals. Indeed, studies in contemporary low fertility settings have typically used aggregated samples that may mask positive wealth-fertility relationships. Furthermore, while much evidence attests to trade-offs between reproductive and socioeconomic success, few studies have explicitly tested the idea that such constraints are intensified by market integration. Using data from Mongolia, a post-socialist nation that underwent mass privatization, we examine wealth-fertility relationships over time and across a rural-urban gradient. Among post-reproductive women, reproductive fitness is the lowest in urban areas, but increases with wealth in all regions. After liberalization, a demographic-economic paradox emerges in urban areas: while educational attainment negatively impacts female fertility in all regions, education uniquely provides socioeconomic benefits in urban contexts. As market integration progresses, socio-economic returns to education increase and women who limit their reproduction to pursue education get wealthier. The results support the view that selection favoured mechanisms that respond to opportunities for status enhancement rather than fertility maximization.

Keywords: contraception; demographic–economic paradox; life-history trade-offs; socio-economic success; somatic capital.

Figure 1.

Household wealth and reproductive outcomes along a rural–urban gradient. (a) Predicted means (and s.e.) for LRS among post-reproductive women (older than 45 years; n = 815). LRS is 12% higher among the wealthiest in all regions. (b) Predicted hazards (and s.e.) for the adoption of contraceptive methods before the birth of the first child (n = 9314). Wealthy women are more likely to adopt contraceptive methods (either modern or traditional) before the birth of their first, second and third child, and particularly so in urban areas. Values are reported for married women, atheists and born after 1967. Squares represent women living in rich households and circles represent women living in poor households.

Figure 2.

The relationship between educational level and fertility. (a) Predicted hazards (and s.e.) for age at first birth (n = 9314). The time to the first birth increases with educational level. Time 0 corresponds to age 15, the beginning of the period of risk exposure. As compared with women with lowest level of education, women with the highest level are less likely to have reproduced before the age of 25 years (time = 10; odds = 0.93), but more likely to have given birth to their first child after that time (time = 11, odds = 1.06). (b) Probability of school attendance (women aged 15–24, n = 2722). Predicted means (and s.e.). Women are approximately 63% less likely to be currently attending school once they have reproduced. Values are reported for women aged 20, married and atheists. (Online version in colour.)

Figure 3.

Ecological variation in the socio-economic returns to educational level among married women (n = 5428). Predicted odds (and s.e.). The average wealth of household headed by husbands is used as a proxy for how education translates into socio-economic returns. In Ulaanbaatar (grey bars), educational level is positively correlated with household wealth, and women with the highest level of education are 12 times more likely to live in wealthy households as compared with women with no education. In rural areas (black bars), women with the highest level of education are four times more likely to live in rich households. The ‘returns’ of education are partly mediated by educational assortative mating.