Socioeconomic status determines sex-dependent survival of human offspring

Abstract

Background and objectives: In polygynous societies, rich men have many offspring through the marriage of multiple wives. Evolutionary, rich households would therefore benefit more from sons, and according to the Trivers-Willard hypothesis, parents invest more in offspring of the sex that has the best reproductive prospects. We determined the sex differences in number of offspring, sex ratio of offspring, offspring survival and offspring weight in rich and poor households in a polygynous population.

Methodology: We studied a population of 28 994 individuals in Northern Ghana during an 8-year prospective follow-up. We determined the fertility rate for both men and women, sex ratio of 3511 newborn offspring and offspring survival in 16 632 offspring up to reproductive age (≤18 years). Also, we collected 9842 weight measurements of 1470 offspring up to the age of 3 years from growth charts of local clinics.

Results: In rich households, men have a lifetime number of 6.0 offspring, while for women this was 3.1. In line with evolutionary predictions, the male:female sex ratio was higher in rich households (0.52; poor households 0.49), sons had lower mortality in rich households (hazard ratio male versus female 1.06, P = 0.64; poor households: hazard ratio male versus female 1.46, P = 0.01) and sons also had higher weights in rich households (P = 0.008).

Conclusions and implications: In rich households, men have higher reproductive prospects in this polygynous society and, in line with Trivers-Willard, we registered more sons in rich households, sons had lower mortality and higher weights, maximizing the reproductive output in this society.

Keywords: Africa; Trivers–Willard; offspring survival; offspring weight; reproduction; sex differences.

Figure 1.

Cumulative survival, age-specific fertility rate and offspring per year for poor and rich men and women of different age groups.

Figure 2.

Offspring per year (a), sex of offspring (b), offspring survival (c) and offspring weight (d) in poor and rich households. Error bars indicate standard errors. SDS = Standard Deviation Score.

Figure 3.

Offspring survival ≤18 years dependent on socioeconomic status (SES) in different strata of wealth (DHS wealth index). (a) Split by median (poor versus rich), (b) tertiles of SES, (c) quartiles of SES and (d) quintiles of SES.