Separating the Bruce and Trivers-Willard effects in theory and in human data.

Objectives: Theories of reproductive suppression predict that natural selection would conserve mechanisms that abort the gestation of offspring otherwise unlikely to thrive in prevailing environments. Research reports evidence among humans of at least two such mechanisms-the Trivers-Willard and Bruce Effects. No literature, however, compares the mechanisms nor estimates their relative contribution to observed characteristics of human birth cohorts. We describe similarities and differences between the Trivers-Willard and Bruce Effects and explore high quality historical data from Sweden to determine which mechanism better describes temporal variation in the ratio of males to females in birth cohorts.

Methods: We measure Trivers-Willard exposures with the death rate among women of reproductive age. We measure Bruce exposures with the death rate among children. We use time-series regression methods to estimate the relative contribution of the Trivers-Willard and Bruce Effects to temporal variation in historical Swedish secondary sex ratio data.

Results: We find that the Bruce Effect appears to be a better predictor of the secondary sex ratio than does the Trivers-Willard Effect.

Conclusions: Attempts to identify mechanisms by which reproductive suppression affects fetal loss and characteristics of human birth cohorts should consider the Bruce Effect as an alternative to the Trivers-Willard Effect.