Evolution Of Human Sex Roles More Complex Than Described By Universal Theory

In 1948, Angus J. Bateman's performed some now famous studies in fruit flies that showed that males exhibit greater variance in mating success (the number of sexual partners) and in reproductive success (the number of offspring) when compared to females. In addition, Bateman demonstrated that there was a stronger relationship between reproductive success and mating success in males than females.

Bateman concluded that, because a single egg is more costly to produce than a single sperm, the number of offspring produced by a female fruit fly was mainly limited by her ability to produce eggs, while a male's reproductive success was limited by the number of females he inseminated. These studies supported the conventional assumption that male animals are competitive and promiscuous while female animals are non-competitive and choosy.

"The conventional view of promiscuous, undiscriminating males and coy, choosy females has also been applied to our own species," says lead study author Dr. Gillian R. Brown from the School of Psychology at the University of St. Andrews. "We sought to make a comprehensive review of sexual selection theory and examine data on mating behavior and reproductive success in current human populations in order to further our understanding of human sex roles."

Dr. Brown and colleagues examined the general universal applicability of Bateman's principles. To test one of Bateman's assumptions, they collated data on the variance in male and female reproductive success in 18 human populations. While male reproductive success varied more than female reproductive success overall, huge variability was found between populations; for instance, in monogamous societies, variances in male and female reproductive success were very similar.

The researchers also examined factors that might explain variations across human populations that are not in keeping with the prediction of universal sex roles. "Recent advances in evolutionary theory suggest that factors such as sex-biased mortality, sex-ratio, population density and variation in mate quality, are likely to impact mating behavior in humans," concludes Dr. Brown. "The insights gained from this new perspective will have important implications for how we conceive of male and female sexual behavior."

Researchers include Gillian R. Brown, University of St Andrews, U.K.; Kevin N. Laland, University of St Andrews, U.K.; and Monique Borgerhoff Mulder, University of California at Davis, CA.