Why are women so intelligent? The effect of maternal IQ on childhood mortality may be a relevant evolutionary factor.

摘要

Humans are an unusual species because they exhibit an economic division of labour. Most theories concerning the evolution of specifically human intelligence have focused either on economic problems or sexual selection mechanisms, both of which apply more to men than women. Yet while there is evidence for men having a slightly higher average IQ, the sexual dimorphism of intelligence is not obvious (except at unusually high and low levels). However, a more female-specific selection mechanism concerns the distinctive maternal role in child care during the offspring's early years. It has been reported that increasing maternal intelligence is associated with reducing child mortality. This would lead to a greater level of reproductive success for intelligent women, and since intelligence is substantially heritable, this is a plausible mechanism by which natural selection might tend to increase female intelligence in humans. Any effect of maternal intelligence on improving child survival would likely be amplified by assortative mating for IQ by which people tend to marry others of similar intelligence - combining female maternal and male economic or sexual selection factors. Furthermore, since general intelligence seems to have the functional attribute of general purpose problem-solving and more rapid learning, the advantages of maternal IQ are likely to be greater as the environment for child-rearing is more different from the African hunter-gatherer society and savannah environment in which ancestral humans probably evolved. However, the effect of maternal IQ on child mortality would probably only be of major evolutionary significance in environments where childhood mortality rates were high. The modern situation is that population growth is determined mostly by birth rates; so in modern conditions, maternal intelligence may no longer have a significant effect on reproductive success; the effect of female IQ on reproductive success is often negative. Nonetheless, in the past it is plausible that the link between maternal IQ and child survival constituted a strong selection pressure acting specifically on women.