Why are women so intelligent? The effect of maternal IQ on childhood mortality may be a relevant evolutionary factor
Bruce G. Charlton
Medical Hypotheses, 2009; 74: 401-2. in the press
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.
Humans are an unusual species because they exhibit an economic division of labour . And most theories concerning the evolution of specifically human intelligence have either focused on the value of abstract thinking and rapid learning in solving economic problems such as hunting, gathering, farming and defence; or on sexual selection mechanisms in producing creative intelligence to entertain the prospective mate (for review see ).
Both types of theory apply more to men than women, and tend to regard high intelligence in women as having been somewhat passively pulled-along by rising male intelligence due to the relative slowness of evolving sexually dimorphic traits . But, while there is evidence for men having a slightly higher average IQ (perhaps about four IQ points ), the sexual dimorphism is not striking except at very unusually high and low levels of intelligence .
However, there is some evidence of a more female-specific selection mechanism which concerns the distinctive female role in child care, especially during the offspring’s early years. I have located two studies which demonstrate a statistical relationship between increased maternal intelligence and reduced child mortality  and . These are supported by a host of indirect studies which find the same negative correlation between proxy measures of IQ such as educational attainment or duration, and measures of child health which might plausibly correlate with mortality (e.g.  and ).
So it seems likely that there would be a selection pressure tending to increase maternal intelligence since (all else being equal) increased intelligence would lead to a greater chance of offspring surviving to adulthood. This would lead to a greater level of reproductive success for intelligent women, and intelligence is substantially heritable – so this is a plausible mechanism by which natural selection might tend to increase female intelligence in humans (all else being equal).
Any effect of a mother’s intelligence on improving survival of her children is likely to be amplified by assortative mating for IQ: in other words people tend to marry others of similar intelligence  and . This suggests that a woman of higher-than-average-intelligence (who is more likely to keep her children safe) is also likely to have a higher-than-average-intelligence husband (who is likely to be a better economic provider). A more-intelligent wife would also be able more rapidly to learn to use these extra resources from her smarter husband in improving the health and survival of their offspring.
Furthermore, since intelligence seems to have the functional attribute of a general purpose problem-solving ‘module’ , 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. In an ancestral society and environment, it is likely that women have been equipped (by natural selection) with the ‘maternal instincts’ necessary for successful child-rearing. But as humans have moved further ‘out of Africa’ and into evolutionarily unfamiliar climates and ecologies, and have developed novel economic systems such as agriculture and cities – then the advantages of higher maternal IQ (including attributes of better ability to solve novel problems and more rapid ability to learn from experience) may have become more important. This would fit with some evidence (also consistent with explanations for higher male IQ) for varying human general intelligence according to multi-generational ancestral experience of higher latitude, agrarian economy, distance from Africa and some other indices of evolutionary novelty , ,  and . The effect of maternal IQ on child survival may make this a special case of Gottfredson’s ‘deadly innovations’ hypothesis .
However, the effect of maternal IQ would probably be of major evolutionary significance only in situations where childhood mortality rates were high: presumably, the higher the childhood mortality rates, the greater would be the differential effect on survival of higher maternal intelligence. Until the industrial revolution all societies experienced high childhood mortality rates due to factors such as birth problems, starvation, disease, parental death, predation and other types of violence . Then in industrializing/developing countries from about 1800, mortality rates fell more rapidly than birth rates and populations began to grow and living standards to rise. The modern situation is that population growth is determined mostly by birth rates, and child mortality rates are low enough that some of the population groups with the highest child mortality are growing rapidly due to even-higher birth rates; and populations with the lowest child mortality may be shrinking due to sub-replacement birth rates  and . Indeed, there is a strongly inverse relationship between IQ and reproductive success in women in modern societies (e.g. ).
So, in modernizing societies, maternal intelligence may no longer have a significant effect on reproductive success, or the relationship may be negative – nonetheless, in the past and especially in agrarian societies rich in evolutionarily novel situations that impact on child-rearing, it is plausible that the link between maternal IQ and child survival constituted a strong selection pressure acting specifically on women (it should, however, be remembered that most newly arising traits are initially inherited by both sexes, and that it takes longer for sexually dimorphic traits to evolve – see  for review).
This idea of maternal influence on childhood mortality driving an increase in female intelligence might potentially be testable. For example, specifically female intelligence might be measured in relation to the quantitatively evaluated degree of environmental evolutionary-unfamiliarity among ancestors (somewhat as in Ref. ), or perhaps in relation to differences in the nature and intensity of child-rearing practices and specific hazards (as in the study of the Ache tribe reported in ). And since the reason for women’s high IQ is postulated to be different from the reason for men’s high IQ (i.e., maternal benefits versus economic benefits), then it may be possible to generate and test predictions of how these sex differences would affect the nature and application not only of abstract reasoning, but also personality and other cognitive attributes.
For instance, the degree of selection probably on maternal psychology in situations of high child mortality in evolutionarily unfamiliar situations may affect the relative level of intelligence in women compared with men, or may affect the personality traits of higher conscientiousness and agreeableness/empathizing self-reported by women (for review see ). The population magnitude of such sexually dimorphic personality traits may vary in ways that can be predicted according to that population’s ancestral experience of maternal natural selection. Furthermore, Cochran and Harpending’s hypotheses that Ashkenazi Jewish men were under exceptionally powerful economic selection pressure for evolving higher intelligence over the past millennium  might be expected to create a larger than usual men-to-women IQ differential in this group.
In summary, differences in maternal psychology (intelligence and perhaps personality) could have had importance fitness effects by their influence on child survival in situations of high childhood mortality. This idea may provide a useful framework for understanding more about the evolution of human intelligence.
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