Sex and Population Differences in Intelligence Are Partly Caused by Sexual Selection: Hn Evolutionary Hypothesis

Davide Piffer
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Abstract

Sexual dimorphism in intelligence suggests that this phenotype is a sexually selected trait. This view is supported by an overrepresentation (compared to the autosomal genome) of genes affecting cognition on the X chromosome. The aim of this study is to test the hypothesis that sexual selection can explain sex and country-level differences in performance on tests of fluid intelligence. Nationally representative samples from N = 44 countries were obtained from the Programme for International Student Assessment (PISA) Creative Problem Solving (CPS), which evaluates the core of intelligence, that is novel problem solving ability. Sexual selection has the double effect of increasing the prevalence of a favored phenotype and reducing genetic variation in sexually selected traits. Matching these predictions from evolutionary theory, the average country fluid intelligence is positively correlated to sexual dimorphism after partialling out per capita GDP and the latter in turn is inversely correlated to variance in intelligence scores within populations. Males have a higher variance than females but there is a negative correlation between male-female difference in variance and sexual dimorphism in intelligence, suggesting that selection reduces variance more in the selected sex. Average country male height is negatively correlated to sexual dimorphism in intelligence, a fact that supports the notion of a trade-off between physical and intellectual competition in the context of access to females. The results of this study, if replicated, imply that genome-wide association studies of cognition may benefit from a focus on sex chromosomes, which so far have been neglected. Another implication of this study is that intelligence has continued to evolve after different human populations migrated out of Africa and possibly up to the 19th century, as suggested by the substantial variability in sex differences even between neighbouring countries.
智力的性别和种群差异部分是由性选择引起的:一个进化假说
智力的两性二态性表明这种表型是一种性选择的特征。这一观点得到了X染色体上影响认知的基因的过度代表(与常染色体基因组相比)的支持。这项研究的目的是验证一种假设,即性选择可以解释性别和国家在流体智力测试中的表现差异。从国际学生评估项目(PISA)创造性解决问题(CPS)中获得了来自N = 44个国家的具有全国代表性的样本,该项目评估智力的核心,即解决新问题的能力。性选择具有双重作用,一方面增加了有利表型的流行率,另一方面减少了性选择性状的遗传变异。与进化论的预测相吻合的是,在剔除人均GDP后,平均国家的流动智力与两性二态性呈正相关,而后者又与人群内智力得分的差异呈负相关。男性的差异高于女性,但男女差异与智力性别二态性之间存在负相关,表明选择在被选择性别中更能减少差异。国家男性平均身高与智力的性别二态性呈负相关,这一事实支持了在获得女性的背景下,身体和智力竞争之间权衡的概念。这项研究的结果,如果被复制,意味着认知的全基因组关联研究可能受益于对性染色体的关注,这一点迄今为止一直被忽视。这项研究的另一个含义是,在不同的人类种群迁出非洲之后,智力仍在继续进化,可能一直持续到19世纪,这一点从性别差异的巨大差异中可以看出,甚至在邻国之间也是如此。
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