Abstract
The most consistent sex differences in cognition are found for spatial ability, in which males, on average, outperform females. Utilizing a twin design, two studies have shown that females with male co-twins perform better than females with female co-twins on a mental rotation task. According to the Twin Testosterone Transfer hypothesis (TTT) this advantage is due to in-uterine transmission of testosterone from males to females. The present study tested the TTT across 14 different spatial ability measures, including mental rotation tasks, in a large sample of 19–21-year-old twins. Males performed significantly better than females on all spatial tasks, with effect sizes ranging from η2 = 0.02 to η2 = 0.16. Females with a male co-twin outperformed females with a female co-twin in two of the tasks. The effect sizes for both differences were negligible (η2 < 0.02). Contrary to the previous studies, our results gave no indication that prenatally transferred testosterone, from a male to a female twin, influences sex differences in spatial ability.
Highlights
Sex differences are small to negligible in most cognitive traits[1,2]
The first aim of the study was to investigate whether previously found sex differences for spatial ability are present across all aspects of spatial ability
Effect sizes ranged from small to moderate, consistent with the previous research which has shown that men outperform women on spatial ability tasks with advantage of up to one standard deviation[7]
Summary
Sex differences are small to negligible in most cognitive traits[1,2]. some measures show differences between males and females[3,4,5,6]. Two recent studies, using data from a large longitudinal twin sample in the United Kingdom ( used in this study), have shown evidence for a uni-factorial structure of spatial ability across a variety of different spatial measures, both phenotypically and genetically[19,21] In both studies, the first order factor explained approximately 42 per cent of the variance across diverse spatial tasks. Studies on the etiology of spatial ability found only negligible differences in genetic and environmental factors driving sex differences in spatial ability in males and females[19], including in mental rotation[28]. Some evolutionary arguments suggest that testosterone is a factor maintaining spatial sex differences[34] According to such accounts, the greater elaboration of the neurocognitive basis of spatial ability, especially in 3-dimensional environments, is due to navigating and tracking movement that had more evolutionary relevance for males than females[34,35]. In line with the evolutionary argument, naturally occurring testosterone levels vary between sexes: typical testosterone levels in clinical assessment, measured in blood, range between 0.5 to 2.4 nmol/L in females, and from 10 to 38 nmol/Lin males[38]
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