Brain lateralization refers to the division of labour between the two hemispheres in controlling a wide array of functions and is remarkably well developed in humans. Based on sex differences in lateralization of handedness and language, several hypotheses have postulated an effect of prenatal exposure to testosterone on human lateralization development, the topic of a long-standing and unresolved debate. Here we demonstrate a clear relationship between prenatal levels of testosterone as assessed from amniotic fluid of healthy pregnant mothers and language lateralization of their offspring at the age of 6 years. Using focused attention conditions in the dichotic listening task, in which the child is instructed to report information from the left ear or the right ear, we were able to differentiate between potential effects of early testosterone on the left hemisphere and effects on inter-hemispheric connectivity. This provides a new method to distinguish between the claims of the different hypotheses. The results suggest that in girls higher prenatal testosterone exposure facilitates left hemisphere language processing, whereas in boys it reduces the information transfer via the corpus callosum.
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