This study examines foetal testosterone (fT) levels (measured in amniotic fluid) as a candidate biological factor, influencing sex differences in systemizing. Systemizing is a cognitive process, defined as the drive to analyze or construct systems. A recent model of psychological sex differences suggests that this is a major dimension in which the sexes differ, with males being more drawn to systemize than females. Participants included 204 children (93 female), age 6-9 years, taking part in a long-term study on the effects of M The systemizing quotient - children's version was administered to these mothers to answer on behalf of their child. Males (mean = 27.79 +/- 7.64) scored significantly higher than females (mean = 22.59 +/- 7.53), confirming that boys systemize to a greater extent than girls. Stepwise regression analysis revealed that fT was the only significant predictor of systemizing preference when the sexes were examined together. Sex was not included in the final regression model, suggesting that Er levels play a greater role than the child's sex in terms of differences in systemizing preference. This study suggests that the levels of fT are a biological factor influencing cognitive sex differences and lends support to the empathizing-systemizing theory of sex differences.