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Fetuses positioned next to littermates of the opposite sex may develop different sexually related traits and aggressive behaviour in later life. These effects of intrauterine position are due to hormonal transfer between fetuses. Testosterone from a male fetus that crosses the fetal membrane of an adjacent female fetus can affect molecular events in the female including modification of the steroid hormone receptors in hypothalamic neurons. The intrauterine environment may also account for the variability of laboratory animals and newborn human twins.