Gender mix, especially the supposed hormone transfer in utero of the male fetus to his female co-twin, is a highly debated controversial subject. It occurs in animals (free-martin syndrome in the cow) but its existence in man has not been convincingly demonstrated. Two aspects of gender mix effects in man, birthweight and cognitive development, were studied in the Belgian East Flanders Prospective Twin Survey, a large population-based registry of multiple maternities, characterised by accurate data on pregnancy, placental structure and zygosity. The birthweight of the female member of the pair is not influenced by the male co-twin but, unexpectedly, the female twin enhances to a slight degree the birthweight of her male co-twin by prolonging the gestation for a few days. Also unexpectedly, in an opposite direction, the cognitive development, as measured by the IQ (WISC-R) of the female twin rises as compared with controls if her birthweight exceeds that of her male co-twin.