Two sides to every story: Sex hormones, brain lateralization and gender development

Sex hormones, like testosterone and estradiol, can affect the development of the brain and subsequently have essential influences on our behaviour. There are two major events during life when dramatic changes in the brain occur: before birth and in puberty. In the first part of the thesis, we investigated the influence of sex hormones on the asymmetrical specialization of cognitive functions of both halves of the brain (i.e. brain lateralization) based on two unique studies: 1) A longitudinal study in adolescents whose sex hormone levels were measured before birth (in the amniotic fluid of the mothers) . 2) A quasi-experimental study of the effects of hormone treatment in persons with gender dysphoria (who experience a mismatch between their sex assigned at birth and their gender identity). This offered us an experimental approach to study the effects of hormone administration in humans. In the second part of the thesis, we investigated the influence of sex hormones before birth and in puberty on gender role behaviour and gender identity from childhood to adolescence. Although the asymmetrical distribution of the brain and gender role behaviour and identity are very different output measures, there are several parallels in the effects sex hormones have. The main overarching conclusions are:1. Exposure to sex hormones before birth affects the influence of sex hormones in puberty. 2. Not only testosterone is important but estradiol plays a major role as well3. The effect of sex hormones depends on the phase of development, the cognitive task, and the sex of the participant.