A series of experiments showing that it is possible to effect the sexual behaviour of animals by manipulating hormones circulating in their bodies during early development is presented. An important message is that the mammallian brain and external genitalia tend to be female unless they are exposed to testosterone in infancy. This finding has important implications for human development which are explored by presenting a study that suggests that girls that are exposed to testosterone in utero exhibit masculine behaviour patterns in adulthood. The activational effects of the hormones testosterone, estrogen and progesterone were discussed in the last lecture.
Activational effects refer to effects in the adult organism.
Organisational effects occur during the early development of an animal Castration of male rats in infancy causes them to become demasculinised and feminised Treatment of female rats with testosterone in infancy causes them to become masculinised and defeminised The organisational effects of the presence or absence of testosterone are only apparent in adulthood when the organism is under the activational effects of testosterone or estrogen and progesterone. Testosterone exposure in infancy affects a range of behaviours not just reproductive behaviours. There are sex differences in human behaviour e.g. aggression in pre-school children that may be the result of neonatal hormone exposure. Some transsexuals seek surgical intervention to change their external genitalia because theior gender identity is at odds with their external genitalia. The blue dot within the rat represents the testes.