Testosterone is a sex hormone that is present in larger quantities in males than in females, and it affects development and behavior both before and after birth. If testosterone is released in the womb at 7 weeks (when‘maleness’ is switched on by the SRY gene on the Y chromosome)it causes the development of male sex organs and also acts on the hypothalamus, resulting in the masculinisation of the brain.It has been found that an area of the hypothalamus at the base of the brain, called the sexually dimorphic nucleus, is much larger in male than in females. Testosterone is believed to be responsible for typically male behaviors, such as aggression, competitiveness and superior visuo-spatial abilities. Males also have a surge of testosterone during puberty, and this is responsible for secondary sexual characteristics such as facial hair and deepening voice.