Drives often occur "within" the individual and may not need external stimuli to encourage the behaviour. An example is the sexual drive which is driven by our hormone system. The desire for sex is wired deep into the brain of all human beings as glands secrete hormones that travel through the blood to the brain and stimulates the onset of sexual desire. The process is started by the brain's hypothalamus, which releases pulses of GnRH (gonadotropin-releasing hormone). This starts a whole chain of reactions which we call "puberty". Longer term, the hormone supporting male sex drive is testosterone, and for women testosterone and oestrogen.