Sex is primarily the process of combining male and female genes to form offspring, however over the past billion years complex systems of behaviour (and the motivation and reward circuits that root such behaviours) have evolved around this process and no where is this complexity more elaborately represented than in the human brain. At some point in their life, everyone will engage in sexual behaviour or at least experience sexual desire. Ultimately the brain is the largest sex organ controlling the biological urge, mediating all thoughts, experiences and physiological responses to sex, so 'Where Does Sex Live in the Brain.  Sexual desire is defined as the behavioural drive that motivates individuals to fantasize about or seek out sexual activity. In contrast, sexual arousal is defined as the autonomic physiological processes that prepare the body for sexual activity (Toledano, Pfaus., 2006). It is important to make clear distinctions between the two definitions as confusion can occur due to, the often simultaneous occurrence of both mechanisms.  Sexual desire is the culmination of several different neural mechanisms, each is controlled in different areas of the brain and is activated at different times of the sexual experience. The euphoric and pleasurable experience of sex stems primarily from the limbic system. The colloquial term for areas including the amygdala, hippocampus and limbic lobe (dentate and cingulate gyrus). This area is common to all mammals and is considered one of the oldest areas of the brain. It regulates emotion and encourages the avoidance of painful of aversive stimuli and the repetition of pleasurable experiences. From a physiological perspective sexual arousal is controlled by the parasympathetic portion of the autonomic nervous system and manifests itself as vasodilation in sexual organs along with several other physiological phenomena including an increase in heart rate. An orgasm and in particular male ejaculation is controlled by the sympathetic portion, this is also accompanied by deactivation of many areas in the brain relating to external stimuli in particular fear, allowing the mind to focus on the task at hand.  Sexual desire is initially modulated by the release of sex hormones, for males testosterone and for females oestrogen, our levels of these hormones are understood to affect our behaviour. Recent studies how found that male testosterone levels increase by approximately 7.8% in males even though they do no consider her attractive. 

Testosterone, a member of the androgen family of steroids, is secreted in the testes of males and the ovaries of females as well as small amounts from adrenal glands. Arousal causes the cerebral cortex to signal the hypothalamus to stimulate the production of testosterone, the production is regulated by a complex chain of events known as the hypothalamic-pituitary-gonadal axis; Gonadotropin-releasing hormone (GnRH) is secreted by the hypothalamus, via the hypophyseal portal system, it travels to the anterior pituitary which then releases luteinizing hormone (LH) in order to stimulate the production of testosterone in the testes. Production levels are controlled by negative feedback. Luteinizing hormones also stimulate the production of oestrogen in the ovaries, some oestrogens are also produced in other tissues such as the liver and adrenal glands. In females, oestrogen synthesis begins in cells in the ovaries by the synthesis of androstenedione, this compound then travels into the surrounding granulosa cells of the basal membrane where it is converted to oestrone or oestradiol, testosterone is also converted to oestradiol at this point.