Looking back in time, what led to an adult’s brain/mind, with its ‘regular’ as well as idiosyncratic features of sexual desire? Considered in this way, the individual has a history in terms of two related aspects:

• human evolution, starting millions of years ago;

• development of the individual from conception to adulthood.

To understand how desire got here, it can be insightful to look at events happening over these two very different time scales, which is done in this chapter and the next. Such questions arise as:

• What are the roles of nature and nurture in determining human sexual desire?

• What evolutionary factors contributed to contemporary sexual culture and how did they do so?

• How does the early experience of the child lead to the later emergence of sexual desire?

• What is the role of interactions between the child and its parents in leading to a brain/mind that finds other humans attractive?

• What sort of individual becomes the object of desire and how does this occur?

• How can an unusual development lead to unusual sexual behaviour?

The development of sexual behaviour can be best understood in terms of general principles of development, described next.

A broad framework: some general processes underlying development

From conception and starting ‘simply’ as a single fertilized egg cell, the newly formed individual develops and grows, by means of cells dividing and thereby their number multiplying astronomically. The growing foetus interacts with its physical environment, by absorbing nutrients, making limb movements and bodily adjustments. This is believed to facilitate the wiring of the motor controls of the body, in forming coordination between brain and muscles.