EXCERPT: 'How Sex Works'

May 4, 2009— -- Have you ever wondered exactly what attracted you to your partner? Or why you act the way you do around the opposite sex?

Evolutionary biologist Sharon Moalem's new book "How Sex Works" takes a look at the scientific reasons why people are attracted to one another.

Find an excerpt of "How Sex Works" below:

Chapter One: Girls Just Want to Have Fun.

If you're a woman, you almost certainly remember the first time you got your period. The first menstruation, called menarche, is only one in a series of events that mark the transformation of girls into young women, but it is one that has been loaded with cultural significance throughout human history. For a long time, menarche was thought to coincide with the onset of fertility. Now we know that most girls do not ovulate with menarche; in fact, it can take a year or two after their first menstruation before ovulation even becomes regular.

Something strange is affecting the age of menarche, making parents and researchers take note and wonder alike. The average age of menarche has crashed from a traditional seventeen to just twelve, in the evolutionarily brisk span of just 150 years. So what is turning young girls into young women so quickly? There are lots of theories, but no clear answers.

According to the psychosocial acceleration theory, the root cause is increased stress, and a few studies have, in fact, found a correlation between increased stress and earlier menarche. Here's the theory: if young girls in our increasingly complex society experience a lot of stressors early on, their bodies take it as an indication that they have been born into stressful times. In earlier eras, stress was usually the result of circumstances that threatened survival, such as conflicts or famine. In those situations, there might be an evolutionary benefit to earlier menarche, because it would give an individual a chance to reproduce faster, perhaps before succumbing to local threats. For most women in the developed world, the source of today's stress is probably not war or famine. But as far as your brain and body are concerned, stress is stress and it produces the same result.