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.

Then there's a theory that menarche can be triggered in girls who spend little time around their biological father and lots of time around unrelated men. A large study involving 1,938 college women, published in 2006 in the American Journal of Human Biology, indicated that both the absence of biological fathers and the presence of half brothers and stepbrothers had an impact on earlier menarche. According to this theory, the absence of one's father and the presence of unrelated men signals a young woman that it's time to start looking for a mate. And how is that signal sent? Well, it might be by scent. As we'll discuss in more detail later, many animals receive chemical signals through smell, and there is real evidence that humans do as well.

Another theory that has been garnering considerably more weight over the last few years revolves around the skyrocketing rates of childhood obesity. One recent study that looked at weight and age of menarche was conducted by Joyce Lee at the University of Michigan. Lee tracked 354 girls from the time they were three until they were twelve. She found that there was a clear link between extra weight and early puberty. In her study, obese girls -- twenty-two pounds or more overweight -- had an 80 percent chance of developing breasts before they were nine years old and reaching menarche before they turned twelve.

One study suggests, however, that it's not how much fat a girl carries, but where she carries it that is driving early puberty. According to William Lassek, a researcher at the University of California at Santa Barbara: "What our findings suggest is that menarche is likely to occur when girls have stored a certain minimal amount of fat in the hips and thighs, and that girls who tend to store more fat around the waist -- who have abdominal obesity -- are likely to have delayed menarche."

Scientists know that sufficient fat stores are key to the onset of menarche. And as Lassek points out, fat located in the lower part of the body is chock full of the omega-3 fatty acids that are so important for fetal brain development. "This fat is protected from everyday use like money deposited in a bank," says Lassek. "You are not allowed to withdraw it until late pregnancy."

Whatever the biological cause, it's clear that many girls are entering sexual maturity long before they reach emotional maturity, especially today when it takes more emotional maturity than ever to navigate a complicated world. "This long period of mismatch is very confusing for young people," says Peter Gluckman of the University of Auckland, New Zealand. His book, "Mismatch: Why Our World No Longer Fits Our Bodies," calls for significant changes in education to help bridge the gap. Gluckman believes that the early age of menarche we're seeing today is likely our set point for menstruation?the norm, given good health and nutrition. According to Gluckman, with the advent of agriculture the overall level of nutrition dropped, resulting in a decrease in nutritional health, and increase in the age of menarche. Early menstruation creates a mismatch for some girls. They may be physically ready, but emotionally and intellectually unable to handle the responsibilities of adult sexuality.

Menarche is essentially the culmination of puberty in girls. Puberty itself is the incredible collection of physiological processes that transform children into adults, with sexually mature bodies capable of reproduction. We're constantly uncovering more of the biological nuances associated with the onset of puberty. For example, scientists have recently discovered a protein called kisspeptin (named in honor of Hershey Kisses by researchers at the Penn State College of Medicine), which plays an important role as a biological signal in starting both puberty and ovulation.

But even though the physical transformation into a sexually mature human being is more or less on biological autopilot, adult sexuality is anything but just biological. Modern sexuality is the intersection of biology, society, and history.

What we need, what we want, what we like, and how we like it are all shaped by a combination of evolutionary imperatives, cultural training, and individuality. Evolution, of course, has a keen interest in encouraging us to have sex, even if it comes at a significant cost. The guiding imperative for any species is survival. At least for us, and for most vertebrate animals, no sex means no babies, and no babies means extinction. Having an interest in our sex lives clearly has evolutionary advantages. But before we get too far into how sex works, let's begin by looking at how girls become women, how women become sexual, and how those changes affect the way men (and other women) perceive them.

Excerpted from "How Sex Works: Why We Look, Smell, Taste, Feel, and Act the Way We Do" by Dr. Sharon Moalem. HarperCollins Publishers, copyright 2009. To Browse Inside, click here.