CHAPTER 1: WHAT IS A WOMAN? Although there are a whole bunch of different ways that a person could answer a question like this one (we could talk about gender identity or social roles or any of the other numerous forces that make you who you are), we're going to look at what evolutionary biology has to say about it. Because it turns out that we can learn a lot about women by understanding the things that our brains were designed to do. You see, each one of us is the result of an unbroken chain of successful survival and reproduction that has gone on now--uninterrupted--for millions of years. If even one  of your ancestors had failed to survive long enough to reproduce, or simply failed to reproduce, you wouldn't be here. This is a pretty remarkable thing to think about. As women, we have inherited from our successful female ancestors traits that allowed them--generation after generation, and without pause-- to make good decisions about everything ranging from whether to approach a snake (no!) to whether to have a clandestine love affair with the hot guy from a neighboring tribe (maybe!). Traits that promote successful survival and reproduction get passed down from one genera- tion to the next. Traits that don't promote survival and reproduction don't. It's that simple. This process of inheritance is called natural selec- tion. And it turns out to be a very powerful explanatory tool when it comes to understanding what it means to be a woman and to have a female brain.  YOU ARE YOUR GAMETES  To understand what it means to be a woman and to have a woman's brain, we first need to define what it means to be female. And in the eyes of evolutionary biology, this is something that's defined by the size of your gametes (a.k.a. your sex cells). If you have a limited supply of large, calorically expensive gametes, you are a female and we call your sex cells "eggs." If you have an unlimited supply of small, metabolically inexpensive gametes, you are a male and we call your sex cells "sperm." And although this designation may sound overly simplistic (and potentially even a little crass), it's at the very heart of almost all reliably occurring sex differences observed in creatures great and small, including human beings. And it's actually incredibly fascinating stuff. For instance, being the sex with the larger, more expensive sex cells means that women-- before they even meet the future fathers of their children --have already invested more in any babies that they may have than their future baby-daddies-to-be have. And in humans (and many other species), this investment asymmetry only grows larger once an egg gets fertilized. We are mammals, after all. And for female mammals, reproduction is costly. So having larger sex cells oftentimes means set- ting the stage for costly reproduction. And costly it is. Women hoping to reproduce have to be willing to share their bodies with another human being for nine months. This is no small request. It's energetically costly. It's uncomfortable. And it's a logistical nightmare for a woman's immune and circulatory systems. Further, despite the wonder that is modern medicine, complications from pregnancy and childbirth continue to kill several hundred women around the world daily . But there's more! You see, there's also lactation. And even though this activity is no longer required for successful reproduction to occur, it was something of a nonnegotiable in our evolutionary past. Women had to lactate to feed their infants, and lactation is also pretty costly. In addition to requiring women to secure an additional six hundred or so calories each day to offset the metabolic expense of milk production, it is also time consuming and would have made things like food acquisition difficult for ancestral women. Although I have never personally attempted to forage for food while having a suckling infant stuck to my chest, I can't imagine that it would exactly help. The takeaway here? Women's minimum level of investment in repro- duction is much greater than men's. Much. And this means that women-- over the course of our evolutionary history--have been confronted with a number of adaptive challenges that are specific to being the internally gestating, greater-investing sex. Ultimately, this is why men and women are different. Evolution by selection has shaped women's and men's psychology differently because sometimes the types of traits that best promote survival and reproduction differ, depending on whether they are present in a male or female body. Similar evolutionary challenges create similar brains. Different evolutionary challenges create different brains. To illustrate this point, I want you to consider the prospect of having sex with a stranger. And I want you to consider this from the vantage point of someone living during the time of our ancient ancestors. Imagine living on the African savanna without all the luxuries of modern living, including birth control. First, let's imagine this scenario as a man. This scenario (keeping in mind men's inexpensive gametes and low minimum investment in reproduction) is a pretty sweet deal. Even if the stranger isn't all that good-looking or fun to be around, if she's interested in sex and nothing more, the costs are very low for men if they wish to oblige. In fact, this type of sexual scenario is actually a pretty big "get" in terms of being an opportunity to pass down genes at almost no cost. This is precisely the type of trait that selection tends to favor. Traits that promote gene transmission get passed down to children, who then possess those traits. And then they pass them on to their children, who pass them on to theirs. And when this process of inheriting traits that pro- mote reproduction goes on for millions of years, you can expect that the trait will begin to characterize the species (or at least one sex in that spe- cies). Modern men's mating psychology should be characterized by a penchant for sexual opportunism, because their ancestors would have passed down more copies of their genes than their more sexually restricted contemporaries. But what about women? With our rare, expensive gametes and minimum nine-month investments, how should we respond? Not like men, that's for sure. To start with, because a woman's own body is the limiting factor in terms of her ability to reproduce, women can't increase access to gene- transmission opportunities simply by finding new partners. No matter how many men a woman has sex with over the course of a week, she can produce only--at most--a single pregnancy. For this reason, women who desire sexual novelty, per se, in their choice of partners won't  pass down more copies of their genes than women who prefer to take a one-partner-at-a-time approach to mating. This isn't to say that there aren't benefits to be gained by women from short-term mat- ing. There are (and I'll tell you about some of them in chapter 3). It  just means that increasing access to reproductive opportunities is not among them. A woman's opportunities for reproduction are limited to the number of children her own body can produce and not by her access to men. So, casual sex hasn't been as beneficial to women's reproductive output as it has been to men's. This alone would be enough to prevent selection from favoring sexual opportunism in women. However, in this case, the fate of this trait as an evolutionary nonstarter has been further ensured by the fact that it has also been historically very costly for women. And this is, again, because of the whole pregnancy thing. Although women nowadays can pretty much have it all--careers, relationships, casual sex without pregnancy--our female ancestors weren't quite so lucky. When they were having sex, there was always the chance that the sex that they were having was going to result in a pregnancy. And this is a bfd,* since the children of single mothers, historically, haven't fared very well. These children are more likely to die from every recordable cause of mortality than are children whose dads stick around to help provide food, care, and protection. Although contemporary laws, contraception methods, and social programs for children and their mothers have helped close these gaps somewhat for contemporary women, we've inherited our mating psychology from women who didn't have these options. Given these differences, we should expect to find that women are coyer and less sexually opportunistic than men. We should also find  that women tend to prefer a more prolonged courtship period than do men and are less interested in sexual novelty for novelty's sake than   are men. And you know what? That's exactly what the research finds. Most women are less sexually opportunistic than most men. Hundreds of studies have now found this to be true. For example, in one of the most-talked-about experiments of its kind, researchers had attractive male and female actors take turns standing in a quad on a college campus in Florida. They were then instructed by the researchers to approach random members of the opposite sex and say in a casual tone, "I've been noticing you around campus. I find you very attractive." The experimenter would then follow this with one of three requests (randomly assigned across encounters): "Would you go out with me tonight?" "Would youcome over to my apartment tonight?" or (the not very subtle) "Would you go to bed with me tonight?" You can see the results below. Fifty percent of both men and women agreed to the date. After that, though, things differed pretty dramatically depending on whether the request was being made to someone who had a male or a female brain. For women, fewer than 10 percent agreed to go back to the man's apartment. And none of the women agreed to sex. Not one. At this point, you might be thinking to yourself, "Of course no one said yes to sex. Who would do that? Only a total lunatic would request sex from a stranger, so--of course--no one would say yes to such a bizarre, apropos-of-nothing sexual invitation." Except 80 percent of men did.* Bizarre or not bizarre, men weren't about to look a sex-bearing gift horse in the mouth. Most men are more sexually opportunistic than most women because sex has historically been very costly for women and far less costly for men. In fact, for men, it has come with the possibility of being able to transmit genes into the next generation without too many follow-up costs. Being a female means that sex is costly, and our psychology reflects those costs. Our psychology has been shaped by the myriad female-specific adaptive challenges that come along with being the sex who is obligated to invest more. Now, before we break into a hymn of the oppressed, it's worth noting that a number of unique adaptive challenges also come along with being male. Take, for example, the issue of paternity uncertainty. Women, being the internal gestators of offspring that they are, always know that any child they have is theirs. Like, theirs , theirs. This means that--over the long span of evolutionary time--women would have al- ways benefited from investing heavily in their children. After all, such an investment would help promote the ultimate success of her genes, since she is certain that every child that she is a bona fide genetic relative. This certainty in relatedness between a mother and her children has made heavy parental investment a no-brainer for women. This is why we invest so heavily in our children, even if they show no overt indicators of being one of our relatives. For men, it's a little more complicated. Since they aren't the ones who gestate babies, they can't know for certain that any child they have is theirs (or "theirs," as the case may be). This poses an adaptive challenge to them that women don't have to confront when making choices about parental investment. This is known as the problem of paternity uncer- tainty. And even though you might think it's not a real problem that men have to worry about, a recent metaanalysis (which is a study of other studies--sort of a super-study) tells us otherwise. Although the average rate of non-paternity (which is what we call it when men think that they're the father of a child that actually isn't theirs) for men who report feeling highly certain about being the biological fathers of their children is somewhere between 2 and 4 percent, for men who feel relatively less certain about their relatedness to their children, the rate is closer to 30 percent. Ouch. Because of this, men tend to be more discriminating than women in terms of how much they invest in their kids, investing more in those whom they feel are more certainly theirs and less in those for whom re- latedness is more dubious. For example, in one study, researchers had external judges evaluate the facial resemblance of children to each of their parents. They also had the parents rate their perceived similarity  to their child and report their psychological and emotional closeness to their children. What they found was that mothers' facial resemblance to their chil- dren did not predict their emotional closeness to their children at all (see the bars on the right of the figure opposite). For the dads? As you can see from the bars on the left, it mattered. A lot. Dads who report feeling the most emotional closeness with their children tend to resemble their children more than those who report low emotional closeness to their children. Several studies have now demonstrated that men's parenting psychology is highly sensitive to cues bearing on the likelihood that their children are genetically related to them. Women's parenting psychology, on the other hand, is not. Men and women have each confronted some survival and reproductive challenges that are unique to their biological sex, and their brains are different from one another as a result. Excerpted from This Is Your Brain on Birth Control: The Surprising Science of Sex, Women, Hormones, and the Law of Unintended Consequences by Sarah Hill All rights reserved by the original copyright owners. Excerpts are provided for display purposes only and may not be reproduced, reprinted or distributed without the written permission of the publisher.