On a recent Saturday, I was flipping through TV channels when I spotted something that took me back to my high school days: the movie Species. If you are unfortunate enough to remember this gem from the ‘90s, you probably know it as the movie in which a hideous, deadly alien disguises itself as highly attractive supermodel Natasha Henstridge. Natasha's only goal? To reproduce by finding a hunky human male to have sex with. Coincidentally, the alien's goal fit quite nicely with the moviemakers' goal, which was to make a really bad movie that would still garner a lot of money and attention because it featured prodigious topless nudity. The nudity wasn't the only thing, of course. Just as important was the fact that the movie was centered on an attractive woman who craved sex so much that she would do it anywhere, anytime, and with almost anyone. When is the last time you met someone like that? Such women are not very common, and frankly, many people would doubt that they exist at all. But why don't they exist?

Evolutionary psychology provides the most sensible explanation. For women throughout evolutionary history, it was not adaptive to have sex with just anyone at any time. Pregnancy, child-bearing, and child-rearing are huge biological burdens for women, which are worthwhile only if they are offset by a partner who helps provide her and her future offspring with resources or high-quality genes or, preferably, both. Thus, on average, women have evolved to be less eager than men to participate in one-night stands, which in the ancestral environment would usually have resulted in the woman becoming pregnant without a male partner to care for her. Women also have higher standards than men when choosing a partner for a one-night stand, and this is especially true of attractive women.

Despite embodying the ultimate male fantasy, Natasha proves embarrassingly inept at reaching her goal. For someone with Natasha's looks, single-mindedness, and low clothing-removal threshold, finding a sex partner proves surprisingly difficult, to the extent that she ends up mating only once and with a decidedly non-hunky man toward the end of the movie. One scene in particular (the only one I happened to re-watch - honest) illustrates both her plight and an intriguing evolutionary concept. In this scene, an attractive, rich guy whom she just met takes her home, and they start frolicking in his swimming pool. At first, things are going swimmingly: clothes are flying off, they are making out, and the guy clearly wants to have sex with her. However, she then makes a huge misstep by declaring aloud that she wants to have a baby. The man's reaction is predictable: "What?? Excuse me?" At that point, the romance screeches to a grinding halt.

Why is that? If people are driven by natural selection to leave as many offspring as possible, then shouldn't they consciously desire to have a baby, especially in circumstances when they are eager to have sex, knowing that the sexual act could result in pregnancy? And shouldn't this be especially true of men, who can reproduce without directly shouldering the load of pregnancy and lactation? If this line of logic seems a little silly, that's because it is - but you would be surprised how often it's used as an argument to show that evolutionary psychology must be "wrong". Our ability to consciously override the chain of events that leads from sex to having a baby in no way undermines the fact that humans have evolved to maximize their reproductive fitness. There are multiple reasons for this. First, to motivate people to have babies, natural selection did not act on the actual desire to have babies per se, because this would not have helped us engage in the actual act of making a baby. Literally writing the directive "have as many babies as possible" into the human brain would be as fruitless as commanding a computer to play a movie, without first installing any software that tells it how to play a movie. Instead, natural selection programmed us to be motivated to have sex with opposite-sex individuals, which in turn led to having babies.

Second, conscious desire is not at all a prerequisite for adaptive behavior. Many adaptations, especially those for sexual behavior, arose very early in our species' lineage and were inherited from our primate ancestors. Most of us would agree that when a male peacock unfurls his opulent tail to attract a female, it is not because he consciously "desires" to have offspring. Similarly, when two people are sexually attracted to one another, it is not because they deliberately want to have a baby (for an uproarious spoof on this idea, see this opinion piece from The Onion). Adaptations like sexual attraction arose earlier than conscious thought in humans, which is a much more recent evolutionary development. Thus, basic and crucial adaptations like sexual attraction do not have to, and indeed cannot, rely on conscious desire in order to work properly.

The third point is that when we consciously curb our evolved desires, this in itself can be adaptive. As psychologist Paul Eastwick argues in a new paper in Psychological Bulletin, some of our newer adaptations, such as the ability to consciously control our behaviors, probably function to curb some of our older ones, such as sexual attraction. Come to think of it, unbridled sexual attraction is not always optimal, especially in the complex, socially nuanced human world. Acting on inappropriate sexual urges - attraction towards someone else's partner, or towards your own partner but in a public place - might lead to being punished or ostracized, thereby lowering one's reproductive success in the long run. Desires like these are better left as mere fantasies, which is precisely what conscious, controlled decision-making allows us to do. In other words, the not-so-rare conflicts between our conscious decisions and more primitive biological inclinations indicate neither a flaw in our adaptations nor a flaw in our theory of adaptations. On the contrary, these conflicts often constitute evidence for different layers of adaptive design.

In case you were wondering, the pool scene in the movie ends when Natasha realizes her guy is no longer interested, and drowns him after impaling him with her razor-sharp alien tongue - certainly not the kind of penetration he was looking for. The evolutionary lesson here, as always: human males are not adapted to a novel environment in which murderous aliens take the shape of beautiful naked women.

About the Author

Andrew Galperin

Andrew Galperin is a doctoral candidate in social psychology at UCLA, working with Dr. Martie Haselton.

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