Research psychologist Jesse Bering's Why Is the Penis Shaped Like That? And Other Reflections on Being Human uses 30 quirky essays to dissect what makes us human. It turns out the results are very strange. Bering talked with PW about our species-wide squeamishness and whether suicide is adaptive.

So. Why is the penis shaped like that?

It really is a type of very specialized tool. In human males, the shape of the penis--particularly the umbrella-like 'lip' under the tumescent glans head (the coronal ridge) was sculpted by natural selection to retract the semen of any other males who may have had sex with that woman within the previous 48 hours, since this is the approximate length of time that sperm remain viable.

And form inspires function. The vigor and depth by which the man thrusts his penis into the vaginal canal results in more effective semen-removal (so size does matter in this sense, for the man as well). And data reveal that such vigorous behaviors are more apparent after the couple has been separated for a period of time, such as during a business trip while a boyfriend or husband has been out of town (or away hunting, in the ancestral past) and has less opportunity to monitor the female's behaviors, since this increases the probability that the woman has been with other men during his time away. None of this is conscious, mind you. Rather, it is simply an arousal response that would have evolved to influence human behavior over tens of thousands of generations. The more intense the arousal, the more powerful the ejaculation and the deeper it lands in the vaginal canal, making it more difficult for any males that follow him to retract his sperm with their penises.

Once ejaculation has occurred, men typically become flaccid fairly quickly, and further stimulation of the penis is even uncomfortable. This is important because continued thrusting would be self-defeating: the man would essentially be removing his own sperm at that point.

Why does brain damage make us “very, very randy”?

All human thought and behavior is based on localized brain processes, so it’s not terribly surprising that alterations to certain parts of the brain, or the introduction of some foreign drug that modifies neurochemical functioning, will significantly alter the sexual response. More often than not, the sex drive is watered down by damage and disease, but the opposite effect—hypersexuality—is occasionally symptomatic of a range of neurodegenerative diseases and abnormal brain conditions. Dementia, temporal lobe epilepsy, Tourette’s syndrome, strokes, lesions, frontal lobotomies, Multiple Sclerosis, Huntington’s disease, Wilson’s disease, Bipolar Disorder, Kleine-Levin and Klüver-Bucy syndromes can all trigger, in some patients, an uncontrollable lust and degree of carnality that would make the Marquis de Sade seem positively demure. In one of the essays in Why Is the Penis Shaped Like That, I turn my attention to some startling cases of hypersexuality in Klüver-Bucy syndrome. I describe a previously prudish woman with this disorder, for example, who now couldn’t keep her hands off family members. When admitted to the hospital, she began performing fellatio on an unwitting elderly man in the emergency room. Such cases strain our moral reasoning abilities because we tend to see sexual decision-making as a clear example of “free will” rather than being causally determined by the physical brain.

Is suicide adaptive?

The rather counterintuitive answer seems to be yes—sometimes. One helpful hint is always to remember that “adaptive” in an evolutionary sense has an entirely different meaning from “adaptive” in a contemporary mental health sense. Very bad things, such as violence and postpartum depression, may also be biologically adaptive while being outrageously maladaptive in the more everyday sense of the word. This is because the former implies simply a mindless, net genetic fitness advantage to the organism (the behavior or trait helps to get a broader representation of the individual’s genes out across subsequent generations), whereas the latter centers on a person’s subjective wellbeing. So while it may sound especially bizarre, and more than a little insensitive, to say that suicide could ever be adaptive, context is everything. It’s a challenging idea, in more ways than one, to get your head around. But in the book I describe how the brains of suicidal people may be unconsciously crunching the numbers, and in some cases, remaining alive may actually compromise one's own net genetic value by interfering with the reproductive success of one’s biological kin.

Did you find any common explanations in the questions you set out to answer, i.e. is there a part of our makeup that is tied to all of these strange human phenomena? Is there any uniting thread in the topics of your book?

Just our species’ squeamishness in facing itself in the mirror, really. I do enjoy shining a light on dark subjects, and there are plenty of awkward stories and unpalatable facts in Why Is the Penis Shaped Like That? It’s precisely the “unspeakable” nature of such questions that have always appealed to me the most. Why is it, exactly, that we’re so uncomfortable with studying ourselves under the light of a nonjudgmental nature? Sure, it's not always flattering, what we see. But still, I’ll take the ugliest truth over some garish cultural myth any day, because empirical reality has a special type of beauty all its own. I want my readers to critically examine themselves as the very strange creatures that they—that we—really are. The trouble in our everyday lives is that it’s so easy to miss this strangeness, and therefore to miss our species’ secrets, because we’re too close to the surface. The best science pans out, disciplines all the distorting emotions, and brings an objective clarity to our searching. Those researchers whose work I’ve chosen to highlight in Why Is the Penis Shaped Like That? are all notable for their courting of recalcitrant truths; they couldn’t care less what these truths ultimately turn out to look like, they just want them, and badly. Don’t get me wrong. The book isn’t all horror stories and penises, certainly—there’s plenty of light-hearted fare in there, too. In fact, probably underlying all of the essays, and behind my approach to science in general, is the comic absurdity of our existence.

What is the single strangest thing about the human race, in your book or otherwise?

There’s actually an embarrassment of riches to choose from in answering this, but I’ll focus on something that I flesh out in particular detail in one of the essays in the book. Without question, of the millions of species roaming this earth, we are the most masturbatory species of them all. Our unique social cognition has enabled the special conjuring ability to create fantasy scenes in our heads that literally bring us to orgasm when conveniently paired with our dexterous appendages. It’s really a psychological magic trick. Such masturbation requires a cognitive capacity called mental representation (an internal “re-presentation” of a previously experienced image or some other sensory input) that many evolutionary theorists believe is a relatively recent hominid innovation. When it comes to sex, we put this special capacity to very good—or at least very frequent—use, about every two days, on average.