The body is absurd. The sounds, smells, textures, impulses, tics, blemishes, engorgements, excretions of our organs—these make up the outrageous conditions of our animal existence, from infancy to old age. We suffer the daily indignities, surprises, and wonders of the flesh because we are stuck with this imperfect vessel. No wonder we make so many dick jokes.

In Why is the Penis Shaped Like That? And Other Reflections On Being Human, research psychologist and science writer Jesse Bering aims to understand the function of human attributes through the lens of evolutionary psychology. In considering sexuality and its physical, emotional, and social aspects, he distinguishes between adaptive traits that benefit survival (like masturbation, which keeps sperm fresh and fit for fertilization) and neutral traits (like female ejaculation, which has no discernable reproductive purpose) that may have resulted accidentally, as “by-products” of other adaptations.

In playful, conversational essays adapted from columns originally written for Scientific American and Slate, Bering uses a strategy of “reverse engineering” to hypothesize why things evolved the way they did. His investigations depend on speculative inference and rarely land on definitive conclusions, but are still convincingly argued. It’s impossible to know, for example, why exactly the penis is shaped like a pickle with a pageboy haircut, but Bering cites experiments that demonstrate how the ridge around the penis’s head removes semen if some is already present, decreasing the chances of fertilization by a sexual rival. To supplement that physical evidence, Bering discusses behavioral studies that show couples tend to have sex more if they’ve been recently separated, possibly because there is an “anti-infidelity” function to our urge to copulate. All this suggests that the genetic advantage accorded males with a greater variation in width between head and shaft resulted in the evolution of the form we know today.

Be warned that the picturing of ancestral gangbangs is one of the more painless leaps of imagination that Why is the Penis Shaped Like That? demands. The book is a compendium of topics normally greeted with skittishness or repulsion. Premature ejaculation, autofellatio, the chemical properties of human semen, cannibalism, masturbation, female orgasm, suicidal thoughts, pedophilia, zoophilia, foot fetishism—nothing is taboo. Bering’s goofy sense of humor makes these concepts easier to digest. The book is studded with puns and self-deprecating asides about Bering’s own sex life, among them tales of his own attempts to self-fellate, musings about how simpler life would be if he could love dogs to the extent that he could “dispense with all those emotional encumbrances that come with being attracted to” another human and settle down with “a sassy little bitch,” and the confession that as a gay man, the female orgasm seems “exotic and foreign . . . like decorative basket weaving in a small African village.” The cover, which features wrinkly walnuts in uncomfortably close focus, perfectly illustrates Bering’s irreverent tone. Indeed, part of the fun of reading the book is what happens when you whip it out in public: everyone from my brother to my mother to total strangers grabbed it out of my hands, all giggling like fifth graders in Sex Ed as they flipped through the pages.

Our laughter is very much the point: Bering’s jokes about the things that make us most squeamish invite us to share his joyful curiosity about human sexuality, to see the world through his eyes. Our capacity for this kind of social cognition, which we call empathy, is in Bering’s estimation our most powerful and defining trait. Since we can weigh our own needs and desires against the needs and desires of other beings, empathy acts as both a compliment and counterpoint to our psychobiological urges. Take, for example, the “problem” of premature ejaculation: though research suggests that the “quickie” is an advantage that evolved in order for human males to expel as much semen in as little time as possible, at some point, empathy—in this case, manifested in the desire to please a partner—entered the equation, and now one adaptation seems to modulate another.

Empathy also characterizes Bering’s approach to uncomfortable topics such as sexual violence, pedophilia (the attraction to pre-pubescent children), hebephilia (the attraction to children on the cusp of puberty), asexuality, zoophilia, and sexual fetishes. Throughout the book, Bering argues persuasively for a more open-minded view of the infinite varieties of human sexuality, and calls for a more rigorous scientific understanding of all sexual impulses, regardless of our moral objections to them. That’s not to say that he thinks child molesters should be excused by the fact of their attraction, or that it’s ever okay to have sex with animals. “People don’t have any say whatsoever about what their genitals respond to,” he writes. “But people do have considerably more control over what exactly they do with these genitals.”

In the final section, Bering appeals to our empathy by offering a “‘what-it-feels-like’ phenomenology of ‘being’ suicidal.” Contextualizing suicide within the framework of evolutionary psychology, he suggests that humans, like bees or certain spiders, may be programmed to take ourselves out of the mix if we feel our existence is not a benefit to the species. “Even the darkest of human emotions may be adaptive if they motivate gene-enhancing behavioral decisions,” he writes. Bering’s idealistic hope is that by learning to recognize the conditions that “favor suicide”—such as increased alienation, self-condemnation, and a “narrow, unemotional focus on the present moment”— we will be better equipped to help friends and family showing these symptoms.

The concept of suicide as an “escape from the self” is a strange and sad counterpoint to the book’s primary concern: the shaping of that very self. But perhaps Bering’s strongest argument for the power of empathy is that though suicidal thinking should seem as aberrant as podofillia or zoophilia, it doesn’t. Suffering is something we experience both together and alone. Like humor, it binds us to each other. Our capacity to understand the pain and pleasure of others defines who we are, both as individuals and as a species. As Bering describes it, the complex interplay between biology, psychology, and culture suggests that what makes us most human—empathy—is also what makes us the most complicated beast of all.

Rose Lichter-Marck is a writer and photographer living in Brooklyn.