Do you think that "true love" really exists and comes instinctually to us humans — or do you think it's just a cultural construct? Do you think monogamy is natural or, rather, that social norms (and Hollywood happy endings) help keep us brainwashed into thinking it is? Would we all be a lot more at peace with our lives if "free love" or polyamory were considered normal instead of aberrant?

These are just some of the questions raised in a new book called Sex at Dawn: The Prehistoric Origins of Modern Sexuality, written by Christopher Ryan, Ph.D., and Cacilda Jethá, M.D., a psychologist and psychiatrist. Given the complicated state of marriage and sexuality in contemporary American society — where the divorce rate is high, marriages that stay together are often unhappy, and single people are frequently confused about how to act on their sexual desires (ahem) — Ryan and Jethá decided to take a look back at what sexuality used to be like, in the caveman days, in the hopes of helping us understand why the hell we're so confused now. Ryan and I recently discussed their investigation into the evolution of our sexual habits.

WHY MEN HAVE FLINGS

Reading your book, I found your theory about why many middle-aged men risk it all for flings with younger women very interesting. Can you tell us a little about that?

We think the role of testosterone (T) deserves more attention when it comes to the question of middle-aged men's eroticism. In their twenties, men's T levels begin a long decline, often experienced as diminished passion. One of the few things that can reliably revive sagging testosterone is exposure to a new woman; even a brief chat with an attractive female can raise men's testosterone levels by 14 percent, as one study found. In Sex at Dawn, we suggest that many men may be confusing the hormonal rush they feel after being with a new lover with actual "love," leading to foolish decisions that damage their families, their marriages, and eventually themselves.

WHY LONG-TERM FIDELITY IS DIFFICULT

Why is long-term fidelity so difficult for many people, men and women alike? And why does sexual passion fade for a couple, even as their love deepens?

Several factors conspire to make long-term sexual monogamy difficult for people. We evolved to be sexually responsive to novelty. In hunter-gatherer societies, our ancestors were genetically predisposed to be attracted to new and unusual partners because that helped them to avoid incest and to have offspring with greater genetic variety — which helped them to become more fit to survive.

Another problem is that many people in the West marry because they're "in love," which is a temporary, possibly delusional state we should not expect to last forever. As the German poet Goethe put it: "Love is an ideal thing, marriage a real thing. A confusion of the real with the ideal never goes unpunished."

The intense infatuation that makes us want to spend all our time together only exists until we really do spend all our time together. Then, like any hunger satisfied, it gradually dissipates. While married life can be deeply satisfying and uniquely meaningful, it cannot sustain the passion of those first months or years. Many American couples have unrealistic expectations about the longevity of this initial phase and consider its passing a sign of a failed relationship, which is unfair, unrealistic, and unfortunate.

WHY LONG-TERM FIDELITY ISN'T IMPOSSIBLE

Is long-term fidelity "natural?" And does it really matter if it's natural or not? Is fidelity more dependent on individual ethics, values, and decisions rather than any kind of biological imperative?

Long-term sexual monogamy clearly doesn't come "naturally" to our species. But that doesn't mean we can't choose it, as long as we fully understand and accept the costs involved in living in a way that conflicts with how we evolved.

WHY HAVING GOOD SEX WELL INTO OLD AGE IS POSSIBLE

How long into old age does sexual passion continue? What helps it continue into this latter stage of life?

Assuming one is physically and mentally healthy, there's no reason sexual passion can't continue throughout life. An open, flexible mind is probably the key to maintaining a satisfying sex life, at any age. Things change, and a willingness to accept and adapt to these physical and emotional changes is essential to aging with grace and passion.

Readers, I wonder what you think about all this. Do you find it encouraging or disappointing to think that monogamy isn't natural to humans — but that we can be monogamous if we make a conscious choice and a concerted effort to be? (Or do you think that's too glib — and even conscious choices and concerted efforts can be sabotaged by certain biological imperatives and subconscious desires?)

What Do You Think?