The Authors/FAQ

The authors/faq

At Angkor Wat (Cambodia, 2003).

With everyone’s favorite primatologist, Frans de Waal—note his book in Casi’s hand (Barcelona, 2010).


Christopher Ryan, Ph.D.

Christopher received a BA in English and American literature in 1984 and an MA and Ph.D. in psychology from Saybrook University, in San Francisco, CA twenty years later. He spent the intervening decades traveling around the world, living in unexpected places working at very odd jobs (e.g., gutting salmon in Alaska, teaching English to prostitutes in Bangkok and self-defense to land-reform activists in Mexico, managing commercial real-estate in New York’s Diamond District, helping Spanish physicians publish their research). Somewhere along the way, he decided to pursue doctoral studies in psychology. Drawing upon his multi-cultural experience, Christopher’s research focused on trying to distinguish the human from the cultural. His doctoral dissertation analyzes the prehistoric roots of human sexuality, and was guided by the world-renowned psychologist, Stanley Krippner.

Christopher has lectured at the University of Barcelona Medical School, consulted at various hospitals, contributed to publications ranging from 
Behavioral and Brain Sciences (Cambridge University Press) to a textbook used in medical schools and teaching hospitals throughout Spain and Latin America. He’s been featured in major national media, both conventional (e.g., MSNBC, Canada’s CBC-TV, Oprah Radio, CNN, NPR, The Washington PostTimeNewsweekThe AtlanticOutside magazine) and Internet-based (e.g.,,, Big Think, and Andrew Sullivan’s Daily Dish blog over a dozen times). He and his work have also appeared in many international newspapers (e.g., The Times of LondonToronto Globe and Mail, Israel’sHa’aretzThe Sydney Morning HeraldSonntagsBlick) and television (U.S., Spain, Russia, Canada, Australia). 

Christopher contributes to both Psychology Today and Huffington Post.


Cacilda Jethá, M.D.

Cacilda Jethá has an Indian face, a European education, and an African soul. She was born in Mozambique to a family that had immigrated two generations earlier from Goa, India. As a child, she fled civil war to Portugal, where she received most of her education and medical training before returning to Mozambique in the late 1980s. A young physician determined to help heal her country, Cacilda spent seven years as the only physician serving some 50,000 people in a vast rural district in the north of the country. While there, Cacilda also conducted research (funded by the World Health Organization) on the sexual behavior of rural Mozambicans in order to help design more effective AIDS prevention efforts.

After almost a decade in Mozambique, Cacilda returned to Portugal, where she completed her medical residency training in both psychiatry (at the prestigious Hospital de Julio de Matos in Lisbon) and occupational medicine. 

She and Christopher currently reside together in Barcelona, Spain, where she is a practicing psychiatrist at Hospital San Joan de Déu and in private practice. She speaks Portuguese, French, Spanish, Catalán, English, and some rusty Tsonga.


selected bibliography

Ryan, C. and Jethá, C. (2005). Universal human traits: The holy grail of evolutionary psychology (comment). Behavioral and Brain Sciences, 28 (2), 292-293.

Krippner, S. and Ryan, C. (2004). Jinns: On the road to Morocco. 
Dream Network, 23 (2), 34-45.

Ryan, C. (2003). Terror management theory and human sexuality. 
Methods (Annual edition, 2003), pp. 73-96.

Ryan, C. and Krippner, S. (2002, June/July). [Review of Mean genes: From sex to money to food, taking our primal instincts by T. Burnham & J. Phelan]. 
Association for Humanistic Psychology Perspective, pp. 27-29.

Jethá, C. and Ryan, C. (2002). Aspecto psicológicos del dolor. In C. Busquets & M. V. Ribera (Eds.). Unidades de dolor. Realidad hoy, reto para el futuro (Pain units. Today’s reality, tomorrow’s challenge). Barcelona, Spain: Acadèmia de Cièncieds Mèdiques de Catalunya I de Balears.

Krippner, S. and Ryan, C. (2001). Food in the Neolithic. In R.-I. Heinze (Ed.), 
Proceedings of the Nineteenth Annual Conference on the Study of Shamanism and Alternate Modes of Healing(pp. 217-219). Berkeley, CA: Independent Scholars of Asia.

Ryan, C. and Krippner, S. (1999). Dreams as a mirror of change in personal mythology. Dream Network, 18 (4), 32-33, 38 (also included as a chapter in 
SAGA, Volume II, The Best New Writing on Mythology, edited by Jonathan Young).

Krippner, S. and Ryan, C. (1999). Water spirits of the Kariri Xoco Indians of Brazil. In R.-I. Heinze (Ed.), 
Proceeedings of the Fifteenth International Conference on the Study of Shamanism and Alternative Modes of Healing (pp. 53-57). Berkeley, CA: Independent Scholars of Asia.

Krippner, S. and Ryan, C. (1998). Chaotic attractors in myth: Stories told by the Kariri-Xoco Indians. 
Dream Network, 17 (1), 30-33.

Jethá, C. and Falcato, J. (1991). A mulher e as DTS no distrito de Marracuene. Acção SIDA 9, Brochure. (Women and Sexually Transmitted Diseases in the Marracuene district.)



Frequently Asked Questions about Sex at Dawn.

1. Why is long-term sexual monogamy so difficult for many couples?

Several factors conspire to make long-term sexual monogamy difficult for people. As a species, we’ve evolved to be sexually responsive to novelty. From a genetic point of view, the lure of new partners (known to scientists as the Coolidge effect) combined with less responsiveness to the familiar (the Westermarck effect) motivated our ancestors to risk leaving their small hunter/gatherer societies to join other groups, thus avoiding incest and bringing crucial genetic vigor to future generations. 

Another problem is that most people in the West marry because they’re “in love,” which is a temporary, blissfully delusional state we should enjoy, but 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.” 

2. Why do you specify “sexual monogamy?” Isn’t all monogamy sexual?

Biologists distinguish sexual monogamy from social monogamy. As DNA testing has grown cheaper in recent years, we’ve learned that most species formerly classified as “monogamous” (primarily birds) are socially monogamous, but not sexually so. In other words, they form pairs that cooperatively care for that season’s brood of young, but the male may well not be the biological father. Applied to humans, we argue that a more flexible approach to sexual fidelity can increase marital stability and thus lead to greater social and family stability.

3. How can you say that humans are the most sexual species? This makes us sound like animals.

Actually, most animals would consider us the sex maniacs. Almost all animals have sex only for reproduction—just when the female is ovulating. But humans (and our closely related cousins, the bonobos) have sex for an endless list of reasons. We do it for fun, for pleasure, for money, to cement a friendship, for ego gratification, to relax, to seal a business deal or political alliance (think of arranged royal marriages), and yes, sometimes even to make babies. If you consider the ratio of copulations per birth, humans and bonobos are off the charts. Then, if you add all the hours spent fantasizing, remembering, planning, masturbating, porn and soap-opera watching, romance novel reading . . . .

4. Even if you’re right that humans aren’t “naturally” monogamous, we’re conscious beings with free will to decide how we live, so what’s wrong with simply choosing to be monogamous?

Nothing, as long as we fully understand and accept the costs involved in choosing behavior that conflicts with how we evolved. For example, you might happily choose to work the night shift, but the resulting disruption of your circadian clock will increase your risk of cancer, cardio-vascular disease, gastric disorders, and so on no matter how committed you are to your decision. Similarly, we can choose to wear tight corsets, or ill-fitting shoes, or to live on chili-dogs and ice cream, but because all these behaviors run counter to our evolved nature they will cost us over time. Like celibacy, lifelong sexual monogamy is something we can certainly choose, but it should be an informed decision.

5. Maybe monogamy isn’t natural for humans, but what about love?

The capacity for love may be the most “human” thing about us. In fact, anthropologists commonly report long term, stable partnerships between men and women, even in many of the most sexually promiscuous societies we discuss in Sex at Dawn. But our book is about the evolution of human sexuality, not our emotional development. The tendency to confuse love with sex (and vice-versa) leads to immense suffering.

6. So you’re recommending the everyone should have an open marriage or not get married at all?

Definitely not. We’re not recommending anything other than knowledge, introspection, and honesty. In fact, as we say in the book, we’re not really sure what to do with this information ourselves. We hope Sex at Dawn advances the conversation about human sexuality so people can focus more on the realities of what human beings are and a bit less on the religious and cultural mythologies concerning what we should be and should feel. What individuals or couples do with this information (if anything) is up to them.

7. You guys are married, right? How do you handle this issue?

That’s definitely a fair question, but one we’ve decided not to answer publicly. Certainly, our relationship is informed by our research, but the details of our own sex life are nobody’s business but ours.

8. What about the neurochemistry of love? Doesn’t research showing increased levels of neurotransmitters (particularly dopamine) and brain activity in certain regions when people look at photos of someone they love demonstrate that pair bonding is natural for our species?

Possibly, but not likely. These effects are seen when people look at their children, close friends, and siblings as well as their husband/wife, so it’s not clear how this research demonstrates much about one kind of love versus another. Perhaps more important, as we demonstrate in Sex at Dawn, it’s a mistake to assume that sexual exclusivity is a standard part of all pair bonds. In many societies that can legitimately be said to practice marriage, neither male nor female fidelity is expected as part of the deal. The notion that the exchange of female fidelity for male provisioning extends to our origins as a species appears to be little more than a projection of contemporary morality into the distant past—what we call 

9. The second chapter of your book is called “What Darwin Didn’t Know About Sex.” Are you arguing against Darwinian evolution?

No, we are not Darwin bashers, by any means. Darwin passionately believed that good theory comes from good data, which is why he spent most of his life collecting and organizing specimens, observations, and precise measurements. Obviously, contemporary theorists have much more data to work with than what was available a hundred and fifty years ago, so it’s no critique of Darwin’s brilliance to question a few of his assumptions in light of all this new information. He’d demand nothing less.

10. Why do middle-aged men risk so much for flings with younger women?

With the caveat that every situation is different, one factor we think deserves more attention is the role of testosterone (T) in middle-aged men’s eroticism. In their twenties, men’s T levels begin a long decline, often experienced as diminished passion and appetite for life. Suppressed T levels are associated with depression, heart attacks, dementia, and overall mortality rates from 88 to 250 percent higher. One of the few things that can reliably and immediately revive a man’s sagging testosterone is exposure to a new woman. One researcher found that even a brief chat with an attractive woman raised men’s testosterone levels by fourteen percent within minutes. In Sex at Dawn, we suggest that many men may be confusing the hormonal changes triggered by an affair with actual “love,” thus leading them to make ill-advised decisions catastrophic to their families, their marriages, and eventually themselves.

11. Does this explain why many men are afraid of commitment?

A lot of men certainly know from experience that variety is an important element in their sexual response and that a lifetime of monogamy—even with the woman of their dreams—is an intimidating prospect. Whether this represents “fear” or self-knowledge is an open question. This 
short essaysums it up pretty well.

12. What does the human body tell us about our sexual evolution?

The human body is full of information about our ancestors’ sex lives. In Sex at Dawn, we explain how women’s breasts, orgasms and reproductive anatomy echo the same story told by men’s testicles, penises, and seminal chemistry. It’s an X-rated tale of the orgiastic origins of our species.

13. If monogamy isn’t natural, why have I read that marriage is universal among all human societies?

Many anthropologists who have argued that “marriage” is universal haven’t agreed on a clear definition of what they mean by the word. In Sex at Dawn, we discuss societies where so-called “married couples” don’t expect sexual exclusivity, exchanges of property, cohabitation, any difficulty in ending the union, a relationship between extended families, or even a hint of paternal responsibility. Yet anthropologists still insist on calling these relationships “marriage.” 

14. If your thesis is correct, then why do almost all industrialized societies prohibit—at least officially—infidelity?

It’s almost impossible for most of us to appreciate how radically different the social world of our ancestors was from what we experience today. Anthropologists agree that pre-agricultural societies almost universally share a passionate commitment to so-called “fierce egalitarianism.” Because they are nomadic, such people accumulate as little personal property as possible, thus resulting in cultures organized around sharing. Food, shelter, child-care, protection from predators . . . all are scrupulously shared.

With the advent of agriculture just 10,000 years ago (less than 1/20th of our existence as anatomically modern Homo sapiens), personal property became all-important. Families accumulated land, buildings, status, and wealth that they wanted to keep in the family. The only way a man could ensure his paternity was through strictly controlling his wife’s (or wives’) sexual behavior. Thus, female infidelity has been ruthlessly punished for millennia. Most evolutionary psychologists assert that male obsession with controlling female sexual behavior is intrinsic to human nature, but the evidence we present in Sex at Dawn shows it to be a response to economic conditions that arose with farming.

15. When I hear my (heterosexual) neighbors having sex, why is it almost always the woman who is loudest?

Believe it or not, there are scientists who follow primates through the jungle with microphones, collecting data on what’s called “female copulatory vocalization.” What they’ve found is that the females of the more promiscuous species tend to have the loudest, most complex vocalizations. Don’t tell the neighbors!

16. Does human nature lead to war or peace, selfishness or generosity?

Asked this way, this question will never be answered. The nature of human nature is changeability. Is the natural state of H2O solid, liquid, or gas? Context is crucial.

17. Aren’t we much healthier than our ancestors were? After all, they only lived into their thirties.

The widely-accepted idea that a thirty five year-old stone age person was “old” is simply untrue. In Sex at Dawn we show that our prehistoric ancestors typically lived into their fifties, sixties, and even seventies.