Sex At Dawn: Summary, Review & Criticism

sex at dawn book cover

Sex At Dawn proposes a different interpretation of human sexuality than the standard “western ideal” of monogamy and paternal investment. The authors challenge the precept that we are mostly monogamous and that parents care about who their children are.
Instead, suggest the author, we are polyamorous and we are used to living in communes where people don’t care who their children are.

Bullet Summary

  • We are (probably) naturally more polyamorous than monogamous
  • Physically speaking, we are in between polyamory and monogamy
  • Men lose testosterone in monogamous long relationships and crave diversity

Sex At Dawn Summary

About the Authors: Christopher Ryan did not originally have a background in psychology, evolutionary psychology, or any of the like. But he later acquired an M.A. and Ph.D. from an online learning program.
Cacilda Jethá, the other author, is Ryan’s wife and is a doctor (M.D.).


We didn’t descend from apes.
We are apes, say the authors right away.
The standard narrative of sexual evolution goes like this:

  1. Boy meets girl
  2. Assess mate: he looks for youth, fertility, and fidelity; she for wealth, status, health, and MPI
  3. They mate and form a long-term bond
  4. He keeps an eye on her and on other girls to impregnate; she’s wary he doesn’t fall in love with another woman and keeps an eye on genetically superior men to have a quick fling (possibly during ovulation)

The authors say researchers claim to find evidence for that narrative, and it makes sense on paper.
But they say there’s no real evidence and monogamy in the west is more an adaptation to social conditions than our true nature.

Part I: On The Origin Of The Specious

The authors say Charles Darwin was a genius and a gentleman for whom they have endless respect.

But as it’s the case for most gentlemen, he was a bit clueless about women.

There are two theories as to why women copulate outside of fertile days. The standard theory from Fischer says it’s to cement the bonding with a constantly horny male mate.

He wouldn’t have a need to look for other females, and since he wouldn’t know when she’d get pregnant he also has an interest in sticking around.

The other explanation is that it evolved to “seed confusion”.
The woman mating with different males would leave them unable to know who was their children, so nobody would want to kill them but, just in the off chance, take care of them.

In either case, it’s a world where both genders want to confuse and cheat each other.
The authors don’t seem to buy into that.

Part II: Lust In Paradise (Solitary)?

The authors say the human specie is obsessed with sex.

And before the agricultural revolution sex used to be more of a shared thing, and not such a big thing.
There was an abundance of sex.

The institutionalized sharing of resources and sexuality spread and diminishes risks and secures a more peaceful and secure social environment.
This is the case in bonobo societies. The availability of women makes males far less violent.

The authors call into question the obsession that evolutionary psychologists give to being the genetic father of a baby.

They say that if “passing the genes” is the most important thing a parent should care for, then seeing his wife sleeping with his brother should anger him less.

But that’s not the case, they say.
Many societies live caring about everyone’s children. And so it was probably with our ancestors.

Part III: The Way We Weren’t

When we evolved in smaller groups people gave with the expectation of receiving because everyone knew each other.
At least up to 150 people (Malcolm Gladwell, The Tipping Point).

Karl Marx’s error was not that of failing to understand human nature, but to apply to large societies when it actually works best in smaller groups.
And it seems like our hunter-gatherer society was more about sharing.

Then things changed with the end of nomadic life and the agricultural revolution no longer than 10.000 years ago -the day after yesterday in evolutionary terms-.

Agriculture is highly dependent on a number of factors that make famine and scarcity much more likely.
Scarcity made people more selfish and drove them to live in families and create a private property. Men and husbands wanted to make sure they were toiling the fields for their own children and nobody else. And that changed everything. Women became, as Darwin said, coy.

That also changed what women look for.

The authors say women look for wealth not genetically but by adapting to a world where men controlled a disproportionate share of resources.
It wasn’t like that in the hunter-gatherer tribes, where people shared resources -and women-.

My Note:
Of course, women don’t look for money genetically, but they might as well look for help genetically, which translates into resources.
This part here betrays, in my opinion, the liberal-idealistic nature of “Sex at Dawn”.

Now here’s the funny thing: women are more malleable than men.
They adapt more easily to the culture. So their adoption of monogamy was a bit easier than it was for men.

Part IV: Bodies In Motion

Males’ testicles are a way of observing a species’ sexual past.

Bigger testicles mean the male needed to fight bigger sperm wars.
Smaller testes mean the male didn’t have to fight bigger sperm wars, telling us females were mostly monogamous or faithful to one man.

Human testicles are in between those of chimps/bonobos and bigger than polygynous gorillas.
The authors say the glass is half full or half empty depending on how you look at it.
We are in the middle, you could choose which view to heed the most, whether Polyamory or monogamy.

But they then add a twist: the testes’ sizes likely changed in the 10.000 years since the agricultural revolution and our testes shrank since then.

The authors say humans produce lots of spermatozoons, so the only question is if they are competing among themselves or among someone else.
And they advance a few reasons why, they say, humans seem to be wired for competition with other men:

  • Men ejaculate more after they’ve been away from their partner or suspect infidelity
  • Women report stronger thrusting in the above cases
  • Porn videos feature lots of gang bang but little reverse gang bang (ie: stressing competition)
  • Men watching more men with a single woman ejaculate more

The authors then seem to suggest that the standard theory of being a mostly monogamous species doesn’t hold up.
We evolved for polyamory, they seem to suggest.

My Note:
The authors don’t cite any study and “women reporting stronger thrusting” is very unscientific. 

If we also add a penis strongly adapted to sperm competition, the authors suggest we are leaning more toward polygeny.

Differences Among Races

The condom’s width for Asia is 49 mm 52 mm for Europe and 53 mm for Africa.

What it means though is difficult to say since there are so many variables at play, including food and food availability.

My Note:
Other sources I read say there are no primate species in Africa that form pair bonding but there are some in Asia.

This seems to suggest that pair bonding developed later, not earlier.
It would also make sense that humans today are more pair-bonding species and that Africans have a bigger penis for sperm wars.

It would also fit with my veeeery anecdotal evidence that black women are (far) easier than white girls.
Even when you won’t be having sex with black women, they are far warmer and more welcoming upon approaching.

Women’s Vocalizations

The authors say that baboons’ female vocalization communicates the female reproductive state and the status of the male copulating.

If you factor in that giving away your location to potential predators is actually dangerous in the animal kingdom, it convenes that a baboon’s female vocalization is actually a call for other males around to stoke sperm competition.

Vocalization in the animal kingdom is associated with promiscuous mating and not monogamy, say the authors.

And guess what human females do?

Part V: Men Are From Africa, Women Are From Africa

The authors poke a little fun at the book Men Are From Mars, Women Are From Venus, and also attack The Moral Animal when Wright says we seem designed to make each other miserable.

They say that’s not true and we are each a fragment of a greater unity.

Male VS Female Sexuality

Groups of men and women were shown different sexual scenes.

The scenes were either gay erotic, straight erotic, people at the gym, and two primates mating.

They were supposed to report how they felt about it as their blood flows were also measured.

Gay and straight men reported their status in line with their blood flow. Women were turned on by gay and lesbian scenes, but they reported less arousal.

When they watched heterosexual couples they were turned on and reported even higher arousal than their body indicated. They reported nothing to Bonobo on Bonobo action but their body… Kinda liked it too.

Basically, gay and straight men and women react according to their preferences.
But heterosexual women like everything.

The author says that’s exactly what the theory of erotic plasticity suggests. Such as, that we -mostly women- are conditioned to like what society expects them to like.
It’s worth keeping in mind why some women report no interest in sex or difficulties in reaching orgasm.

The Dangerous Pill

Women smelling shirts worn by males tended to prefer men whose Major Histocompatibility Complex best differs from theirs, which makes genetical sense.

Women on the pill though failed to pick the man who best fit her MHC or, worst, picked the one most similar to hers.

Repression and Violence

The authors cite a study saying that cultures repressing sexual behavior, especially in childhood and adolescence are closely related to violence.

Monogamy Drains Testosterone

Testosterone levels go down as the years advance, but it’s not just age, the authors say, it’s monogamy as well.

Married men have lower levels of testosterone compared to single men of the same age.
Fathers of young children have even less.

Affairs raise testosterone instead. Even just talking to an attractive woman raises a man’s testosterone.

Correlation does not necessarily imply causation, as the authors themselves admit.
Albeit the study shows that simply talking to a pretty woman seems to point toward the fact that female novelty raises testosterone levels. 

For men, it seems, sexual monogamy leads to monotony.
That’s why, the authors say, so many marriages end up being low-sex or no-sex marriages.
Novelty itself is the attraction.

Most men don’t understand it until they’re married (without a prenup) and have children and joint properties.

What’s A Couple To Do?

The authors seem to criticize the current trend in the western world that cheating equals “packing your backs because the marriage is over”.

They suggest staying wary of “one size fits all” recommendations but don’t propose much more in the way of solutions.

Except, maybe, understanding better human nature and being more ready to… Understand?

sex at dawn book cover

Real-Life Applications

Don’t Meet Guys On The Pill!
The pill will change your perception of man. Once you go off of it, you might not like your partner as much.

Watch Out For Monogamy
Monogamy is not easy. Especially for men.

Sex At Dawn Criticism

Albeit I enjoyed “Sex at Dawn”, there is also much to criticize here.

The authors mention Rousseau and talk about “socialism working for smaller groups”.
They talk a lot about sharing, sexual freedom, and female libido being repressed.
To me, it feels like they are falling for that idealistic “back then it was great and today all sucks” narrative.
Altogether, it makes me a bit suspicious of progressive, left-wing ideology-driven thinking.

And this idea that bonobos are nice and peaceful isn’t even true. Robert Sapolsky who actually studied bonobos says they are incredibly nasty to each other.

Sex At Dawn indeed received a flood of criticism.
Steven Pinker calls it pseudo-science in a tweet (but then links a terrible book).
And I am personally very very skeptical that fathers used to care less about the paternity of children, as the authors claim. Well, lemme paraphrase: maybe fathers did not care indeed. And that’s why those fathers are not well represented today :).

And I’m really not sure that societies were much more open toward sharing sexual partners.
At the of the day, without looking for evidence and data, which the authors don’t seem to do too deeply, we’re just discussing personal conjectures.
From my point of view, presenting unsubstantiated claims as facts is unethical and manipulative.

For more on the ethics of manipulation, see:

Manipulation: Techniques, Strategies, & Ethics

More criticism:

  • Lots of Inferring

Lots of the authors’ theories are not based on research.

Of course, evolutionary psychology cannot always and fully rely on research, but I wished they underlined that a bit more, which would have reduced their critics as well.

  • Straw Man 

It’s such a human tendency to build an enemy where there’s none.

The authors discuss the “standard narrative” and then “blame” it for covering up the truth with the monogamy ideology.

But there’s no such standard narrative.

It seems to me that mainstream Evolutionary Psychology is quite clear in highlighting that extra-marital affairs have been part and parcel of our past and that monogamous relationships are often short-lived.

Sex at Dawn Review

I enjoyed “Sex at Dawn” when I first read it, and it taught me a lot.

Among the good point, it makes:

  • monogamy is difficult
  • monogamy lowers men’s testosterone
  • we are somewhat physically and emotionally wired for sperm competition
  • heterosexual women are attracted to other women -albeit maybe not always consciously aware of that-

And albeit the book does not make that point, it’s obvious to me that:

  • Humans are highly plastic in their sexual expressions. They can go from monogamy to polyamory to partible paternity

Still, I found “Sex at Dawn” one-sided and, even more so, idealistic. This is not science, this is more like personal opinions and wishful thinking with a smattering of data to make it sound more credible.

Check out the best evolutionary psychology books or Get Sex At Dawn on Amazon

Scroll to Top