Those Cheating Testicles, or Who's Your Baby?

Benjamin Franklin once quipped, "Where there's marriage without love there will be love without marriage." His affairs are well known in American history, however this founding father may have been stating a truth extending to evolutionary history as well.

Christopher Ryan (author of the forthcoming Sex at Dawn) offers some thoughts on the role of novelty in the sex lives of our favorite primate. He suggests that men are drawn to variety in sexual partners while women are drawn to variety in technique:

When researchers decided to look at this issue to develop a Sexual Boredom Scale, they found that for men, sexual boredom was correlated with variety in partners (or lack thereof), while for women, it was more related to variety in activity. In other words, women were more likely to be satisfied by changes in the sexual what, while men (gay or straight) were more likely to respond to a changes in the sexual whom. It's a simple, unavoidable truth almost everyone knows to be true, but few dare to discuss: variety and change are the necessary spice of the sex life of the male of our species.

Ryan goes on to suggest that these attitudes today are the result of our evolutionary legacy. While I'm willing to agree with his basic conclusion about males (the data from biology and anthropology strongly suggest that males are concerned more with quantity than quality) his post overemphasizes male sexuality at the expense of females (though he says more will follow). Afterall, since statistics suggest that women are more likely to be in committed relationships at any one time than men are, if men are seeking additional partners they're more likely to find someone that's already involved.

According to The Janus Report on Sexual Behavior 35 percent of men (1 in 3) reported at least one case of infidelity with their spouse, but 26 percent of women (1 in 4) also did. However, these figures need to be viewed with some skepticism since men are more likely to exaggerate their number of sexual partners while women are more likely to understate them. It could very well be that, for every Fred Astaire swinging away on the dance floor, there's a Ginger Rogers following him step for step.

Just as multiple partners may be a part of male sexual strategy, so too could it be for females. As David Geary writes in Platek and Shackleford's Female Infidelity and Paternal Uncertainty, an average of 10% of children around the world are produced through "extra-pair copulations."

Because men vary in quality . . . and are readily available in multi-male, multi-female communities, women have the opportunity to cuckold their social partner and can sometimes benefit from doing so.

While the evolutionary strategy of males has often been the pursuit of multiple sexual partners, female strategy has often been one of increasing the genetic diversity in their offspring. If they're also able to have that child raised by an unsuspecting partner who isn't the father, or even gain support from multiple fathers, so much the better.

In the comments on Ryan's post many people expressed their outrage that any infidelity could be "justified" by showing it has roots in our evolutionary history. However, considering that the evolutionary proscription against infidelity is, 1) a female having to share her mate's resources with unrelated offspring or, 2) a male investing in offspring that aren't his, such emotional baggage over infidelity doesn't make a whole lot of sense in the age of contraception. Nevertheless, old habits die hard (and ancient instincts especially).

To understand human mating we have to understand the mating system of our species. Chimpanzees and bonobos (who share around 99% of our DNA) have what's referred to as a multimale-multifemale mating system. Females have sex with multiple individuals in their troop and make positive choices about which males they're most interested in. The evolution of sexual jealousy is seen in nascent form in our evolutionary cousins when a low-ranking chimpanzee is caught mating with a female that a higher ranking male is sweet on. The forest isn't at peace for some time afterwards. In bonobos the situation is a little different. Females largely call the shots and have been known to harass males (and other females) for mating with their preferred partners. Throughout the primate world males also compete with each other in what biologists refer to as "sperm competition." Large amounts of ejaculate will be produced in order to "wash out" a previous males' contribution. Chimpanzee and bonobo males are extraordinarily well endowed in the testicle department as a result. A large testicle-to-body size ratio is therefore a strong predictor of a multimale-multifemale mating system.

In contrast to this, gorillas live in a single male-multifemale mating system and the large bodied males have testicles so small that anatomists have reported difficulty in even finding them. This is because there was no selection pressure from the gorilla mating system to produce a large amount of ejaculate (but this doesn't mean that gorilla females always mate exclusively with their "harem leader," with predictably jealous tantrums if the alpha male discovers the tryst). Gibbons are monogamous "lesser" apes and likewise have relatively small testes for their body size. In all of these cases the mating system of the primate species in question can be predicted based on male testicle size.

So this leads to the obvious question: are humans more like chimpanzees and bonobos or more like gibbons and gorillas. Unequivocally, (and as you would expect from the genetic evidence) human testicles are more like chimps and bonobos. In fact, in their analysis of the seminal protein genes SEMG1 and SEMG2 (genes that code for semen coagulation or "mating plugs" found exclusively in multimale-multifemale systems) Sarah Kingan and Steve Dorus found a direct correlation between the average number of sexual partners that females of a species will have and the selection for this gene in males. Humans lie closer to chimpanzees and bonobos at both of these loci and this strongly suggests that humans evolved with a multimale-multifemale mating system.

The anthropological literature also shows this pattern in that 83% of all societies are classified as polygynous (one male marrying multiple females) and that infidelity in both sexes is fairly commonplace. In many societies women adopt a strategy defined as "polyandrous motherhood" in which females will have children by several men and receive support from each. As Miriam Zeitzen writes in her book Polygamy:

While a woman may not have several simultaneous husbands, she may have several simultaneous recognized fathers to her children, thus effectively managing several men at once. Because these men typically have different lineage affiliations, their children have access to different kinship networks, thereby optimizing the woman's and her children's access to resources.

Primatologist Sarah Blaffer Hrdy has done a good deal of work on this very topic and her books The Woman That Never Evolved and Mother Nature are must reads for anyone interested in the evolution of human sexuality.

After looking at the various arguments supporting an ardent strategy for human males Ryan concludes his post asking the following question:

If men evolved over millions of years to seek sexual novelty (thus avoiding genetic stagnation in small groups of foragers in a sparsely populated planet), is it fair to condemn them now for responding to these ancient, evolved appetites?

It's a fair question, but one that needs to be asked both ways. For every man that wants to justify his dalliance with the secretary, he should also ask himself whether he's prepared to help raise the milkman's son?

While it's certainly not the case that all men or women will cheat on their partners, it does appear that sex is the primary cause of a happy or unhappy relationship. According to an ABC News Poll of American sexuality men are more likely to have sex outside of their relationship to fill a sexual need while women will in order to fill an emotional need. Satisfaction in their sex lives is also the best predictor of a happy relationship:

Regression analysis finds that Americans' satisfaction with their sex lives is a significant predictor of their satisfaction with their marriages or committed relationships. . . . While directionality is hard to establish, it seems more plausible that satisfaction with sex fuels satisfaction with marriage than the reverse. That's because more people are very satisfied with their marriages than are very satisfied with their sex lives. If satisfaction with marriage drove satisfaction with sex, this gap would not exist.

While Benjamin Franklin moved to France to spice up his love life, for those who aren't satisfied -- but don't want to lie to their partner -- there is a growing movement of people looking for alternatives to a cheating culture. Books on open marriage and polyamory are regular features at major bookstores across the US and Europe. If indeed multiple partners are the norm for our species, openness and honesty about a person's needs is clearly the best approach. Claims to moral outrage appear to be falling on deaf ears as humans in modern societies are moving closer to the kind of sexuality that existed in our species for millennia; that is, before civilization imposed religious strictures on who was allowed to bonk whom. I'm eagerly looking forward to Ryan's book (co-authored with his wife Cacilda Jethá) where I trust they'll elaborate on the strategies for women in human mating systems.


Kingan, S., Tatar, M., & Rand, D. (2003). Reduced Polymorphism in the Chimpanzee Semen Coagulating Protein, Semenogelin I Journal of Molecular Evolution, 57 (2), 159-169 DOI: 10.1007/s00239-002-2463-0

Dorus, S., Evans, P., Wyckoff, G., Choi, S., & Lahn, B. (2004). Rate of molecular evolution of the seminal protein gene SEMG2 correlates with levels of female promiscuity Nature Genetics, 36 (12), 1326-1329 DOI: 10.1038/ng1471

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I think there will be no easy solution in sight, because we all (but perhaps mostly men?) want for ourselves what we deny of our partners in terms of sex. There are feelings involved that can't be ignored, no matter how many books on open marriage we read. Open and honest would mean that both men and women said (but perhaps for different reasons) "Accept that I have the extramarital affairs that I crave, while you can't have any."

It's hard for me to envision open or polyamorous relationships that are in any sense lasting? It would seem inevitable that often relationships would end as new loves came along. And of course emotional wounds would result. Perhaps this really wouldn't be any different than it is now with peer pressure monogamy, but I guess I'm suggesting that for the human species as a whole we would always gravitate toward monogamy.

What's fascinating to me about Ryan's post, your response, and many thoughts I encounter from like-minded individuals, is the complete lack of engagement with mostly any culturally influenced behaviors over the past few millenia. Sure, you pay a little lip service to culture, but the truly important considerations are evolved behaviors. And when the received cultural practices appear to you to have the slightest hint of religion, of course it must be a case of lemmings following the religious "strictures" imposed upon them by civilization's religious elite? Why such a simplistic analysis of the origin and influences behind monogamy?

@ Korey.

I have to ask why you find it hard to envision a lasting polyamorous relationship. Have you ever tried one?

I have maintained a polyamorous relationship for some time now and it is both lasting and loving. tHere are a lot of definitions to 'polyamorous' so I'll start with mine.

I live with my girlfriend, though to prevent arguing over who gets to use the bed when we bring someone new home we do have separate bedrooms (we usually alternate between which we share on any given night). We both believe that faithfulness in a relationship consists of honesty, not keeping your junk locked away. We are both equally free to pursue not only sex outside the relationship, but other relationships as well.

We talk about our relationships and flings with each other, and I make a point of introducing her to my other girlfriends and she does the same with her boyfriends. We also talk with each other before pursuing a new relationship.

In the beginning I will freely admit there was jealousy. But we talked about it and examined it together. We realized that the jealousy came from our own insecurities, and when we had faced those insecurities, and examined them, they went away, so much dust in the wind. With them went the jealousy. It is not something we experience now (though we sometimes do experience competition, which is a very different and potentially dangerous emotion if not discussed openly).

I find our relationship to be even stronger then I could have imagined.

Part of why its so strong, and why I believe polyamoury works, is it is patently ridiculous to expect one person to fulfill every need you have.

At the risk of sharing too much information, in our relationship I am the more dominant one (sexually at least, every other dimension we are pretty equal). We we break out the silk rope and fluffy handcuffs I',m the one doing the knots. We have tried to reverse this, to let my girlfriend indulge her dominant side, and it just feels wrong, it goes against our dynamic. However she has another boyfriend who she does get to play the dominant with, and I have an other girlfriend who I am submissive in sex too. In this way we get to fulfill ourselves without having to force each other into roles that don't suite us.

I've used sex as an example her because its the one I think most people will understand the easiest, however this give and take carries on into every aspect of our loving and long term relationship.

No I haven't tried it. I'm married now, with no plans for polyamory. So you can interpret my comments given whatever biases I may have, although I'll attempt to be fair for whatever that is worth.

Thanks for sharing. It is interesting to consider. I still suspect that polyamory is more unsustainable long term than monogamy, particularly given that either of you could be drawn to another lover at some point because it's in the very nature of your relationship. But then again your open relationship may be strong enough that you'll continue to be committed to one another because you are after all similar in that you've embarked upon such a relationship. Although your other partners (assuming they are also aware, which it seems they are) would also be amenable to such openness. I'd also suspect there is a risk that the other individuals could very well be jealous and hurt emotionally.

Also if greater numbers of people were polyamarous I would guess that the emotional harms done and resultant instability would be a greater net negative than any harms inflicted by monogamy. However, I think it unlikely that polyamory would ever be wide spread as I think the reasons for monogamy are perhaps more complex (and not entirely problematic) than only religious strictures.

"... particularly given that either of you could be drawn to another lover at some point because it's in the very nature of your relationship."


If that happens, and you love your "old" partner, why not stay at least friends with them... and more?

If one person finds they're happier with another person... well, then they are. Aren't they? Why is denying yourself or your partner higher and deeper loves considering the epitome of "loving someone"?

IT SEEMS TO ME THAT THE DESIRE TO CONTROL WHO YOUR PARTNER MATES WITH is an instinctual, genetic, evolved, and -- yes -- selfish desire. In the case of the male, it's so the offspring are your DNA's, and not another's. In the case of the female, it's so your man doesn't spend his time, energy, and resources on both another female and the other female's offspring.

From a purely genetic evolutionary point of view.

How is this controlling and selfish (but natural) desire any worse than other evolved instincts such as the sexual attraction to other people?

By Christoph (not verified) on 21 Aug 2009 #permalink

* considering = considered

By Christoph (not verified) on 21 Aug 2009 #permalink

So actually DNA testing would limit the benefits of polyamoury for women? Maybe technology IS going to be a driving force in our evolution.

By Jan-Maarten (not verified) on 22 Aug 2009 #permalink

@ Korey

What I think confuses most monogamous people is the idea that love is not a zero-sum game, that by loving one person I do not decrease the amount of love I can share with another. Parents are not required to love one of their children more then the others, why should I be required to love one partner more then the others?

I do not think that polyamoury works for everone, but I'm certain that monogamy also doesn't work for everyone (as evidenced by any and ever study on cheating ever done and its prevalence). What most polyamorous people I know want, and what I want, is to see it become mainstream that people have the choice. Fallen madly in lobe with your spouse to the exclusion of all others? Fine, monogamy for you. Love your partner but still have the urge to play the field sexually, if only for the variety (or because your lover doesn't share your particular sexual fetishes?)? Fine, open relationship. Love multiple people equally? Fine, polyamoury for you.

The key is for people to be more aware of their sexuality and find the lifestyle that suits their needs.

You are also ignoring the possibilty of triads. Groups of three lovers who love each other equally. These can be all male, all female, or two of one and one of the other. In the triad each member loves the other two equally, and they all three share in the strength of that relationship. Remember, in geometry the triangle is the strongest shape.

I have to strongly disagree that if polyamoury were more prevalent it would lead to greater instability. For one thing, there isn't any evidence for it, for another, if you look at rates of cheating and divorce these days, its pretty obvious that monogamy is not stable at all.

@ Christoph

I fully agree that letting someone go to love someone else and remaining friends with them is both more noble and a greater expression of love, but I have to point to my response to Korey, love is not a zero-sum game. In your scenario, at worst, my original relationship gets bumped from primary status to secondary, but I do not lose my lover, I just share her. Perhaps I don't see her quite as often, but when I do she is happier, and that improves the time we spend together. It also gives me more time to focus on my other relationships, which improves them. The fluidity of that is what makes our relationship stronger and lasting.

As for why the selfish desire to keep your mate is worse, as an evolved trait, then sexual attraction is as an evolved trait? It is worse because the selfish desire to keep my mate all to myself decreases her happiness. Any time I prevent someone from doing something for such a pathetic reason as exclusivity I am decreasing their happiness. This makes itr, in my eyes, worse.

In counterpoint, sexual attraction seems to bring me, and those I share it with, mostly joy.

Evolution is not an ethical or moral yard-stick, and should never be looked at as such. We do not derive our moral compass from the Laws of Physics, neither should we derive them from the Laws of Biology.

My entire moral compass is based on happiness. In an action of mine brings me happiness, harms someone, then I should do not it. If it brings me happiness and has no effect on anyone else, I should do it. If it brings me happiness and brings someone else happiness as well, then I should go out of my way to do it.

Whether or not something is an evolved trait has no bearing on whether or not it is moral. Both love and murder are evolved (possibly quite closely), but no one would argue they are equally moral because of that.

Oh how I miss the days when monogamy was the only choice... Yes, I am aware that things weren't any different than now but were only done in a more private manner, but still... I am probably one of the last romantics, or however you may call us, who truly believes in monogamy. In that one person whom you've picked to share the good, the bad, and most importantly - whom you'll make love to exclusively. You say that's selfish, well maybe it is my ego which needs to be fed this way, but I see it from another point of view...

It astounds me that every article that is written about this topic is filled with monkeys. Yes I do know from whom we derived from, but are we really going to use that as a proof for something? What about our hearts? Our souls? What about that one thing that makes us humans and separates us from all the other species? What about their kind of sex? Are we really just having sex for the hell of it? I would like to think that we are bigger than that.

Do you remember your first kiss? I can remember it even now. That anticipation of the rush I was supposed to get, the butterflies flying in my belly and the scene from a favorite romantic movie spinning in my head... Yes, that was something! The kiss alone however (as probably for many other people) wasn't all that... But nevertheless remained a special memory in my mind. The years went by and the kisses soon became something random. Yes, every kiss was special, but they their original worth by the number of persons whom I shared them with.

It was then that I decided never to do the same mistake again. And I didn't. I didn't wait for marriage to sleep with my boyfriend, but I didn't change many sexual partners. And now the sex still remains something special.
I think that's the main point which we ''pro-monogamists'' want to make. With all the other people on the side you loose the specialness of sex with your partner. That's just the way things are - if you take or get too much of something you'll soon get pretty fed up with it.

So I'm still routing for monogamy. I'm routing for the people that are brave enough to make a commitment to that one person, and keeping the fire burning without implicating other people on the side. I do realize that it's not easy, especially since you have about 1h day which is NOT spent on thinking about sex. But I think that sometimes you just have to see that it's worth it. :)

Best wishes for you and your girlfriend(s)! ;)

Oh! Haven't any of you studied the risks of STDs from sex with multiple sexual partners who all also have multiple sexual partners? The bacteria and viruses are thriving, multiplying and diversifying whether or not you care what they may do to you, or your partners, or your offspring.

Ok I too believe im momogamy. I think for the sake of families, for children, for the idea of a soulmate. I don't know. I think when you have so many lovers life is so all about you and your appetites. We know, looking at America, appetites are fickle and are never satisfied. There are a lot of Eastern Philosophies that say that the reason why humans are stuck in the treadmill of misery is because we are so obsessed with ourselves and our own appetites and pleasures. I find it exhausting to try to constantly bring myself pleasure and find it much more rewarding to have very strong, intimate, loyal bonds and to exercise some self control and disipline with my biological appetites. Those of us who value self control are a dying breed though if you look at the health, weight, and relationship problems in our country.

For myself, monogamy is the only "lifestyle". As far as what other people should do or should not do? "To each his own" has always been one of my favorite sayings. I see both positive and negative aspects of each of the above relationship types. There are also variables involved that I do not believe were discussed in the previous posts - children. What is best for children? In a household where lovers of both adult parties come and go, I would think that having children to care for would be among many things, " a stick in your spokes". Parents have a moral obligation to care for their children first and their personal wants and desires last. If you have a child that needs feeding, changing (parenting!} at 2:00 am and both parents are in separate bedrooms with their lovers attending to their sexual "needs", who is taking care of the child? Do you make a schedule? Then the child becomes an intrusion, an inconvenience.My husband and I have three children (and are monogamous) and have never felt our children were an intrusion. Kids need stability,love, security and most of all, a mother and a father that would lay down their life for that child! To expect a child to grow and thrive in a household with a revolving bedroom door and a mother and a father that are "giving" only a portion of themselves would be irresponsible, immoral and abusive! Yes, infidelity, divorce and disgruntled parents are not the best conditions for children either but this is where our separation from the "apes" comes into play. Our superior intelligence should tell us that happiness and emotional fulfillment are achieved not when our own needs and desires are met but when we help to meet the needs and desires of those closest to us, our loved ones. For instance, take gift giving at Christmas time. When we are children getting gifts is the greatest thing on earth! Giving them? not so important to us. This is because we are young, immature and are only interested in our own satisfaction. Yes, we may, along with mom or dad's help give a gift to another but in no way is it as gratifying as receiving. It is not until we mature and realize that giving a gift to someone you care deeply for is the most gratifying of all! This is maturity and superior intelligence- seeing past ourselves will in the long-run be what gives us eternal joy! Not self gratifying sex with every willing ape (oh, I mean person) around.
One day each of us will be old, possibly infirm and in need of basic care on a daily basis and personally, I would rather know that the person caring for me, holding my hand, is my husband, the man I devoted myself to for most of my life. As I will do the same for him if and when it is needed. For the one who does not have this committed partner, aging seems possibly a scarier, more uncertain way to live out your final days. Sex is a fun activity, but unless it is truly meaningful love with your life partner, it is just an activity like baseball, checkers, knitting, you get my drift!

I agree with everything in the above post from Kim, and that for myself, monogamy is the only lifestyle. I live in San Diego where it seems like everyone is a swinger. I find it very disheartening that I can flip through the phone book and come across 20 or so pages for escort services and only a tiny section on 1 page for marriage counselors. Like it's okay to pay to get laid but it's not okay to try and work on the problems in your marriage. The "you must be sexy and have sex with whoever and whatever at whatever cost" culture makes me want to throw up. I am pro-monogamy and pro-self control. Anything else I have to say about this would get ugly.

The people who are pro-monogamy here appear to be so based on idealistic and unrealistic values, particularly relating to "the family". Family is a social construction and as we say over the past century, the ideal "nuclear family" never existed and never will. So perhaps polygymy would be a positive shift? I have never experienced such a relationship so cannot comment, but being pro-monogamy for the sake of family values and soul mates is being pro-monogamy for the wrong (and often deluded) reasons that are spurred on by the Western society's ideals.
Also, pro self-control? Why?

Respect in a relationship is what counts but when people allow people to use them like they are nothing but a sex object how can their be love in that anybody can be a sex object. Love suppose to be caring about someone else's feelings not trying to be selfish with one another theirs no love in that. People do what they think they can get away with and thats the bottom line. Listen polygamy can only bring disease to one another cervical cancer and god knows what else thats why their are so many diseases going around. Monogamy is the best way to be don't use your partner as a sex object listen man if you have a problem with being faithful than don't expect someone to love you for it you are just a selfish human being. Now do you think that their is science about selfishness come on excuses always excuses as to why some cheats guess what women cheat to is their science for that to. The bottom line is some people are selfish and don't care about noone else's feelings but their own. So women please don't let a man use you unless you want to use him back. You know what they say two can play the same game but one can always play it better and find a scientific excuse as to why you want to play.

By Sugar Baby (not verified) on 02 Mar 2010 #permalink