Monogamy Is Human

By Robin Hanson · November 23, 2009 11:00 pm · Comments (25) · ·

It easier to maintain pair bonding in larger primate social groups if males can’t easily tell when females are fertile.  In turn, monogamy makes it easier to keep the peace in larger groups.  And since folks in large groups have more uses for big brains, and more resources to pay for them, monogamous social apes should have bigger brains.  So monogamy encouraged by hidden female fertility may have been the key to humans succeeding far beyond other apes.

Why might we think this?  Chimps are humans’ closest living relatives, splitting apart 5-7 million years ago.  The Ardipithecus ramidus proto-humans of 4.4 million years ago were bipeds with a broad diet in woods and grasslands, and with a brain

about the same size as a modern bonobo or female common chimpanzee brain … The less pronounced nature of [their] upper canine teeth … has been used to suggest that the last common ancestor of homonids and African apes was characterized by relatively little aggression between males and between groups.

A recent Science article persuasively elaborated this argument:

Elimination of the [upper canine teeth] in hominids is unique among all higher primates and occurred long before Australopithecus. … Available evidence now suggests [it] was, as is theoretically most likely, a social adaptation … consistent with a strategy of increasingly targeted provisioning. …. Males would benefit from enhanced male-to-male cooperation …. Foraging could be achieved most productively by cooperative male patrols … Provisioning would reduce female-to-female competition … and would improve (or maintain) social cohesion. …

A large brain is not our most unique characteristic. … The combination of [upper canine teeth] elimination, habitual bipedality, and reproductive crypsis (each in itself an extreme rarity) is unique among all known mammals. Conversely, simple brain enlargement is readily explicable in myriad ways.

They plausibly suggest that these three key uniquely human features appeared together over 4 million years ago, leading over time to our uniquely large human social groups and brains, and all else they imply.

If monogamy is this essential to human success, that does make me a bit more concerned about current trends away from monogamy.  Of course hunter-gatherer monogamy may only have been for 4+ year periods, and we are in some ways moving more toward that.  But still, it gives me pause.

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  1. [...] Make the Best Parents Monogamy and Human Success November 29, 2009 Robin Hanson comments on Monogamy: It easier to maintain pair bonding in larger primate social groups if males can’t [...]

    By Monogamy and Human Success « Incredible Wampum November 29, 2009 at 11:10 am

  2. [...] that overt dominance is rare, and long distances make war rare (as 4 million year old fossils suggest). Foragers live in tight quarters and use language to express and enforce social norms on food [...]

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23 Comments

  1. michael vassar
    November 24, 2009 at 2:08 am | Reply

    Doesn’t the previous post tend to contradict this one? Also, aren’t both of those posts… da, da, DAAA… contrarian? How do you go about justifying taking positions that differ as these do from the consensus among academics?

  2. Jake
    November 24, 2009 at 3:04 am | Reply

    Your current trends away from monogamy? Sounds pretty scandalous…

  3. Chris Pine
    November 24, 2009 at 4:23 am | Reply

    Are you sure humans are naturally monogamous?

    Kanazawa talks about this in a number of posts. In short: some great apes are monogamous, but humans are not among them.

  4. Barkley Rosser
    November 24, 2009 at 4:24 am | Reply

    Presumably you are referring to the rising rates of unwed motherhood in saying we are moving away from monogamy? In the Muslim world the trend is towards monogamy, despite Islam allowing for polygamy. I would remind that in many ancient (and not so ancient) societies, polygamy was allowed and even widespread. You are a bit off on your facts in this post.

    • Chris
      November 24, 2009 at 8:29 am | Reply

      Monogamy was the de-facto situation for the vast majority of people. Only the richest could afford multiple wives. Additionally, only men were permitted to have multiple partners in most polygynous societies.

      The important trait is that in nearly all societies (including, for example, polygamous Islam), a man could be reasonably certain that a certain subset of women were having sex only with him and that their children were his.

      I believe it might be the decline of this trait that Robin is concerned about.

  5. Doug S.
    November 24, 2009 at 5:26 am | Reply

    It seems to me that monogamy and non-monogamy are both common human reproductive strategies; we’re both naturally monogamous and naturally non-monogamous. We generally prefer that our partners be exclusive to us, but we would prefer not to have that restriction imposed on ourselves. (Note that this applies to both sexes.) This is a problem, because it is impossible to simultaneously satisfy everyone’s desire for multiple exclusive mates. As a result, monogamy tends to be something that’s negotiated on either the individual or societal level, rather than set in stone by biology. Some people have monogamous relationships, some have half-monogamous relationships, and some have non-monogamous relationships. And some people lie about being monogamous.

  6. AA
    November 24, 2009 at 7:12 am | Reply

    But is it monogamy as such or stable families? In a sense, if every male know which females are his and which are his neighbour’s that would also make it so much easier to maintain peace and produce the claimed adaptations.

    Also, there was a paper I can’t track now that tries to explain the evolution of sex. The approach was to assume that all members of a species are hermaphrodites who can switch between male and female. One of the conclusions is that if a member is surrounded by a lot of females it is best for that member to switch to male but not the other way around. This seems to indicate that polygamy is natural.

    Given that the adaptation you talk about is really about males knowing for sure which females are theirs, I don’t see why we have to jump all the way to making monogamy the primary explanation. This also would fit neatly with your previous observations about paternity.

    Of course, monogamy is a strong result of requiring males to have declared partners. This follows from sex ration at birth and the fact that un-partnered males would cause too much trouble.

    But if this explanation of monogamy is true, then it follows also that monogamy should prevail in societies with almost uniform age distributions. For societies where the age distribution is strongly pyramidal (high growth), polygamy can work fine if the standard is for males to marry younger females (of which there is more than the older males). So, as societies stabilise their population growth monogamy phases out.

    Rosser’s observation about Muslim societies is also easily explained this way. In fact, I’d hypothesize that the trend to monogamy is a middle class construct since these are the strata with stabilised growth and a strong incentive to marry in-group.

    I’ll track that paper on sex evolution (I think I read the manuscript that was submitted to Nature).

    Thanks!

  7. Stuart Armstrong
    November 24, 2009 at 8:08 am | Reply

    If monogamy is this essential to human success, that does make me a bit more concerned about current trends away from monogamy.

    Should be “If monogamy was this essential…” There’s no evidence here that monogamy is a currently useful adaptation. It might well be like our preferences for sugars and fats: deliterious in our current society.

  8. John Seiffer at http://www.BetterCEO.com
    November 24, 2009 at 8:12 am | Reply

    Assuming it’s true that “monogamy makes it easier to keep the peace in larger groups.” I think you could argue that our cultural evolution has changed the dynamics of keeping the peace and even the definition of “larger groups.” To a digression away from monogamy (even if true) might not be as dangerous as it would have been 3 million years ago. That’s the glass half full version. It could also be worse. My point is that’s only one part of the dynamic that is changing.

  9. Justin Martyr
    November 24, 2009 at 9:32 am | Reply

    The primatologist Frans DeWaal makes a similar point in Our Inner Ape which is a running comparison of humans with chimps and bonobos. Chimps do not have disguised fertility so there is heightened male violence and aggression in order to control sexual access to female. Even though chimps generally have to form coalitions to seize power, the alpha male takes the lion’s share of sexual access. Bonobos do have disguised fertility which reduces the incentive for males to fight to become alpha. Females are dominant. But they are not egalitarian. They seek status in the social arena and the alpha females secure the most food for their offspring.

    The problem with bonobos is that disguised paternity also means that males do not have an incentive to contribute to the group. Instead they are fairly indolent and rely on handouts of food from females.

    Promiscuous sexuality is like being a free rider on society. You can maximize your own reproductive success, but to the extent that a society is promiscuous males will tend to disengage from contributing to the cooperative surplus of society and instead invest their time and energy into gaining sexual access to females.

    • anon
      November 24, 2009 at 12:49 pm | Reply

      The problem with bonobos is that disguised paternity also means that males do not have an incentive to contribute to the group.

      On the contrary, the most peculiar thing about (systematically) disguised paternity is all males are genetically incented to cooperate towards the same group goal. It’s a form of higher-level selection–sort of like the worker ants in a colony. You’re not going to see anything like this with chimps or humans.

      • Justin Martyr
        November 24, 2009 at 2:06 pm |

        I understand what you are saying about the group level of selection and in theory it could work. But bonobos, like all promiscuous species, have a high reproductive skew in males. That means that bonobo males generally direct their energies to keeping low status males away from females. Meanwhile low status males direct their energies to sneaking around and finding a female on the sly.

        Ants and other eusocial species have an even higher reproductive skew. But they are true group selectionists with separate reproductive and working castes. Workers in a eusocial species have no option of diverting energy into the reproductive arena. Thus they work hard for the group because all other choices have essentially been permanently removed from the table.

        Monogamy is a different (and more egalitarian) method of accomplishing the same goal. They both have very strong ex ante boundaries of reproductive access. For members of a working caste in a eusocial species the boundary is “you will never reproduce”. For monogamy the boundary is one partner. But either way, if these ex ante boundaries are strong then people will work at the group level rather than becoming status-seeking free riders.

      • TGGP
        November 24, 2009 at 3:03 pm |

        Actually, as haplodiploids (which gives rise to greater relatedness) workers can have male offspring without being fertilized. This is typically done by descendants of a previous queen who aren’t interested in the general welfare of the colony under the new queen.

  10. TGGP
    November 24, 2009 at 11:19 am | Reply

    There are reasons to be skeptical of Lovejoy’s thesis. He’s been pushing for years before this evidence came in.

  11. Zach Kurtz
    November 24, 2009 at 11:30 am | Reply

    Robin – I suggest you take a look at this alternative theory by Paul Bingham. It suggests a much cleaner game theoretic approach to human evolution:

    Social cooperation between non-kin evolved out of violence, specifically from evolving the ability to throw, and kill from a distance.

  12. baconbacon
    November 24, 2009 at 3:08 pm | Reply

    But is it monogamy as such or stable families? In a sense, if every male know which females are his and which are his neighbour’s that would also make it so much easier to maintain peace and produce the claimed adaptations.

    Its many things, but simply having each male know which females are his leads to disaster in a 50/50 male/female population. Take an extreme example- our society has decided that only men with a net worth of > 10 million dollars can reproduce. Women may freely choose any man to mate within this group and every other man works to support this situation. On the surface this seems like a plausible way to select for the traits that make successful individuals- but you get massive gambling addicts in the future. What happens is that any steady, hard working lower class person has a near zero chance of passing on his genes while his crazy, compulsive gambling neighbor could win the lottery/go on a hot run in vegas/win the trifecta and get to pass his genes along. Gambling becomes a far better strategy and it gets passed on into the gene pool.

    That sounds like a bizarre and unrealistic example- but in a 50/50 society polygamy automatically leaves explicit losers in the gene leaving race (its even worse when males can reproduce indefinitely but women have a cut off point). If there are 50 males and 50 females and 1 male has 2 wives then 1 male has to have none. Then that last male has a huge incentive to steal a wife, or murder another man, and those kinds of activities create conflict. If 1 man has 10 wives then there are 9 buys out there with tons of sexual energy and all the genetic incentive to start murdering/stealing wives.

    • AA
      November 24, 2009 at 8:02 pm | Reply

      But I did discuss the dependence on the shape of the demographic distribution! In a 50/50 society monogamy always prevails!

  13. Violet
    November 24, 2009 at 4:30 pm | Reply

    If you are concerned about monogamy and different sexualities than in EEA shouldn’t you be more concerned with contraception? It allows heterosexual sex without babies and thus totally changes the game.

    Also don’t most studies point to a direction of monogamous pairing with lots of cheating. Wouldn’t requiring paternity tests also change the game in “unnatural” ways.

    If the issue is men ending up single, shouldn’t we promote polyamorous relationships or some form of polyandry?

  14. TruePath
    November 24, 2009 at 6:31 pm | Reply

    Uhh, it seems to me this argument is totally backwards.

    The reason monogomy is important on your account is because it reduces the level of competition/violence inside a group. Yet, it seems to be that the reason that (open) non-monogomy is on the rise is largely because the violence has been so effectively controlled.

    If we ever start to slide back to a state where men fight duels because someone stole their girlfriend people will return to greater monogomy.

  15. Jamie_NYC
    November 24, 2009 at 9:42 pm | Reply

    The difference in average sizes of men and women tells us that human species historically was not completely monogamous. The species that are monogamous (“mate for life”), such as wolves, geese etc., have males and females of nearly identical size. The completely polygamous, where the dominant males appropriate ‘harems’, have vastly different sizes – gorillas are an example. Humans fall somewhere in between the two extremes of complete monogamy and the ‘harem’ arrangement.

  16. Shiraz Allidina
    December 3, 2009 at 7:15 pm | Reply

    Bigger brain, smaller jaw, loss of canines – I prefer this guy’s hypothesis.

  17. Elin Whitney-Smith
    October 17, 2010 at 4:56 pm | Reply

    just came upon your blog.

    On monogamy It seems based on the representation of variety in our DNA that male vs. female “effective population size,” suggests that Polygyny was the norm. since that most men throughout human evolution never reproduced and, in strictly genetic terms, had mysteriously vanished without a trace.

    see: http://j.mp/aFpxvg
    http://j.mp/doP6mC
    http://j.mp/btIHjf

    On hunter/gatherer lifestyle see my book in progress being previewed at http://information-revolutions.com especially chapter 1

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