Did Pairbonds Evolve to Be Asymmetrical?

“I know you belong to somebody new, but tonight you belong to me.” Rose and David 

“I wanted love, I needed love. Most of all, most of all.” Auerbach and Carney (1)


Several researchers have attempted to reconstruct the very difficult problem of why pairbonding may have evolved in humans (I wrote about this here, but a decent guess is that this began around 2 million years ago. Maybe). Those reconstructions often come with an addendum, which predict that pair-bonds would not be completely symmetrical. Instead, either males or females would have more of an incentive to initiate or maintain a long-term bond, and this would depend on the reasons that pairbonding evolved in the first place.

For example, here is David Buss, explaining why males are more likely to fall in love first:

“Because love is an emotion tethered to long-term mating; because fecundity and reproductive value is so critical to men in selecting a long-term mate; and because physical appearance provides an abundance of cues to a woman’s fecundity and reproductive value, we can predict that men will experience “love at first sight” more often than women. The empirical evidence supports this prediction.” (Buss, 2006: 69)

Australopithecus afarensis ("Lucy"), out with her date. Source.

Australopithecus afarensis (“Lucy”), out with her date around 3.6 mya. This portrayal of an early pairbond is speculative, but it ‘s probably not accurate. Source: AMNH.

In this scenario, the cognitive-emotional process of falling in love should move quicker for males, while females are more able to bide their time before committing. I’m sure that most people have an opinion on whether men or women are more likely to be indifferent or overly romantic, based on some combination of our personal experiences, gendered stereotyping, and perhaps even some empirical data thrown in. Is this true? If so, how well might this apply today, as opposed to our hypothetical, ancient, heterosexual ancestors? Or, are these predictions too heteronormative and based on overly essentialized notions of gender?

One wrinkle with the above explanation is that it takes a somewhat generic approach, which could, in theory, apply to any pairbonding species. Gray and Garcia (2013: 69) noted that pairbonds in other species run the gamut, from aloof to intimate, with: “all the highs (romantic love?) and lows (divorce, infidelity), and even variation (life-long, short-term) that we see in human mating.” But these also vary among pairbonding species. Incorporating more specific reasons for the evolution of human pairbonds could help us better answer such questions. But that’s not an easy problem to solve.

In his 2012 book “The Science of Love and Betrayal,” Robin Dunbar described four possible scenarios for the evolution of pairbonds across species:

(1) when males try to monopolize females to increase their chances of paternity,

(2) when females solicit help from males to protect young from predators,

(3) when females solicit protection from males for themselves and/or their young from harassment (or worse) from other males, and

(4) when males and females both contribute to raising offspring.

This gun’s for hire. Apparently he just needs a spark.

I won’t rehash all of his arguments here, but Dunbar considered how these might apply to the origins of pairbonding in humans, then lays out problems with each. In his view, scenario number #3 — known as the “hired gun” or body guard hypothesis — is the best possible explanation for the origins of pairbonding in our species. In sum, he wrote:

I am led inexorably to the conclusion that pairbonds evolved in humans to solve a problem of male harassment and infanticide risk, probably under circumstances where there were very large numbers of rival males around, and that this arose initially through females latching onto individual males as hired guns.”(Dunbar, p. 255)

Opie et al (2013) came to a similar conclusion, where infanticide was the best predictor of social monogamy across primate species, generally (Part 9). Dunbar’s conclusion sounds similar to a hypothesis proposed in a 1999 paper from Wrangham and colleagues (though Dunbar did not cite it). They suggested that pairbonds could have been favored as a side effect of the early use of fire for cooking, possibly as far back as 1.9 million years ago. The argument here was that use of cooking in slow, stationary food preparation increased vulnerability to theft by larger individuals (of either sex, but probably by larger males). By pairing with a specific male – the “bodyguard” – females could have received extra protection from theft.

The cooking idea also neatly ties together several other components of human evolution: changes in nutrition allowed by cooking, increases in hominin brain size, and several other changes in hominin sexuality, including reduced sexual dimorphism, how sex became less about reproduction and more about facilitating long-term relationships, and how hominins developed some level of pairbonding while also living in large communities (a unique pattern among primates). Greg Laden, one of the coauthors of the Wrangham paper, gives a nice summary here.

In support of the bodyguard idea, Dunbar pointed to the rather long and shameful record of males around the world engaging in verbal harassment and physical assault of females. Furthermore, he noted, this problem becomes worse when social order disintegrates, such as during times of war, when the possibility of legal punishment is reduced (for example, see Gottschall 2004). He could have added the many awful examples of harassment faced by women online.

Dunbar also noted that in each of the four scenarios, one sex would have greater motivation to maintain the bond. For example, in scenario #1 above, males who were primarily motivated by paternity certainty would tend to be the more possessive sex. Alternatively, in the hired gun hypothesis, it is females who would have stronger motivations to keep a male around as protection from harassment, and Dunbar argues that this is reflected in differences in the ways males and females bond in the present day: 

“Recent work in the psychology of attachment indicates that men, and especially younger men, are more likely to have avoidant or dismissive attachment styles than women… Not surprisingly, they seek less intimacy with partners, and feel less need of close relationships. This is not a peculiarity of our Western culture: a similar tendency for men to be broadly more dismissive-avoidant than women was reported from a large cross-cultural sample of sixty-two different cultural regions. Although there was considerable variation in the level of dismissiveness across cultures, this particularly study found that cultural differences (e.g., in terms of gender inequality or gender stereotyping) explained relatively little of the within- or between-sex differences in relationship dismissiveness.” (Dunbar, p. 254-5)

Here he refers to work by David Schmitt et al (2003), which concluded  that it was “a near universal” (but not completely) that males were more likely than females to be dismissive toward the idea of romantic attachment. Additionally, Dunbar refers to research indicating that women tend to be choosier when searching for a romantic partner and that they feel emotional rejection more strongly than do men, making pairbonding in humans “female biased” (I know what you may be thinking. Hold that thought).

However, the Schmitt paper also found that there were exceptions to the above pattern, and in some countries females tended to be the more dismissive sex (ex. Ukraine, Botswana, Zimbabwe, Ethiopia, Fiji). And even where differences were present, whether it was males or females who were more dismissive, they were only “small in magnitude,” indicating that the sexes were far more similar than not.

Gray and Garcia (2013: 107) reminded us that “There is no essential female or male. Rather, there are degrees of feminine and masculine traits.” We can easily imagine stereotypes of avoidant males and quick-to-attach females, but that overlooks a good deal of variation in sociosexuality and attachment styles among different genders and sexualities. For example, Calzo (2014) found that, contrary to stereotypes, college males were not all universally sold on the idea of uncommitted sex. Rather, “nearly half of the men in the sample abstained from or did not engage in high levels of uncommitted sex.” 

The variation we find might be more consistent with one evolutionary explanation for pairbonding than another (such as the bodyguard idea), but it’s also true that however pairbonds in humans originated, they didn’t have to remain fixed that way. Instead, once pairbonds were in place, they could accumulate new parts and functions over time, including the possibility for biparental care and to increase a male’s confidence in paternity. And then there are added benefits like companionship and romance, which can apply to people of just about any age, sexuality, or reproductive status. Love is flexible.

Finally, it makes sense that males and females should overlap in terms of attitudes toward pairbonding if this was an important aspect of our species’ mating behavior, since a bi-directional bond is certainly more durable than one that is one-sided. It might not be equally strong in both directions at all times (how could it?), and it might not last forever, but if one side should waver even temporarily the bond stands a better chance of enduring if the other side was emotionally invested. The stereotype of the aloof male makes less sense in this light. Even the hired gun may need a love reaction.


(1) I tried to pick a couple of PG-rated songs representing different attachment styles. Not sure that I got the right ones, but it was fun trying to think of them. I’m open to suggestions.


Buss DM 2006. The evolution of love, In RJ Sternberg and K Weis (eds) The New Psychology of Love, pp. 65-86. New Haven: Yale Univ. Link

Calzo JP. 2014. Applying a pattern-centered approach to understanding how attachment, gender beliefs, and homosociality shape college men’s sociosexuality. Journal of Sex Research 51(2):221-33. Link

Dunbar R. 2012. The Science of Love and Betrayal. Faber and Faber. Link

Finnin D. 1993. “Australopithecus afarensis, “Lucy” reconstruction diorama by John Holmes and Ian Tattersall,” AMNH Digital Special Collections, accessed February 12, 2014, http://images.library.amnh.org/digital/items/show/4609.

Gottschall J. 2004. Explaining wartime rape. Journal of Sex Research 41(2):129-36. Link

Gray PB, Garcia JR. 2013. Evolution and Human Sexual Behavior Hardcover.  Link

Opie C, Atkinson QD, Dunbar RIM, Shultz S. 2013. Male infanticide leads to social monogamy in primates. PNAS PNAS 2013 110 (33) 13229-13230. Link

Schmitt DP. 2003. Are men universally more dismissing than women? Gender differences in romantic attachment across 62 cultural regions.  Personal Relationships 10: 307-331. Link

Wrangham RW, Jones JH, Laden G, Pilbeam D, Conklin-Brittain N. 1999. The raw and the stolen. Cooking and the ecology of human origins. Current Anthropology 40(5):567-594. Link


15 thoughts on “Did Pairbonds Evolve to Be Asymmetrical?

  1. By coincidence, Barbara King just wrote about Lucy’s species and our projections of them as being pairbonded. More importantly, she also discussed the recently discovered 800,000 year old footprints discovered in England, and whether this represented a pairbonded nuclear family.

    “At this, my skeptical radar kicked in. We’re often too ready to assign a particular type of social organization to evidence from the past, going further than the evidence really allows.”

    Great stuff from her, as always.


    • I remember when the two papers came out, one researcher — a primatologist named Eduardo Fernandez-Duque — said this: “Many times we forget that this is not math…It’s unlikely that one size will fit all.” I thought this was an important point, that we can find statistical associations explaining monogamy, but I wonder if it overlooks variation among species. Is this the right approach? Could biparental care actually be the primary driver in *some* monogamous species?

      One more thing: Robin Dunbar emphasized that prevention of infanticide wasn’t the only thing a female would be looking for from a ‘bodyguard.’ It would also be protection for herself from aggressive males. But that isn’t included in most models, as far as I know. Try as we may (as we must), it’s hard to squeeze nature into a database.

      • Quite so. There will always be variation we can’t control for! With reference to this particular debate, a letter to the editor and a response are (I believe) currently in prep – so will be interesting to follow the dialogue as the discussion progresses.

  2. Nice post, but of course all the ideas are wrong except ours!

    But seriously, I came to a somewhat different (but not contradictory) conclusion to Wrangham et al even as we were publishing that paper, and that idea has cooked along for a while now. Your post inspires me to maybe write it up as a blog post.

  3. Pingback: Morsels For The Mind – 21/02/2014 › Six Incredible Things Before Breakfast

  4. Pingback: 2014 in Review | Patrick F. Clarkin, Ph.D.

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