Mothers And Others (With Benefits)

Understanding the existence and persistence of homosexuality in the face of its apparently reproductive fitness costs has left many evolutionary researchers scratching their heads. Though research into homosexuality has not been left wanting for hypotheses, every known hypothesis to date but one has had several major problems when it comes to accounting for the available data (and making conceptual sense). Some of them lack a developmental story; some fail to account for the twin studies; others posit benefits that just don’t seem to be there. What most of the aforementioned research shares in common, however, is its focus: male homosexuality. Female homosexuality has inspired considerably less hypothesizing, perhaps owing to the assumption, valid or not, that female sexual preferences played less of a role in determining fitness outcomes, relative to men’s. More precisely, physical arousal is required for men in order for their to engage in intercourse, whereas it is not necessarily required for women.

Not that lack of female arousal has ever been an issue for this fine specimen.

A new paper out in Evolutionary Psychology by Kuhle & Radtke (2013) takes a functional stab at attempting to explain some female homosexual behavior. Not the homosexual orientations, mind you; just some of the same-sex behavior. On this point, I would like to note that homosexual behavior isn’t what poses an evolutionary mystery anymore than other, likely nonadaptive behaviors, such as masturbation. The mystery is why an individual would be actively averse to intercourse with members of the opposite sex; their only path to reproduction. Nevertheless, the suggestion that Kuhle & Radtke (2013) put forth is that some female homosexual sexual behavior evolved in order to recruit female alloparent support. An alloparent is an individual who provided support for an infant but is not one of that infant’s parents. A grandmother helping to raise a grandchild, then, would represent a case of alloparenting. On the subject of grandmothers, some have suggested that the reason human females reach menopause so early in their lifespan – relative to other species who go on with the potential to reproduce until right around the point they die – is that grandmother alloparenting, specifically maternal grandmother, was a more valuable resource at the point, relative to direct reproduction. On the whole, alloparenting seems pretty important, so getting a hold of good resources for the task would be adaptive.

The suggestion that women might use same-sex sexual behavior to recruit female alloparental support is good, conceptually, on at least three fronts: first, it pays some mind to what is at least a potential function for a behavior. Most psychological research fails to think about function at all, much less plausible functions, and is all the worse because of it. The second positive part of this hypothesis is that it has some developmental story to go with it, making predictions about what specific events are likely to trigger the proposed adaptation and, to some extent, anyway, why they might. Finally, it is consistent with – or at least not outright falsified by – the existing data, which is more than you can say for almost all the current theories purporting to explain male homosexuality. On these conceptual grounds, I would praise the lesbian-sex-for-alloparenting model. On other grounds, both conceptual and empirical, however, I have very serious reservations.

The first of these reservations comes in form of the source of alloparental investment. While, admittedly, I have no hard data to bear on this point (as my search for information didn’t turn up any results), I would wager it’s a good guess that a substantial share of the world’s alloparental resources come from the mother’s kin: grandparents, cousins, aunts, uncles, siblings, or even other older children. As mentioned previously, some have hypothesized that grandmothers stop reproducing, at least in part, for that end. When alloparenting is coming from the female’s relatives, it’s unlikely that much, if any, sexual behavior, same-sex or otherwise, is involved or required. Genetic relatedness is likely providing a good deal of the motivation for the altruism in these cases, so sex would be fairly unnecessary. That thought brings me neatly to my next point, and it’s one raised briefly by the authors themselves: why would the lesbian sex even be necessary in the first place?

“I’ll help mother your child so hard…”

It’s unclear to me what the same-sex behavior adds to the alloparenting equation here. This concern comes in a number of forms. The first is that it seems adaptations designed for reciprocal altruism would work here just fine: you watch my kids and I’ll watch yours. There are plenty of such relationships between same-sex individuals, regardless of whether they involve childcare or not, and those relationships seem to get on just fine without sex being involved. Sure, sexual encounters might deepen that commitment in some cases, but that’s a fact that needs explaining; not the explanation itself. How we explain it will likely have a bearing on further theoretical analysis. Sex between men and women might deepen that commitment on account of it possibly resulting in conception and all the shared responsibilities that brings. Homosexual intercourse, however, does not carry that conception risk. This means that any deepening of the social connections homosexual intercourse might bring would most likely be a byproduct of the heterosexual counterpart. In much the same way, masturbation probably feels good because the stimulation sexual intercourse provides can be successfully mimicked by one’s hand (or whatever other device the more creative among us make use of). Alternatively, it could be possible that the deepening of an emotional bond between two women as the result of a sexual encounter was directly selected for because of it’s role in recruiting alloparent support, but I don’t find the notion particularly likely.

A quick example should make it clear why: for a woman who currently does not have dependent children, the same-sex encounters don’t seem to offer her any real benefit. Despite this, there are many women who continue to engage in frequent to semi-frequent same-sex sexual behaviors and form deep relationships with other women (who are themselves frequently childless as well). If the deepening of the bond between two women was directly selected for in the case of homosexual sexual behavior due to the benefits that alloparents can bring, such facts would seem to be indicative of very poor design. That is to say we should predict that women without children would be relatively uninterested in homosexual intercourse, and the experience would not deepen their social commitment to their partner. So sure, homosexual intercourse might deepen emotional bonds between the people engaging in it, which might in turn effect how the pair behave towards one another in a number of ways. That effect, however, is likely a byproduct of mechanisms designed for heterosexual intercourse; not something that was directly selected for itself. Kuhle & Radtke (2013) do say that they’re only attempting to explain some homosexual behavior, so perhaps they might grant that some increases in emotional closeness are the byproduct of mechanisms designed for heterosexual intercourse while other increases in closeness are due to selection for alloparental concerns. While possible, such a line of reasoning can set up a scenario where the hits for the theory can be counted as supportive and the misses (such as childless women engaging in same-sex sexual behaviors) dismissed as being the product of some other factor.

On top of that concern, the entire analysis rests on the assumption that women who have engaged in sexual behavior with the mother in question ought to be more likely to provide substantially better alloparental care than women who did not. This seems to be an absolutely vital prediction of the model. Curiously, that prediction is not represented in any of the 14 predictions listed in the paper. The paper also offers no empirical data bearing on this point, so whether homosexual behavior actually causes an increase in alloparental investment is in doubt. Even if we assume this point was confirmed however, it raises another pressing question: if same-sex intercourse raises the probability or quality of alloparental investment, why would we expect, as the authors predict, that women should only adopt this homosexual behavior as a secondary strategy? More precisely, I don’t see any particularly large fitness costs to women when it comes to engaging in same-sex sexual behavior but, under this model, there would be substantial benefits. If the costs to same-sex behavior are low and the benefits high, we should see it all the time, not just when a woman is having trouble finding male investment.

“It’s been real, but men are here now so…we can still be friends?”

On the topic of male investment, the model would also seem to predict that women should be relatively inclined to abandon their female partners for male ones (as, in this theory, women’s sexual interest in other women is triggered by lack of male interest). This is anecdotal, of course, but a fairly-frequent complaint I’ve heard from lesbians or bisexual women currently involved in a relationship with a woman is that men won’t leave them alone. They don’t seem to be wanting for male romantic attention. Now maybe these women are, more or less, universally assessing these men as being unlikely or unable to invest on some level, but I have my doubts as to whether this is the case.

Finally, given these sizable hypothesized benefits and negligible costs, we ought to expect to see women competing with other women frequently in the realm of attracting same-sex sexual interest. Same-sex sexual behavior should be expected to not only be cross-cultural universals, but fairly common as well, in much the same way that same-sex friendship is (as they’re hypothesized to serve much the same function, really). Why same-sex sexual interest would be relatively confined to a minority of the population is entirely unclear to me in terms of what is outlined in the paper. This model also doesn’t deal why any women, let alone the vast majority of them, would appear to feel averse to homosexual intercourse. Such aversions would only cause a woman to lose out the hypothesized alloparental benefits which, if the model is true, ought to have been substantial. Women who were not averse would have had more consistent alloparental support historically, leading to whatever genes made such attractions more likely to spread at the expense of women who eschewed it. Again, such aversions would appear to be evidence of remarkably poor design; if the lesbian-alloparents-with-benefits idea is true, that is…

References: Kuhle BX, & Radtke S (2013). Born both ways: The alloparenting hypothesis for sexual fluidity in women. Evolutionary psychology : an international journal of evolutionary approaches to psychology and behavior, 11 (2), 304-23 PMID: 23563096

7 comments on “Mothers And Others (With Benefits)

  1. On this point, I would like to note that homosexual behavior isn’t what poses an evolutionary mystery anymore than other, likely nonadaptive behaviors, such as masturbation.

    Hmm… but is masturbation really nonadaptive? For sure, it doesn’t lead to babies, so genetically, it is nonproductive.

    In men, for example, masturbation seems an obvious ‘safety valve’ to balance ‘sexual pressure’ while negotiating access to female sexuality. Sex without commitment, given that men, in the traditional role play is the one who needs to commit in order to get ‘real sex’.

    In women, it seems (c.f. Baumeister & Twenge 2002) that there is a power advantage for women in suppressing female sexuality. Masturbation and lesbian sex (as long as it’s covert so that it doesn’t reveal that the woman is more interested in sex than she claims to be) can serve as alternative outlets. Again, sex without commitment, in terms of agreeing to have children with the man who is given the keys to the castle. I realize that this theory has many flaws, but I’m just throwing it out as a sketch of how the phenomena can be described as adaptive. I’m happy to be demonstrated wrong with the help of good references.

    Regarding male homosexuality, there is the Italian theory that the same genes that promote homosexuality in men, seem to increase the number of offspring when present in women.

    The findings represent the best explanation yet for the Darwinian paradox presented by homosexuality: it is a genetic dead-end, yet the trait persists generation after generation.

    “We have finally solved this paradox,” says Andrea Camperio-Ciani of the University of Padua. “The same factor that influences sexual orientation in males promotes higher fecundity in females.”

    • Jesse Marczyk on said:

      “Balancing sexual pressure” might sound plausible until one wonders why such sexual pressure exists in the first place. If that sexual pressure serves some function (such as motivating an individual to seek out sex), a mechanism which diffuses that drive could well cause an individual to behave in ways that aren’t adaptive. Sure, the frustration of not having sex might feel unpleasant, but that feeling unpleasant needs an explanation as well, not just a mechanism to avoid that feeling.

      And I have seen the sexually-antagonistic idea. I reviewed a few times in the past. One of the reasons that explanation isn’t good is that it misses the entire developmental story. More goes into determining sexual orientation than just genes, as with any trait. Further, sexually antagonistic selection would need to explain why homosexuality – the orientation – only really seems to exist in humans and rams, as the story could apply equally as well to any sexually reproducing species. Curiously, it doesn’t seem to.

      • “Balancing sexual pressure” might sound plausible until one wonders why such sexual pressure exists in the first place.

        Uhm… to reproduce?

        An urge to create offspring doesn’t need to be rational in order to be effective from an evolutionary standpoint.

        Or maybe I misunderstood your point?

        And women suppressing their own sexuality does make sense from an evolutionary standpoint as long as it serves to increase the level of commitment from the man in exchange for sexual favors. It works if the male sexual urge is strong enough, which it appears to be.

        • Jesse Marczyk on said:

          Allow me to rephrase with a new example:

          It was recently suggested to me that a possible function for female masturbation might be to loosen the cervical mucus. This prompted my asking why the cervical mucus would need loosening in the first place. The response was that cervical mucus can serve as protection against infectious agents and, further, naturally tends to thin around the time when conception is most likely. So there’s a trade-off there between protecting against infection and fertilization, and natural selection already seems to have done the math, so to speak, weighing the relative costs and benefits of thinning versus thickening. The account of female masturbation serving to thin the mucus, then, seems to be less plausible. It would only serve to unbalance the equation between risk of infection and fertilization, and there’s no guarantee that it unbalances it in a productive way. The initial account of female masturbation failed to fully consider whether the “problem” it was trying to solve was actually a problem at all.

          The same goes for sexual desire. There is an urge to have sex, and sex typically leds to reproduction. On that front, we’re definitely in agreement. Now, presumably, that pressure/desire/urge to have sex is the result of selective forces: there are probably trade-offs between being focused on sex all the time versus taking care of other tasks, like eating, cultivating social status, and so on. This suggests that selection has already been trying to balance these competing pressures via the creation of cognitive mechanisms that regulate them. Individuals that spent all their time pursuing sex and individuals that spent none of their time pursuing sex probably didn’t do so well, on the whole, relative to those who could better balance these competing demands. Now if masturbation dampens this sexual pressure, it raises two questions: (1) does it do so in an adaptive way and (2) if it does, why hasn’t selection already settled on that calculation itself?

          Is that making more sense?

  2. Pingback: La hipótesis aloparental de la homosexualidad femenina | neurociencia neurocultura

  3. Nuclear Wheelchair on said:

    Why are bonobos bisexual? What purpose does it seem to serve?

    • Jesse Marczyk on said:

      Bisexuality isn’t terribly evolutionary problematic, insomuch as it doesn’t lead to a nonadaptive aversions towards otherwise adaptive heterosexual mating. As for why bonobos engage in same-sex behavior – what purpose it serves – I would have to say that I have no idea.