Post edited (5/31/13) See below.
As promised, here is my post on this fascinating, and rather mysterious topic.
Who doesn’t love bisexual women? Before my foray into the HBD world, I thought that that was a trick question. In any case, I’m certainly quite fond of them. 🙂
But bisexuality, or for that matter lesbianism—like homosexuality in men—doesn’t seem, at first glance, to make sense evolutionarily.
One might imagine that attraction to both sexes confers no additional benefit, and would not have been selected for. And for this reason, like male same-sex attraction (both strict homosexuality and bisexuality, hereafter abbreviated SSA), it is suspected to not be evolutionary at all.
However, all the evidence shows that female SSA (both lesbianism and bisexuality) is quite heritable—indeed, more reliably heritable that male SSA (at 0.33 vs 0.22, respectively).
But, at the same time, confusingly, its prevalence seems vary widely in time, as this NY Times chart shows. Indeed, recent info indicates that female bisexuality is now incredibly common, as now as much as 15-20% of women identify as being non-heterosexual, the vast majority of those being bisexual.
Several others have looked at this phenomenon, including, recently, Peter Frost and Jason Malloy. They found the same rather sharp increase in women reporting having sex with women from generation to generation.
Interestingly, Jason Malloy also found that, at least among the U.S. ethnicities, there seems to be a negative correlation between ancestral latitude and the incidence of homosexuality in both sexes. This means that non-Whites are considerably more likely to identify as LGBT, at least in the U.S. However, I doubt that these numbers are truly indicative of the true global distribution of SSA, due to the unreliability of self-reported ethnicity, the unrepresentativeness of Americans of their ancestral peoples, and subsequent evolution in the States.
Since non-heterosexuality is still frowned upon (depending on who’s doing the frowning—or smiling), the true numbers of same-sex orientation may be even higher than noted. The rapid increase in female bisexuality could not be due to evolution. The trait cannot be that much more prevalent now than it was just a generation ago, because evolution does not occur that quickly. It is a phenotypic rather than genotypic change.
What must be occurring is that the proportion of women who are genetically potentially bisexual is quite high. Perhaps permissive attitudes towards homosexual sex has encouraged more women to embrace their SSA.
Interestingly, as noted in the earlier posts by myself and Dr. Frost, the same doesn’t appear to be true for homosexuality in men. The proportions there have remained constant or in fact have declined, possibly for the reasons I’ve stated.
So why the high prevalence of female bisexuality? Its high prevalence suggests rather strongly that, quite unlike male SSA—female SSA was selected for; or at the very least, that it is a side effect of some other trait that was selected for—a side effect which was itself not selected against.
This other trait may have something to do with the very nature of female sexual arousal, which itself is qualitatively quite different from and far more complex than the comparatively straightforward, black-and-white nature of male sexual arousal.
One clue to this is the fact that there is a strong positive correlation between a woman’s sex drive and her chances of being bisexual. Bisexual women (but not lesbians) have considerably higher sex drives, on average, than heterosexual women (at least, among Whites).
Women have been discovered to be physically aroused (by measuring vaginal responses, as similarly done with men) to a wide range of stimuli, as are men. However, these women do not report being mentally aroused anywhere near as often as their superficial physical responses might indicate. Apparently, in the female psyche, unlike that of men, there is a higher barrier between physical and mental arousal. Physically, most women are found to have a bisexual arousal pattern, but this does not translate emotionally. Hence, most—and perhaps almost all women have the physiological potential to be bisexual, but it may take a special set of conditions for that to translate into actual sexual desire.
But now, imagine what might happen as sex drive increases: that barrier between physical and mental arousal might be easier to surmount, and may activate the mental responses to attraction to women within women. Yes, for high-sex drive women, the “dormant” circuitry for attraction to women may become active, and manifest itself as bisexual behavior.
The link between female bisexuality and sex drive might go a long way towards explaining why most bisexual women seem to ultimately end up with men, as all of the above women have. Since sex drive peaks in young adulthood, SSA among women is likely to be highest at this point, but would slowly tend to subside with age, leaving sexual attraction to men as the primary influence.
This would explain why the trait would, at the very least, not be selected against, as it ultimately didn’t (at least in the pre-modern world) detract from a woman’s primary goal (procreation).
However, it is important to note that female bisexuality has been found to be a true sexual orientation, stable throughout life, and not just a passing phase that is abandoned after college, as some have suggested.
Male arousal does not exhibit this duplicitous pattern as sex drive increases—that is, men are either straight, gay, or bisexual. The prevalence of SSA does not appear to be correlated with sex drive. Hence, this may explain the relative stability of the numbers of non-heterosexual men: there simply isn’t a large portion of latent bisexual men who may be swayed one way or another by changes in environmental factors.
Another way of looking at the above hypothesis is like this: evolutionarily, attraction to the female form would be selected for (in that, the most sexually aggressive men would have, for most of human history, in most cultural settings, left the most descendents). Perhaps nature has placed such a premium on male sexual desire that the daughters of these men may have been left with some residual sexual desire for women. This type of desire, provided it didn’t interfere with attraction to men, would not be selected out (since, really, female sex drive is less important to reproduction, so long as a willing mate is present).
But, is the absence of negative selection the whole story? Could female bisexuality have in fact undergone positive selection?
Consider that most bisexual women, despite maintaining SSA throughout life, tend to ultimately end up with men. And, of course, let’s not forget to consider that most men (those who don’t lie) are massively aroused by girl-girl sex (myself included), as the enormous popularity of lesbian porn attests. Perhaps then female bisexuality is so common because it serves some function. Perhaps the fact that men like (love) it is a clue.
The male attraction to female bisexuality could be simply just what it seems to be: that it makes polygyny easier. Obviously a woman who is sexually interested in other women can supply a man with opportunities to have additional women for himself, something that, evolutionarily speaking, is an opportunity men would be selected to seize (more or less).
But if so be the case, what do the women get out of this? The answer is simple: men. As Peter Frost has discussed, female-female competition has been important part of human evolutionary history, more so for some groups than others. Anything that gave a woman a leg up against her rivals (and didn’t put the woman at too much of a disadvantage in the process) would have been selected for. Indeed, this may have helped less attractive and otherwise less desirable women gain a leg up on their competition: bisexual women may have been able to grant a man the promise of greater quantity of mates even if this came at the cost of quality somewhat. As with polygyny in general, the cost of potentially having to share a man with girlfriends may have been offset by the ability to snag a higher quality man (additionally, she has the ability to influence who those other “wives” are). This may be evidenced by the fact that bisexual women have had a greater number of male partners than straight women. For women, bisexuality may be all about getting men.
(Of course, the above would predict that bisexual women are less attractive, on average, than heterosexual women. I know of no data either way on that point.)
Indeed, there is evidence that bisexual women are more orgasmic, fantasize more about sex, and are open to more sexual experimentation even with men. This is consistent with their higher sex drives. There is even some evidence that suggests that bisexual women enjoy stabler marriages than their heterosexual counterparts (possibly because male infidelity is less of an issue). Female bisexuality, if it is indeed an attraction strategy, may however lean somewhat towards the r-side of the r/K spectrum (as high sex drive does in general). Either that, or it may be what I call an “artist” strategy (or more technically, an evolutionary stable strategy), which, like mild forms of certain mental disorders (which will be discussed in a future post), is successful, but only so much so, because it may confer wild success to some but marginal success to others, leaving the trait a smallish (but steady) minority in the gene pool.
I do believe that it is likely that female bisexuality was, at the very least, not selected against—at least not on average. If it was in fact selected for, this may have been only on the average. This is would because of the hit-and-miss nature of an r-strategy in general—or because of the hit-and-miss nature of “arty” strategies. In this case, because encouraging your husband to practice de facto polygyny would only be effective if he can either afford to invest in his extra “wives” (difficult in most societies outside of tropical Africa), or at the very least doesn’t desist from investing in you and your children to go off with his extra girlfriends. I’m sure time and more research will clear the picture up one way or another.
Edit, 5/31/13: Also see the “Alloparenting Hypothesis” by Barry X. Kuhle: