This came up a while ago and I assumed the idea would die the usual quick and painless death, but the idea seems to be either so fascinating or so irritating to people (mainly in various blog comment sections) that it still twitches and still has a heartbeat, but only as a result of the repeated flogging it is getting.
The research was reported in Science and quickly popularized in a post by Brian Alexander. Please read this review of the tear research and a critique of Alexander’s post by Christie Wilcox. The idea that tears are a mechanism to avoid rape is mainly proffered in comments in various posts. Also, please have a look at this post by Mike the Mad, which is not about tears but about the evolution of rape avoidance mechanisms in general.
Here’s the idea. It appears that a steroid hormone found in human female tears reduces sexual arousal in human males when they are exposed to those tears. Is this a mechanism to stave off rapists?
Let’s start out by giving the idea the best chance we can. Rape matters. Female hominids in a semi-pair bonded or monogamous species who are raped will give birth to young that will not garner the normal level of male parental investment because the male parental unit is not the father. The female that would be raped is better off not being raped for this reason and thus we would expect adaptations to arise to avoid it. Any rap-avoidance mechanism would be strongly selected against. Bla bla bla.
OK, that’s about as good as I can do. Now, rather than making a cogent argument against this hypothesis (which I could do, honest) I’m simply going to list a number of questions I would ask about this idea before deciding anything.
1) Does this work with male tears? Do man-tears make heterosexual men less aroused? If so, then the steroid hormone is just something in tears.
2) Does this work on the females? Do woman-tears make the woman herself less aroused? Does this work with male tears for homosexuals? Do gay men have reduced arousal when their partner cries? This may not mean much, but would also indicate that this is not really an adaptation, because the production and delivery of a hormone is only part of the process. Receptors matter too. If the tears are working in any of these ways to reduce sexual arousal, then that means that the recipient of the putative signal is responding as the hypothesis predicts, inappropriately or non-specifically, and thus, both the hormone delivery and the reception of the hormones is incidental.
3) Do rapists have reduced sexual arousal when exposed to these hormones? I.e, does the adaptation work in this way? (See below.)
4) Does reduction in sexual arousal in rapists lead to a reduction in likelihood of rape? How does this vary across different kinds of rape?
5) Do rapists who get cried at become safer or more dangerous?
6) Does the presence of the hormone vary with who is getting signaled to?
7) Is the tear-born steroid hormone level in the same range across females, or is it tied to each female’s baseline? Hormone amount is not the factor that matters in their function; What matters is receptor site frequency and distribution and relative hormone amount. Hormone signaling within an individual works because hormone production levels and hormone receptor site frequency and distribution co-develop. This is one of the reasons why those who study endocrine systems establish individual baselines and measure hormone levels in relation to those baselines. Using steroid hormones set to internal baselines to signal between organisms would be difficult, though not impossible.
8) Yes, pheromones exist and work, and these limitations can be compensated for, but they remain two-way devices. A female animal may send out a scent to attract a male, and the male may get attracted, but both parties benefit from this communication and the communication serves a common interest. Why would male rapists have a mechanism for getting “turned off” as it were? If rape is of evolutionary significance, it would be trivial, I would think, for male rapists to avoid a “don’t rape me” pheromone interference mechanism (such as simply not developing the receptor sites for it).
9) Why does the effect happen even though the tears used in the study were not from frightened about to be raped women, but rather, women who were watching Bridget Jones’ Diary or something?
The Science study does show an effect. Perhaps something is going on here. The evidence that it is an anti-rape adaptation is very poor, and reason suggests that it is not. The idea that one facies of emotional state (sad/happy/etc) is linked to another (aroused/not aroused/etc) is reasonable, and we may be seeing that here.
One problem is that human tears are a unique human trait. And, human rape is probably unique as well, though forced copulation is not (human rape is so very much more, in a bad way, than that). This makes it difficult to do the sorts of comparative work that helps to parse out the adaptive from the incidental.
Christie, in her post, discusses a more nuanced hypothesis that does involve communication of emotional states in a way that is probably more reasonable given that tears occur in a much wider range of social settings than either the intercourse-related or the rape-related. In fact, to narrow down her idea a bit, tears in infants may be a powerful way of eliciting care-taking behavior. That alone could cause tears in adults to have similar effect for no particular adaptive reason, but rather, as a ride-along and incidental effect. (Christie discusses all of this).
Finally, while rape is important in human affairs, I’m not so sure how important it is in long term evolutionary history. Most tropical and subtropical foragers are relatively monomorphic in body size, and there is little evidence of rape being important. Rape is much more common in so-called “middle range societies” which a lot of modern evolutionary psychologists lump in with foraging societies. They are wrong to do this, and I find it very annoying. So annoying that I’ve started to write about it a few times but my keyboard burst into flames every time. Eventually, I’ll address that issue, but I may have to skin a few colleagues.
Gelstein, S., Yeshurun, Y., Rozenkrantz, L., Shushan, S., Frumin, I., Roth, Y., & Sobel, N. (2011). Human Tears Contain a Chemosignal Science, 331 (6014), 226-230 DOI: 10.1126/science.1198331