One of the bugbears of our times is PMS, a physical condition that’s used as everything from a punchline to an idiotic reason why women shouldn’t be in charge of something important. Why does PMS exist? Is there any advantage to PMS, one that could have been selected for over the course of our evolutionary history?
A recent paper hypothesizes that the evolutionary advantage of PMS is that it “increased the chance that infertile pair bonds would dissolve, thus improving the reproductive outcomes of women in such partnerships”. Michael Gillings bases this idea around the fact that women don’t get PMS when they’re pregnant, so PMS is a sign of an infertile pairing.
There are three legs of his argument detailed in the paper published in Evolutionary Applications: Genetic variation affects the severity of PMS; PMS is directed preferentially at current partners; and PMS increases the likelihood of changing partners.
He cites studies that show that genetic variation affects the severity of symptoms of PMS, and that these symptoms are hereditary. This variation is normally put down to differing sensibility to the hormones, since “the absolute levels of progesterone, oestrogen and testosterone do not differ between individuals at high or low risk of PMS”. Since this genetic variation is heritable, it could be subject to natural selection.
Gillings cites a number of studies showing that severe PMS has a negative impact on marital relations, which is no surprise, but only one study which shows that it affects family life greater than social, school and occupational life. Specifically, “Functional impairment tended to be highest at home, followed by social, school, and occupational situations.” (source) This could be explained by the tolerance for functional impairment going from highest to lowest in those situations. At work you do your job, even if you’re in pain – at home you leave the dishes for later and just order pizza for dinner, because who’s going to say otherwise.
However, the crux of his argument is that PMS encourages a change of sexual partners, and for this he has little support. Most of his references in this section are of tendency towards risk, whether or not women want to have sex at this point in their cycle, and whether visible signs of menstruation would encourage other males to attempt to mate with a woman (this connection is not only a stretch, but irrelevant to PMS). This section is full of segues and studies only loosely related to what he’s trying to show.
It’s not a strong hypothesis, and ScienceNews interviewed a number of researchers who were generally unimpressed. A very strong point was made by Jane Ussher, a women’s health psychologist at the University of Western Sydney in Australia, who said: “Women are fertile before the premenstrual phase of the cycle…so any function of PMS in terms of repelling males would have no impact on fertility.” She goes on to add that PMS directed at partners is probably a symptom of other issues the woman is unhappy about.
For me, the most compelling argument was made by Mark Elgar, who studies evolutionary biology at the University of Melbourne in Australia. He pointed out that there’s no association between PMS symptoms and how many children a woman has. “In other words,” he explains, “there doesn’t seem to be a very compelling reason to construct an evolutionary explanation of PMS in the first place.”
Speculative hypotheses are integral to the advancement of scientific knowledge, but this seems to go off on an unnecessary side tangent. Hormones and their receptors play important roles in a variety of traits that are under selective pressure, while menstruation is a vitally important aspect of human reproduction. Menstruation can be painful, and pain makes people irritable; menstruation also requires a change in the levels of hormones, which naturally changes their other functions. PMS is just a byproduct of these biological aspects. Gillings – who lectures on human biology and the science of sex – seems to be seeking an evolutionary explanation for a trait where none is required. There is an unfortunately common idea that every single trait must have an evolutionary explanation, and must have conferred some sort of benefit that was selected for. In 1979 Stephen J Gould and Richard Lewontin published a paper (pdf) arguing against this position (which I’ve remembered because it has the fantastic title of: The Spandrels of San Marco and the Panglossian Paradigm). They made the point that many traits are simply byproducts of other things that have been selected for, and convey no inherent advantage themselves. This is probably what PMS, since it doesn’t seem to affect the reproductive capabilities of women.
Also (getting input from someone who actually menstruates), my wife told me that PMS is a largely modern and western phenomenon, and is made worse by low levels of exercise and the use of hygienic products made from synthetic materials.