What would be the evolutionnary purpose of PMS (premenstrual symptoms which include bloating, headache, non negociable need for junk food, not wanting to be touched, painful breasts, pimples, wanting to kill your husband for breathing, etc)?
Why does it need an evolutionary purpose?
If it doesn’t hamper our ability to produce a successful new generation, it’s not necessarily going to be selected against.
There is no evolutionary purpose for it. There is no reason to believe that it should or does have an evolutionary purpose.
Well, to be fair, it is not unreasonable to think that a condition that impairs people’s judgement and leads them to behave irrationally and often to alienate the people around them (particularly mating partners) might be expected to have been selected quite strongly against? The question, surely, is why hasn’t evolution eliminated it? Some women seem to suffer from it much less than others, so the requisite variation is there.
Ahem: I have a HUGE spike in my sex drive when my period is coming (in addition to bloating, cramps, tenderness and a non-negotiable need for good chocolate and salty snacks). I got yer next generation right here, you gorgeous thing – c’mere! NOW!
P.S.: AHEM! :o 
Those pesky symptoms do occur at a less than optimal time to fertilize an egg. Any chance that the state of being less receptive to breeding reduces the chances of ectopic pregnancies which can result in death of both fetus and mother in the event of a ruptured tube?
Variation doesn’t imply genetic variation.
So, the first question has to be, is it genetically related?
And some reasonable followups:
Even if it is genetically related, are genetics the primary cause of PMS? If it’s 80% “other” causes (environment, diet, hormonal levels) there’s not going to be a way to select against it by natural selection in any case.
It seems the severity of symptoms can vary from cycle to cycle. So, even if it is genetic in origin, is it really that much of a problem over the course of an entire lifetime for selection purposes?
Also, even if it is genetically related, does it provide enough of a disadvantage to producing a viable next generation of offspring to be selected against compared to those women who don’t suffer from it?
Also, the question seems to presume that mates would have to deal with symptoms alongside the women. At the risk of sounding misogynistic, would men historically be around women long enough for it to be an issue? As opposed to, say, toiling in the fields all day or segregating women around their periods or not being in a monogamous relationship with a woman who suffers from it or simply dealing even more abuse back?
I’ve heard of 2 theories.
One is that when PMS-ing, you refuse your partner’s advances, therefore insuring that there will be no jostling or moving around too much, insuring that the fertilised egg will have more chances of attaching itself correctly. I don’t lile that hypothesis, because doesn’t PMS happen when you are about to have your period, when your body knows you are not pregnant?
The second one makes more sense to me, evolution wise, although it seems a bit weird. When your body senses that you haven’t been fertilised, then you realise that your mate is not prime reproduction material, and so your behavior insures that he will leave you, and then you can be fertisied eventually by some other stud who WILL get you pregnant…
On a similar thought, maybe it discourages sex during a non-fertile time? (Isn’t fertilisation more likely if you concentrate sex during the fertile period, instead of spreading it out evenly?)
Sex during menses leads to a higher risk of STD transmission, so refusing sex may have had an advantage.
I also challenge the base assumption that women, in general, act significantly differently enough while PMSing to be described as being so very different than the rest of the cycle. Some women do suffer a lot, I know, for most I know (including me!), its a minor side ffect of the necessity of fluctuating hormones that doesn’t affect my life that dramatically.
Furthermore, women in times past probably didn’t menstruate nearly as often as women do now. They were probably pregnant or nursing most of the time.
Maybe it encourages women to have as many babies as they possibly can. You can’t get PMS if you are always pregnant.
But I doubt there is an explanation. PMS is simply evidence that we have imperfect bodies.
I think you are vastly overestimating the impact of PMS. For most women it ranges from “nothing at all” to “slightly uncomfortable and a maybe a little irritable.” I think in many cases there is attribution bias- when you are feeling moody during PMS time, you’ll blame it on PMS, but if you are moody during other times it’s just because you are tired or stressed. But for the most part, PMS isn’t really much more than the discomfort and irritability you’d feel after having a poor night’s sleep or other normal everyday stressors. It certainly doesn’t normally “impair judgement” or cause us to “behave irrationally.”
Not to say that some people don’t suffer greatly- when I was younger, I certainly had terrible cramps, but even these were limited to a few hours. Some people have it worse. But that’s pretty much a medical problem if it’s really getting in your way to the point where you are acting crazy. And who knows why we have various medical problems? The human body isn’t a perfect machine.