When a normally sweet-tempered woman starts lashing out or bursting into tears for apparently no reason, there can only be one explanation: she’s “PMSing.”

At least, that’s long been the way that comedians like to explain it. But a team of researchers from the University of Toronto says PMS, or premenstrual syndrome, might be a myth.

That’s not to say that the physical symptoms of menstruation – the cramping, bloating and headaches – aren’t real.

But the researchers say our culture likely over-attributes women’s mood swings to their menstrual cycles.

The team of psychologists and psychiatrists recently conducted a review of the studies on how menstrual cycles affect mood, saying they could find no evidence that women typically become cranky and irritable just before they menstruate.

And it was not for a lack of trying. The U of T team dug through 646 studies on PMS, some dating all the way back to 1806, finding that most of those studies were a mess.

Only 47 met their criteria of properly performed studies. And among the 47, many had different definitions of the menstrual phases and different ways of assessing moods. Even then, only seven found a link between negative mood and the premenstrual phase, while 18 found women were moody both before and during their period.

Another eight could find no evidence of negative mood at any time of the month, and four studies found women tended to be in more negative moods when they weren’t premenstrual.

“Taken together, these studies failed to provide clear evidence in support of the existence of a specific premenstrual negative mood syndrome in the general population,” the authors write in the journal Gender Medicine. “This puzzlingly widespread belief needs challenging, as it perpetuates negative concepts linking female reproduction with negative emotionality.”

The authors acknowledge there is a legitimate mood syndrome called premenstrual dysphoric disorder. PMDD is listed in the Diagnostic and Statistical Manual of Mental Disorders -- the “psychiatric bible” – and has been shown to cause depression, insomnia and other behavioural and physical symptoms in the second half of the menstrual cycle.

But lead author Dr. Sarah Romans tells The Atlantic that the disorder is relatively rare, affecting as little as 1.5 per cent of the female population.

The rest of what we think of as “PMSing” might actually have nothing to do with menstrual periods at all. Instead, she and her team explain, it might come from long-held beliefs that a woman’s reproductive system controls much of her behaviour and mood.

Romans believes that PMS might simply be how society chooses to explain “unladylike” behaviours in women. The term allows us to attribute rage and crankiness to a temporary blip caused by body changes beyond a woman’s control.

She says PMS might be society’s way of excusing such behaviour by implicitly saying: “We'll let you be cranky and bad-tempered now, but just for one or two days. The rest of the time you've got to be like a true woman.”

Romans is seemingly unswayed by society’s belief that PMS plays as big a role in women’s mood: "The idea that any emotionality in women can be firstly attributed to their reproductive function,” she told The Atlantic. “We're skeptical about that.”