It's one of the most widespread stereotypes that half the population will face at some point in life: women can't think clearly during menstruation. But scientists have shown in one of the largest studies of its kind that this is nothing more than prejudice.
It's not uncommon for men to be freaked out or even disgusted by periods. Many women aren't that fond of them either because of the cramps, headaches and joint pain they can bring. It's not easy to tease apart what is a sexist idea about menstruation, and what is actually happening to women's bodies each month.
Several hormones fluctuate during menstruation: oestrogen, progesterone and testosterone levels change and they can feel like they're going haywire. For some women this is linked to heightened levels of depression and anxiety and other changes in mood.
But the ability to think clearly is something else entirely. Women may often experience having their opinions dismissed if they are thought to be on their period, but their cognitive ability doesn't change a bit from the rest of their monthly cycle, according to a study in Frontiers in Behavioural Neuroscience.
A total of 68 women in Germany and Switzerland were tracked for two months, taking cognitive tests at different stages of their monthly cycle. Over the two menstrual periods this included, the women's working memory, cognitive bias and ability to focus on two things at once were no different.
Exactly where the myth came from is likely to be a mixture of prejudice and poor science. Study author Brigitte Leeners of University Hospital Zurich found that previous research on the topic was riddled with scientific fallacies and biases in its methods.
Many studies were based on very small numbers of women, which give unreliable results. Others take only two measurements of women over time, rather than several. Individual differences between women are rarely taken into account.
But a rigorous study with a larger number of women and multiple measurements throughout the menstrual cycle has gone a long way towards putting those problems to rest.
"The hormonal changes related to the menstrual cycle do not show any association with cognitive performance," said Leeners.
"Although there might be individual exceptions, women's cognitive performance is in general not disturbed by hormonal changes occurring with the menstrual cycle."