Women are more willing to share their resources with strangers and exhibit a cooperative behaviour during and shortly after menstruation, new research suggests.

Previous studies have shown that fluctuating hormone levels change a woman’s social behaviour over the course of the menstrual cycle.

Now psychologists at the Goethe University Frankfurt have discovered that the willingness to share one’s own resources with strangers also fluctuates with hormone levels.

"While we are firmly convinced that the variation in the willingness to share over the course of the cycle is a real and systematic effect, we still have to determine whether or not it is really caused by oestrogen as the present data suggest," said lead author Christine Anderl.

In the course of the two studies involving over 400 German and US women, researchers compared the willingness to cooperate between women in the time during and shortly after menstruation – when the levels of the hormones oestrogen and progesterone are low – and a few days after ovulation, when the oestrogen and progesterone levels are high.

The researchers measured the subjects’ individual willingness to cooperate using a well-established psychological scale, the "Social Value Orientation".

The findings showed that the women were significantly more inclined to share their own resources with a stranger during and shortly after menstruation than they were a few days after ovulation.