Menstrual synchrony
Strassmann argued menstrual synchrony defined as menstruation overlap should be quite common. For example, the expected difference by chance between two women with 28-day cycles—which is approximately the average length of menstrual cycles of women at the age —is 7 days. Considering that the mean duration of menses is 5 days and the range is 2 to 7 days, the probability of menstruation overlap by chance should be high.
Sexual attraction
Research has also found that menstrual cycles affect sexual behaviour frequency in pre-menopausal women. For example, women who had weekly sexual intercourse with men had menstrual cycles with the average duration of 29 days, while women with less frequent sexual interactions tended to have more extreme cycle lengths.