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.
Menstrual cycle
The different phases of the menstrual cycle correlate with women's moods. In some cases, hormones released during the menstrual cycle can cause behavioral changes in females; mild to severe mood changes can occur. The menstrual cycle phase and ovarian hormones may contribute to increased empathy in women. The natural shift of hormone levels during the different phases of the menstrual cycle has been studied in conjunction with test scores. When completing empathy exercises, women in the follicular stage of their menstrual cycle performed better than women in their midluteal phase. A significant correlation between progesterone levels and the ability to accurately recognize emotion was found. Performances on emotion recognition tasks were better when women had lower progesterone levels. Women in the follicular stage showed higher emotion recognition accuracy than their midluteal phase counterparts. Women were found to react more to negative stimuli when in midluteal stage over the women in the follicular stage, perhaps indicating more reactivity to social stress during that menstrual cycle phase. Overall, it has been found that women in the follicular phase demonstrated better performance in tasks that contain empathetic traits.