if not for the difference in the ratio of testosterone-to-estrogen in males and females, we couldn't explain different reactions when men and women are exposed to the same situation. we can also see behavior changes when men or women take steroids (testosterone).
i read a study recently about women, during ovulation, being exposed to male pheromones (in the form of male sweat, odor masked, and applied under the women's nose). the same questionare before and after the exposure found that the women were more submisive after being exposed to the sweat at the same time the womens' ratio of estrogen increased. THEIR theory was that this behavior, when women are near men's armpits, increase female submisiveness to promote mating.
you can relate this to estrogen levels being high while breastfeeding.
* i can find that study in the 2nd paragraph if anyone is interested. but you should be able to google it.