Even where (and if) such differences do exist, why should such differences justify sexist oppression? *
Biological determinism is one form of essentialism which has been used to argue for male superiority for all of recorded history: that men are naturally stronger, smarter, more rational and more trustworthy and thus are entitled to rule both politically and domestically. The more science discovers about biology the more this male biological superiority is shown to be utterly without foundation: for any quality measured there is far more variation among the group of all men and among the group of all women than there is on average between individuals of opposite sex.
A common corollary belief is that while men are physically and rationally superior, women are morally superior. At times influential groups of both men and women, both feminists and anti-feminists, have subscribed to this view. It is equally without evidentiary foundation, and has often been used to give women a sense of power in the role of morality enforcer which acts to support the larger social system of male dominance (and which especially excuses the male sexual exploitations of women as due to a baser moral nature which can't be changed, but which "good" women have the duty to "tame").
Masculine and feminine traits have been culturally placed in opposition to each other, and claimed to thus complement each other and result in harmony when men and women are constrained within the accepted sex roles. Masculine roles differ across societies, but are always portrayed as not only different from but also superior to the feminine. Women and men who transgress the boundaries of the accepted sex roles are considered "not real" men/women, and usually denigrated and sometimes abused and punished by outraged defenders of normative sex roles. It is this rigid ghettoising of masculine and feminine, and the assigning of superiority always to the masculine, that feminism challenges.
* Spot-the-strawfeminist: It is often claimed that feminists say there are no differences between men and women, by people who tend to condescendingly point to women's chest area as they "debate". Rubbish - feminists are, on the whole, not blind. What feminists say is that neither the size of the fatty glands on one's pectoral muscles, nor whether one's reproductive organs are innies or outies, are indicators of deeper essential differences, and nor such indicators of sexual dimorphism relevant when discussing rights, equity and sexual egalitarianism.
Winter (Mind the Gap!): Biological Determinism - A Rant
Kathleen Trigiani - Out of the Cave: Exploring Grays Anatomy - a series of essays ripping the veil off the romanticised submission of Venusians in John Gray's odes to essentialism and thus male dominance, the Mars/Venus canon.
Evidence vs Myth:
A classic debunking from Mark Liberman(Language Log): the popular claim is that women utter 20000 words per day compared to men's 7000 (recently resurrected by Louann Brizendine). A survey of linguistic studies show no such evidence - men and women are found to utter roughly equivalent numbers of words and more often as not the men talked more than the women.
Recommended Reading Offline:
Myths of Gender: biological theories about women and men
By Anne Fausto-Sterling 1992 ISBN 0465047920
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