Cardinal Joseph Rat– zinger's recent letter to Catholic bishops, while

championing women's rights to equality in the workplace, sharply criticizes feminists who seek to advance women's interests by treating men as adversaries and by denying psychological and spiritual differences between the sexes.

But the thousands of books and articles in science and social science that I read in preparation for writing my book Taking Sex Differences Seriously support the main themes of the Vatican letter.

In particular, the Vatican letter emphasizes the crucial importance of feminine love for the well being of children and families. Women are inclined, by nature, to be superior nurturers within families. As the Vatican letter notes, feminist theorists do tend to explain sex differences by reference to culture, and they promote a fluid, polymorphous understanding of sexuality.

Yet many feminists change their minds about these matters upon having a child of their own. For example, when feminist author Naomi Wolf had her first child, she said, “The ways in which the hormones of pregnancy affected me called into question my entire belief system about ‘the social construction of gender.’”

Oxytocin is a peptide that promotes bonding, nurturing and a calm, relaxed emotional state. It is released in large quantities during pregnancy and breastfeeding. Women have more neural receptors for oxytocin than men do and get even more of them during pregnancy. When nursing releases oxytocin in the mother, it is believed that some of the oxytocin reaches the child through the breast milk.

By inducing a mutually plea surable experience for mother and child, oxytocin increases the feeling of mutual attachment.

Infants become familiar with their mothers’ voices and heartbeats in utero, and both calm the babies after birth. A mother is better than a father at distinguishing a cry of pain from one of hunger or of anger. Mothers are also better than fathers at reading body language and other nonverbal signals. Indeed, women in general are quicker and more accurate than men at identifying infant emotions such as joy, interest, sadness, fear, surprise and distress.

Prior experience as a mother or babysitter does not explain these sex differences.

Women are better listeners than men, and the evidence shows that time with mom is also important after infancy. Children's reactions to their parents are often remarkably gendered. For example, lower maternal work hours increase student achievement, but lower paternal work hours decrease it.

The Vatican letter argues that the interrelationship between family and work has, for women, “characteristics different from those in the case of men.” It warns that women who work should not have a schedule that forces them to relinquish their family life or endure “continual stress, with negative consequences for (their) own equilibrium and the harmony of the family.”

The evidence strongly supports the Vatican on these points. Studies of married couples show that employed mothers with minor children feel far more psychological stress than do employed fathers, and this is true even if their husbands are fully supportive of their wives’ careers. The stress of their wives’ work also takes its toll on fathers. Husbands of wives who work long hours are more depressed and less satisfied with their jobs, marriages and life in general. There is no difference in these reactions between men with nontraditional sex role attitudes and other men.

In this country, in families where mothers and fathers try to share child-care equally, both husbands and wives say that the mothers are more emotionally involved with the children and find it harder to concentrate on other tasks when away from them. Even in Sweden, which has made concerted efforts to meld the roles of fathers and mothers, women are far less likely than men to report pleasure at returning to work at the end of their parental leaves.

A father's chemical makeup does not promote nurturing nearly as well as a mother's does. In fact, in the words of one study, there is evidence of “an inverse relationship between free testosterone and nurturance — both within and between the sexes.” Thus, the bonding and nurturing instincts are not as strong in men as in women.

This is the case even though testosterone drops when men have a child, facilitating nurturing. Fathers do not like childcare as much as mothers do. When men do care for children, they are more apt to play than to do less agreeable chores. Mothers do the less agreeable, as well as the agreeable tasks, and yet they, not fathers, report significantly higher satisfaction with parenting.

Interestingly, the Vatican letter suggests that all family members, husbands as well as children, learn to love by seeing how women love within the home. In support of this understanding, three separate studies have concluded that father-son attachments are less secure “when non-maternal care is initiated on a full- or near-full-time basis in the first year (after birth).” This suggests that men's paternal love is strengthened by observing the example of maternal love. In other words, in addition to naturally loving their children, mothers facilitate the entrance of new love into the world.

Steven E. Rhoads has taught public policy at the University of Virginia for more than 30 years. His new book is

Taking Sex Differences Seriously.