In case you hadn't noticed, boys are different from girls. No snickering please. We're not just talking about the viva-la-difference difference.

We're talking about the kind of deep-seated, attitudinal, emotional, behavioral differences that led Jane Austin to allow as how she (or her heroine) sometimes thought men might belong to a different species.

I'm as feminist as the next one, but there have been times when I've suspected the same. Some of us who have had both a boy and a girl baby have seen the differences from birth -- well before any sex-role indoctrination can come into play.

Someone -- a man -- wrote an article once about whether men can have friends. Its thrust was this: "I'm a man. I have no friends. But that's okay, my friends have no friends. They're men too."

Science writer Maggie Scarf quotes that article often. It underscores a thesis she developed about men and women after several years of interviewing several hundred women to find out why some two thirds -- or more -- of the nation's approximately 40 million depressed people are women.

She did not find, as some say, that statistics simply reflect the tendency of women to seek help more readily than men.

She did not find, as has been propounded, that women are more often labeled depressed because of sexual bias. p

What she did find (to the skepticism, even hostility, of some feminists) was that men and women are different. "If you say that," she sighs, "a lot of feminists take it as a political statement. It's not a political statement. It's a public-health issue."

And, in her best-selling book, "Unfinished Business," (Doubleday, $14.95), Maggie Scarf sets out to justify her thesis.

It isn't that men don't get depressed, says Scarf, 48. It's just that their depression is usually over, or at least related to, a career crisis of one sort or another.

When women get depressed, it is almost always over a relationship.

voila !The difference.

Women, found Maggie Scarf, "give over more of themselves to the keeping of others. They generously ante up more of who they are into the keeping of people, where they expect good feedback from it.

"I think that the most fundamental, basic question in terms of self-esteem and self-judgment for a woman is 'Am I lovable?'"

Then: "'Am I loved?' 'Am I valued? 'Am I valuable?' 'Am I worthwhile?'

"And being worthwhile is very involved with those issues of intimate relationships."

Women's depressions tend to cluster around life events, "Separation" events, Scarf found in her interviews with women in mental hospitals, in clinics, with neighbors, friends, colleagues. Once the word got out she was working on women and depression, she was approached by more than she could handle, all eager to talk about their own depressions. And Scarf's own personality emanates sympathy and compassion: the instant-friend. I've always been the one," she concedes, "people call when they need help.

From the interviews, she selected those she felt most typical of the event-oriented depressions, decade by decade from the teens to the 60s. Always with self-esteem dependent on a relationship -- with parent, lover, fetus, child, husband -- in life's unending cycles of coming together and breaking apart.

"So," she says, "we are talking about a series of differentiated life tasks, but they all do have a common, underlying theme, and that is the breaking up of relationships which have either gotten out of date, or sour, or just not working, or somebody has to no longer be a child and can't face it, or somebody is angry at a husband and is afraid to damage the relationship. . ."

Among Scarf's more controversial conclusions: Menopausal depression is unrelated to hormonal imbalance (and therefore estrogen therapy is not indicated).

Menopausal depression, she maintains, isn't as bad as some other cyclically-induced varieties, "but if you want to make women mad, tell them there's no menopausal depression. It's something people are just committed to."

Maggie Scarf does not suggest that men and women are of different species. The differences, she believes are just a matter of genetic bias.

And it's that saber-tooth tiger again; you know, the one that gave us (the now inappropriate) "fight or flight." It gave women a tendency to (a now inappropriate) protective bonding.

"After all," reminds Scarf, "99 percent-plus of human evolution took place in a wild, primitive environment. And no young are more vulnerable than the human. If a mother leaves a newborn infant for minutes (in the wild), she may not have it when she comes back.

"Over the long millenia of our evolutionary history, those mothers who set up intense attachments with offspring have the offspring who survive to reproduce . . . the genetic bias would have solved a real life and death problem.

"So that when you love somebody, you stay close by. . ."