Out of deep human compassion? Probably.

But compassion only goes so far unless you do the math. And Bill Gates can do the math.

The mega-billionaire who ran Microsoft knows that treating people with AIDS, making up their lost productivity and caring for their orphaned children will bankrupt the world unless we can get a handle on the spread of the disease. He knows that the key to stopping that spread is to empower women.

And he knows that the Bush administration's insistence on stressing abstinence in global AIDS prevention work, while it can help many, will never reach a great number of those who are the most vulnerable.

In Africa and Asia, AIDS is a rampaging heterosexual disease, no matter how much that confuses the homophobic right in the United States.

There, the spread of the AIDS virus is most likely to be halted by women who can choose who they marry (or at least who they have sex with), demand that their men be faithful (or at least use condoms), and, as we get them invented and distributed, when they can use microbicides - gels or barrier creams that a woman can use before sex and that could destroy the virus.

The Bill and Melinda Gates Foundation is funding, among many other things, research into such chemicals. It is receiving help in that endeavor from another of the world's richest men, Warren Buffett, who can also do the math.

Simple abstinence stops the virus cold, of course. But, as Gates pointed out at this week's 16th International AIDS Conference in Toronto, too many women don't have the power to choose healthy behaviors.

In too many places around the world, women are considered chattel of their husbands or fathers. Or they have to trade their bodies for survival. Or men can rape and otherwise abuse them without fear of retribution.

Says Gates: "We need to put the power to prevent HIV in the hands of women. This is true whether the woman is a faithful married mother of small children or a sex worker trying to scrape out a living in a slum. No matter where she lives or what she does, a woman should never need her partner's permission to save her own life."

That does compute.