Sound familiar?

Brace yourself for yet another round in America’s perennial teen-pregnancy wars. On Monday, GOP vice presidential nominee Sarah Palin confirmed that her 17-year-old daughter Bristol is five months pregnant. Faster than you could say “condoms,” liberals and conservatives lined up in predictable battle formations. To the liberal camp, of course, the news about Bristol Palin simply confirmed the need for comprehensive sex education in the schools. On the right, meanwhile, Palin’s pregnancy spurred new calls for abstinence-only instruction.

They’re both wrong.

Let’s start with conservatives, and their stubborn demand for abstinence-only education. Last year, an exhaustive five-year study confirmed that kids receiving this instruction are no more likely to delay sexual intercourse than their peers.

But the abstinence-only sex education program still draws $175 million in federal money and untold sums from states and localities. As governor of Alaska, indeed, Sarah Palin supported abstinence-only education and denounced “explicit sex-ed programs” in the schools.

Yet we still don’t have any evidence that these explicit programs work, either. As University of Pennsylvania sociologist Frank Furstenberg confirmed last year, in an exhaustive review of the literature, efforts to prove the effectiveness of comprehensive sex education are “generally unimpressive, to say the least.”

We know that these programs can enhance students’ knowledge about risky sex behaviors and change their attitudes toward these same behaviors. But can sex education actually influence what kids do? As best we can tell, it can’t.

There’s only one point on which both sides seem to agree: Teen pregnancy is a big problem. They differ on their solutions, of course, but everyone seems to believe that pregnancy hurts the life chances of teenage moms and their children.

Again, the data suggest otherwise. As Furstenberg has shown, bearing a child as a teenager doesn’t hurt a woman’s prospects for education, job advancement or marriage. Ditto for her kids, who don’t suffer any measurable consequences from having a teenage mother.

Instead, they suffer for a much more basic reason: They’re poor. About two-thirds of teenage mothers live at or below the poverty line at the time they give birth. The less income and opportunity that you have, the more likely you are to become a teenage parent.

So Americans have it exactly backward. Teen pregnancy doesn’t deprive our kids of life chances; instead, kids who lack those chances are the ones who get pregnant. Why? Nobody knows for sure. But it seems that young women who have a sense of power and confidence in their lives are more likely to use contraception. Impoverished girls often lack that confidence, so they don’t take measures to protect themselves. They are also less likely to have abortions, which are often too expensive or heavily tabooed in poor communities.

And so the war rages, largely untethered by facts. For in the end, this struggle isn’t really about facts at all. It’s about rival views of sex itself. Left-leaning Americans view sex as a normal part of human development, so they want to give adolescents the information that will help them make responsible decisions about it. But social conservatives think sex should be reserved for one population alone: married people. Everyone else should abstain, especially if they’re teenagers.

That helps explain why Sarah Palin – in revealing Bristol’s pregnancy – also announced that her daughter will marry Levi Johnston, the 18-year-old father of Bristol’s unborn baby. To drive the point home, Johnston has joined the Palins at the GOP convention. It’s a family affair, and now he’s a part of it.

The decision won immediate acclaim from conservatives, who regard unwedded childbearing as the greatest plague on the land. And there’s a significant body of research showing that children raised by two parents do better than those in single-parent homes.

But we also know that so-called “shotgun” marriages – that is, unions forged in response to a pregnancy – are heavily prone to divorce. That’s one reason why divorce rates are so much higher in so-called red states, where young people are more likely to marry after conceiving a child.

All things being equal, of course, it’s still best for our teenagers – and for their offspring – to delay parenthood. But all things are not equal, and that’s the whole point here. The hype over teen pregnancy diverts us from the truly serious problem in American society, which is the growing poverty of teenagers themselves. Last year, for example, UNICEF ranked the United States second to last among 21 developed Western nations in child health, safety and material well-being. Changing the teen pregnancy rate won’t change any of that.

So don’t feel sorry for Bristol Palin or her unborn child, who will probably turn out OK. So did Ann Dunham, who bore a son when she was just 18. You’ve probably heard of him: Barack Obama. He seems to have done pretty well, too.

Instead, think about the teen parents who lack the social and material advantages that you do. Remember that in most cases they’re parents because they’re poor, and not the other way around. The more we fight about teen pregnancy, the less we’ll focus upon teen poverty. And that’s bad news for all of us.

Jonathan Zimmerman teaches history and education at New York University. He is the author of “Innocents Abroad: American Teachers in the American Century” (Harvard University Press).

This article appeared on page B – 7 of the San Francisco Chronicle