A little while ago, a national study authorized by Congress found that abstinence education programs don't work. That gave liberals a chance to feel superior because it turns out that preaching traditional morality to students doesn't change behavior.

But in this realm, nobody has the right to feel smug. American schools are awash in moral instruction ? on sex, multiculturalism, environmental awareness and so on ? and basically none of it works. Sex ed doesn't change behavior. Birth control education doesn't produce measurable results. The fact is, schools are ineffectual when it comes to values education. You can put an adult in front of a classroom or an assembly, and that adult can emit words, but don't expect much impact.

That's because all this is based on a false model of human nature. It's based on the idea that human beings are primarily deciders. If you pour them full of moral maxims, they will be more likely to decide properly when temptation arises. If you pour them full of information about the consequences of risky behavior, they will decide to exercise prudence and forswear unwise decisions.

That's the way we'd like to think we are, but that's not the way we really are, and it's certainly not the way teenagers are. There is no central executive zone in the brain where all information is gathered and decisions are made. There is no little homunculus up there watching reality on a screen and then deciding how to proceed. In fact, the mind is a series of parallel processes and loops, bidding for urgency.

We're not primarily deciders. We're primarily perceivers. The body receives huge amounts of information from the world, and what we primarily do is turn that data into a series of generalizations, stereotypes and theories that we can use to navigate our way through life. Once we've perceived a situation and construed it so that it fits one of the patterns we carry in our memory, we've pretty much rigged how we're going to react, even though we haven't consciously sat down to make a decision.

Construing is deciding.

A boy who grew up in a home where he was emotionally rejected is going to perceive his girlfriend differently than one who grew up in a happier home, even though he might not be able to tell you why or how. Women who grow up in fatherless homes menstruate at an earlier age than those who don't, and surely perceive their love affairs differently as well.

Women who live in neighborhoods with a shortage of men wear more revealing clothing and are in general more promiscuous than women in other neighborhoods. They probably are not conscious of how their behavior has changed, but they've accurately construed their situation (tougher competition for mates) and altered their behavior accordingly.

When a teenage couple is in the backseat of a car about to have sex or not, or unprotected sex or not, they are not autonomous creatures making decisions based on classroom maxims or health risk reports. Their behavior is shaped by the subconscious landscapes of reality that have been implanted since birth.

Did they grow up in homes where they felt emotionally secure? Do they often feel socially excluded? Did they grow up in a neighborhood where promiscuity is considered repulsive? Did they grow up in a sex-drenched environment or an environment in which children are buffered from it? (According to a New Zealand study, firstborns are twice as likely to be virgins at 21 than later-born children.)

In other words, the teenagers in that car won't really be alone. They'll be in there with a whole web of attitudes from friends, family and the world at large. Some teenagers will derive from those shared patterns a sense of subconscious no-go zones. They'll regard activities in that no-go zone the way vegetarians regard meat ? as a taboo, beyond immediate possibility.

Deciding is conscious and individual, but perceiving is subconscious and communal. The teen sex programs that actually work don't focus on the sex. They focus on the environment teens live in. They work on the substratum of perceptions students use to orient themselves in the world. They don't try to lay down universal rules, but apply the particular codes that have power in distinct communities. They understand that changing behavior changes attitudes, not the other way around.

They understand that whether it's in middle school or the Middle East, getting human nature right is really important. We're perceivers first, not deciders.