Philip Morris has adopted the role of good citizen these days. Its Web site brims with information on the dangers of smoking, and it has mounted a campaign of television spots that urge parents, oh so earnestly, to warn their children against smoking. That follows an earlier $100 million campaign warning young people to “Think. Don’t Smoke,” analogous to the “just say no” admonitions against drugs.

All this seems to fly against the economic interests of the company, which presumably depends on a continuing crop of new smokers to replace those who drop out or die from their habit. But in practice, it turns out, these industry-run campaigns are notably ineffective and possibly even a sham. New research shows that the ads aimed at youths had no discernible effect in discouraging smoking and that the ads currently aimed at parents may be counterproductive.

That disturbing insight comes from a study just published in The American Journal of Public Health by respected academic researchers who were supported by the National Cancer Institute, the National Institute on Drug Abuse and the Robert Wood Johnson Foundation. Using sophisticated analytical techniques, the researchers concluded that the ads aimed directly at young people had no beneficial effect, while those aimed at parents were actually harmful to young people apt to see them, especially older teenagers. The greater the teenagers’ potential exposure to the ads, the stronger their intention to smoke and the greater their likelihood of having smoked in the past 30 days.

Just why the costly advertising campaigns produce no health benefits is a rich subject for exploration. The ads are fuzzy-warm, which could actually generate favorable feelings for the tobacco industry and, by extension, its products. And their theme  that adults should tell young people not to smoke mostly because they are young people  is exactly the sort of message that would make many teenagers feel like lighting up. (Trial testimony has made it clear that the goal of Philip Morris’s youth smoking prevention programs is to delay smoking until adulthood, not to discourage it for a lifetime.)