They found, as Mr. Peter put it in a recent telephone interview, that “when teens watch more porn they tend to be more dissatisfied with their sexual lives. This effect is not really a strong effect, though. And teens with more sexual experience didn’t show this effect at all.”

The pair also found that adolescents who watch more porn than their typical peers are generally less averse to casual sex.

But almost as soon as Mr. Peter was done summarizing results, he started listing caveats. Pornography can never be described as the sole predictor of an attitude about sex, or any behavior; it’s always part of a constellation of predictors. Further, he said, he and Ms. Valkenburg have limited their surveys to Dutch teenagers, and extrapolating from their experience is not a good idea. Even the questions these teenagers get asked are different than the ones asked in the United States. In the Netherlands, Mr. Peter said, they tend to assume teenagers are having sex and focus on whether it’s safe. In the United States, there is more focus on when a teenager starts having sex, and whether that initiation is earlier because of pornography.

“I would be very cautious saying that what we found in the Netherlands is applicable in the U.S.,” Mr. Peter said. “Our findings are in a country that is pretty liberal when it comes to adolescent sexuality.”