Starting late last year, Internet service providers in Britain made “family-friendly filters,” which block X-rated websites, the default for customers. Now any account holder who wants to view adult material needs to actively opt in — effectively raising a hand to say, “Bring on the naughty.”

The initiative, which was conceived and very publicly promoted by the government, is intended to prevent what Prime Minister David Cameron called the corrosion of childhood, which, he argued in a speech last year, happens when kids are exposed to pornography at a young age. In the same speech, he seemed to toss teenagers into the group in need of protection, referring to “young people who think it’s normal to send pornographic material as a prelude to dating.”

And here is where the topic starts to get very murky. It turns out that the research suggesting that teenagers and pornography are a hazardous mix is far from definitive. In fact, many of the most comprehensive reports on this subject come to conclusions that amount to “we can’t say for sure” shrugs. One of the most recent is surely known to Mr. Cameron because it was produced by the office of the Children’s Commissioner for England. In May, the commissioner released a report titled “Basically ... porn is everywhere,” which examined 276 research papers on teenagers and pornography.

After sifting through those papers, the report found a link between exposure to pornography and engagement in risky behavior, such as unprotected sex or sex at a young age. But little could be said about that link. Most important, “causal relationships” between pornography and risky behavior “could not be established,” the report concluded. Given the ease with which teenagers can find Internet pornography, it’s no surprise that those engaging in risky behavior have viewed pornography online. Just about every teenager has. So blaming X-rated images for risky behavior may be like concluding that cars are a leading cause of arson, because so many arsonists drive.