I know this will interest no one.

Porn is a big business. Every year, Americans spend $4 billion on video pornography, which makes the industry larger than the N.F.L., the N.B.A. or Major League Baseball. When you include Internet Web sites, porn networks and pay-per-view movies on cable and satellite, phone sex, and magazines, the porn business is estimated to total between $10 billion and $14 billion annually. As Frank Rich notes, “People spend more money for pornography in America in a year than they do on movie tickets, more than they do on all the performing arts combined.” Sex sites are estimated to account for up to forty percent of all Internet traffic.

But how does it work — in the brain? That is the question science asks.