WASHINGTON (AP) — People who use Facebook and Twitter are less likely than others to share their opinions on hot-button issues, even when they are offline, according to a surprising new survey by the Pew Research Center.

The study, done in conjunction with Rutgers University in New Jersey, challenges the view of social media as a vehicle for debate by suggesting that sites like Facebook and Twitter might actually encourage self-censorship. Researchers said they detect what they call the “spiral of silence” phenomenon: Unless people know their audience agrees, they are likely to shy away from discussing anything controversial.