You may not disclose personal information, but your online friends and colleagues may do it for you, referring to your school or employer, gender, location and interests.

“Personal privacy is no longer an individual thing,” said Harold Abelson, the computer science professor at M.I.T.

people really can judge you by your friends

Computer scientists and policy experts say that such seemingly innocuous bits of self-revelation can increasingly be collected and reassembled by computers to help create a picture of a person’s identity, sometimes down to the Social Security number.

By examining correlations between various online accounts, the scientists showed that they could identify more than 30 percent of the users of both Twitter, the microblogging service, and Flickr, an online photo-sharing service, even though the accounts had been stripped of identifying information like account names and e-mail addresses.

In a paper published last year, Alessandro Acquisti and Ralph Gross reported that they could accurately predict the full, nine-digit Social Security numbers for 8.5 percent of the people born in the United States between 1989 and 2003 — nearly five million individuals.