What you 'like' on Facebook can be revealing

London — Clicking those friendly blue "like" buttons strewn across the Web may be doing more than marking you as a fan of Coca-Cola or Lady Gaga.

It could out you as gay.

It might reveal how you vote.

It might even suggest that you're an unmarried introvert with a high IQ and a weakness for nicotine.

That's the conclusion of a study published Monday in Proceedings of the National Academy of Sciences. Researchers reported analyzing the likes of more than 58,000 American Facebook users to make guesses about their personalities and behavior, and even whether they drank, smoked, or did drugs.

Cambridge University researcher David Stillwell, one of the study's authors, said the results may come as a surprise.

"Your likes may be saying more about you than you realize," he said.

Facebook launched its like button in 2009, and the small thumbs-up symbol has since become ubiquitous on the social network and common across the rest of the Web as well. Facebook said last year that roughly 2.7 billion new likes pour out onto the Internet every day — endorsing everything from pop stars to soda pop. That means an ever-expanding pool of data available to marketers, managers, and just about anyone else interested in users' inner lives, especially those who aren't careful about their privacy settings.

Stillwell and his colleagues scooped up a bucketful of that data in the way that many advertisers do — through apps. Millions of Facebook users have surveyed their own personal traits using applications including a program called myPersonality. Stillwell, as owner of the app, has received revenue from it, but declined to say how much.

The study zeroed in on the 58,466 U.S. test takers who had also volunteered access to their likes.

When researchers crunched the "like" data and compared their results to answers given in the personality test, patterns emerged in nearly every direction.

The study found that Facebook likes were linked to sexual orientation, gender, age, ethnicity, IQ, religion, politics and cigarette, drug, or alcohol use. The likes also mapped to relationship status, number of Facebook friends, as well as half a dozen different personality traits.

Some likes were more revealing than others. Researchers could guess whether users identified themselves as black or white 95 percent of the time. That success rate dropped to a still impressive 88 percent when trying to guess whether a male user was homosexual, and to 85 percent when telling Democrats from Republicans. Identifying drug users was far trickier — researchers got that right only 65 percent of the time, a result scientists describe as poor.

Jennifer Golbeck, a University of Maryland computer scientist who wasn't involved in the study but has done similar work, endorsed its methodology, calling it smart and straightforward and describing its results as "awesome."

But she warned of what the work showed about privacy on Facebook.

"Even if you think you're keeping your information private, we can learn a lot about you." For the unknown number of users whose preferences are public, Stillwell had this advice: Look before you like.

The like button is "quite a seductive thing," he said. "It's all around the Web, it's all around Facebook. And it's so easy."