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 analysing the likes of more than 58,000 American Facebook users to make guesses about their personalities and behaviour, 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 realise," 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 on to 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 programme 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 United States 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 per cent of the time. That success rate dropped to a still impressive 88 per cent when trying to guess whether a male user was homosexual, and to 85 per cent when telling Democrats from Republicans. Identifying drug users was far trickier - researchers got that right only 65 per cent of the time, a result scientists generally describe as poor. Predicting whether a user was respectively a child of divorce was even dicier. With a 60 per cent success rate, researchers were doing just slightly better than random guesses.
The linkages ranged from the self-evident to the surreal.
Men who liked TV song-and-dance sensation "Glee" were more likely to be gay. Men who liked professional wrestling were more likely to be straight. Drinking game aficionados were generally more outgoing than, say, fans of fantasy novelist Terry Pratchett. People who preferred pop diva Jennifer Lopez usually gathered more Facebook friends than those who favoured the heavy metal sound of Iron Maiden.
Among the more poignant insights was the apparent preoccupation of children of divorce with relationship issues. For example, those who expressed support for statements such as "Never Apologise For What You Feel It's Like Saying Sorry For Being Real" or "I'm The Type Of Girl Who Can Be So Hurt But Still Look At You & Smile" were slightly more likely to have seen their parents split before their 21st birthday.
For the unknown number of users whose preferences are public, Stillwell had this advice: Look before you like.