Screengrab showing results of Five Labs personality test. (Courtesy of Five Labs)

A new online tool purports to tell you how much you have in common with Bill Gates, Oprah Winfrey, LeBron James and any of your Facebook friends. It also has scary implications for the future of online advertising.

A startup called Five Labs created the online tool using five dimensions many psychologists use to describe human personality: openness, extraversion, neuroticism, conscientiousness, agreeableness. (You can check out the tool here and use it to scan your own profile).

But instead of asking you to fill out a questionnaire, the tool scans your Facebook wall posts, photo captions and comments for words that predict the “Big Five” personality traits. With the click of a button, you can scan your friends’ pages too.

The results appear as a warped pentagon. Each side is assigned a percentage based on how much your personality reflects each of the five traits.

The tool was inspired by a study conducted at University of Pennsylvania published last year in PLOS One journal. Researchers led by H. Andrew Schwartz analyzed 700 million words, phrases and topics collected from the Facebook posts of 75,000 volunteers. They noticed linguistic patterns in the volunteers’ posts — some words were strongly associated with certain personality traits.

Example: “Sick of” and “nightmare” were associated with neuroticism, while words like “chillin” — or simply a string of exclamation points — were associated with extroversion.

The researchers built a model, trained it to look for those words — and found the method predicted personality almost as well as traditional questionnaires.

Five Labs created a similar tool using data from volunteers. The model scans your Facebook activity for the past three or four months — or earlier if you’re an infrequent poster — and compares your posts to the words their model associates with the Big Five.

Pretty cool. But also pretty scary. Nikita Bier, the founder of Five Labs, said the biggest response from volunteers was “this is kind of creepy.” Are Internet companies doing this sort of analysis on us? volunteers wanted to know.

For better or for worse, the app shows people what companies can do with their online data. “It creates balance in what was previously asymmetrical,” Bier told The Washington Post. “Consumers didn’t know exactly what companies were doing with that data and didn’t understand the capacity.”

The tool was used to scan 110 million profiles within three days of its release last week. That’s a lot of data. However, Bier says Five Labs has no plans to sell users’ data or use it to create targeted ads. In fact, he says, they don’t keep user data at all. Five Labs is creating an alternative social networking app, due out this August, that does not allow for this type of data analysis.

But a tool like this could mean big bucks for companies that do targeted advertising. It would let them get inside your head and create ads that appeal to you based on your psychological profile.

Targeted ads are plenty controversial already. Take for example, Facebook’s recent announcement that it will start watching what other Web sites you visit to better target the ads you see on Facebook. Advertisers already use off-site information to decide which ads to show Facebook users, but the company has not previously used that information to choose ads for users’ feeds.

Facebook also introduced a new tool on its site that allows users to get more information about why they’re seeing certain ads, but the news was still met with criticism from privacy advocates. After taking flak for invading privacy, Facebook has tried to be more transparent about changes on their site by announcing them upfront and giving users an easy way to opt out.

“It’s important that people know how their private data is being used and consent to it,” said Andrew Schwartz, a University of Pennsylvania researcher who advised Five Labs on the project. “People probably want to know if an advertisement, for example, was specifically selected for them based on their personality … it’s important such applications have policies to inform people how their data is being used and provide means of agreeing to such use.”

The tool has other consumer applications as well, The technology could be used by businesses to determine whether an applicant is suited to an available position, or by a dating service to identify compatibility. Schwartz said it also has potential health applications — for example, as a way to monitor the mood of a person battling depression by looking for warning signs in his or her social media posts.

Correction: Where the article said Five Labs would be introducing a mobile app version of its online personality tool, it has been corrected to indicate the mobile app is not related to the tool. It is a social networking app.