What friends of friends reveal about us online

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At a time when social network privacy is in the news, new research shows there are more ways than previously realized to reveal certain traits we might be trying to conceal.

…even if a person does not reveal their age, race, or political views, friendship studies can easily and accurately infer these traits.

The work builds on one of the main threads in privacy research, which is to understand how different traits are correlated.

The authors based the paper on databases available specifically for research. These reflect the kinds of information that websites make available to advertisers or reveal to outside groups when people allow third parties to access their social profiles.

Given the prevalence of such data, the researchers sought to better understand what sorts of statistical inferences might end up revealing traits people have sought to conceal.

Friends and personal information

“In social data, some things are more predictable than others,” says Johan Ugander, assistant professor of management science and engineering at Stanford University. “We set out to study the relationship between friend networks and predictability, and ended up uncovering an inference mechanism that hadn’t been noticed before.”

At the simplest level people reveal information about themselves based on how they behave online. If a person buys diapers online, for example, they probably have a baby. That is a direct inference.

A second form of inference is based on looking at our friends, or indirect inference. Researchers who have studied social media relationships have found that we tend to friend people of roughly our same age, race, and political belief.

So even if a person does not reveal their age, race, or political views, friendship studies can easily and accurately infer these traits. Researchers call this tendency homophily, which stems from the Greek words for love of sameness.

But not all unknown traits are easy to predict using friend studies. Gender, for instance, exhibits what researchers call weak homophily in online contexts.

“If an unknown person in a social network has mostly male friends there’s an almost equally good chance they could be female, or vice versa,” says Kristen Altenburger, a PhD student in Ugander’s lab.

Social network privacy

The group’s new research shows that it’s possible to infer certain concealed traits—gender being the first—by studying the friends of our friends.

This technique works because Ugander and Altenburger have described a new social structure they call monophily, Greek for “love of one,” where people have extreme preferences for traits but not necessarily their own trait.

“For example,” Ugander says, “on average it might be the case that men don’t have a clear preference for male or female friends, but that average may be obscuring the fact that some men have strong preferences for male friends while others have strong preferences for female friends.”

They observe that when there’s monophily in a network, it becomes possible to predict traits of individuals based on friends of friends, even in situations where there’s no homophily.

The researchers relied on standard network datasets widely studied by academics. These datasets map friendship networks and contain complete information about all of the traits of all of the individual traits, including gender. The researchers then erased the gender data for certain individuals, creating artificial unknowns, and then used their “friends of friends” analysis to see if it could make a prediction.

“It’s a fill-in-the-blanks problem,” says Ugander. “And while we find that your friends don’t tend to predict your gender, the people those friends choose to associate with, your friends of friends, tend to be more similar to you than even your friends are.”

The researchers says that the power of their new perspective, of looking at the friends of our friends, highlights the importance of protecting network data from prying hands. Any policy solution to preserve network privacy will need to consider the information contained among one’s friends of friends. They are now reapplying their technique to other unknowns to see what else may be disclosed by friends of friends.

We will give up privacy for convenience (or free pizza)

“We’re not sure what else might be revealed in this way,” Ugander says. “Unfortunately, it looks like the realm of network privacy is even smaller than we previously thought.”

The researchers report their findings in the journal Nature Human Behavior.

Source: Stanford University