You have significantly fewer friends than you think you do, according to researchers at Tel Aviv University (TAU) and the Massachusetts Institute of Technology (MIT).

Most of us think of friendship as a mutual feeling — you wouldn’t really consider someone your friend unless they felt the same way about you. In reality, said the study, this two-way street only occurs in about half of our friendships. This means some 50 percent of the people you consider a buddy may not feel the same way about you. The study authors wrote that this poor perception of friendship ties can lead to significant limitation of a person’s ability to influence these “friends,” and failure in “establishing compatible norms, acting together, finding compromise solutions, and persuading others to act.”

“It turns out that we’re very bad at judging who our friends are,” said Dr. Erez Shmueli, who directed the study in collaboration with Dr. Laura Radaelli from TAU’s industrial engineering department, MIT professor Alex Pentland, and MIT research assistant Abdullah Almatouq, in a press release. “Our difficulty determining the reciprocity of friendship significantly limits our ability to engage in cooperative arrangements. We learned that we can’t rely on our instincts or intuition. There must be an objective way to measure these relationships and quantify their impact.”

The team’s research was two-pronged: They examined several friendship surveys taken by more than 600 students to assess friendship levels and expectations of reciprocity, and they conducted widespread social experiments, examining their data alongside that from previous experiments. After collecting the information, the researchers developed an algorithm to analyze objective features of friendship like the number of common friends between two people, or a person’s total number of friends. The algorithm was even able to determine if a friendship was unidirectional or reciprocal.

“We found that 95 percent of participants thought that their relationships were reciprocal,” Shumeli said. “If you think someone is your friend, you expect him to feel the same way. But in fact that’s not the case — only 50 percent of those polled matched up in the bidirectional friendship category.”

Shumeli explained that reciprocal relationships are important for social influence. One of the experiments included in the research showed that friendship often outweighs other incentives, such as money, for persuading people to exercise.

“We found, not surprisingly, that those pressured by reciprocal friends exercised more and enjoyed greater progress than those with unilateral friendship ties,” Shumeli said.

If researchers could learn to understand our limitations regarding friendship, the paper said, groups and companies that depend on social influence could improve their interventions and strategies for reaching others.

Source: Almaatouq A, Radaelli L, Pentland A, Shumeli E. Are You Your Friends’ Friend? Poor Perception of Friendship Ties Limits the Ability to Promote Behavioral Change. PLOS One. 2016.