For the study, Eletra Gilchrist-Petty, an associate professor of communication arts at the University of Alabama in Huntsville, and Lance Kyle Bennett, a doctoral-degree student at the University of Iowa, recruited 346 people, ranging in age from 18 to 64, who were or had been in a heterosexual relationship with someone who had a different-sex best friend. When they surveyed participants’ attitudes toward cross-sex best friendships, they found that people who are engaged to be married look more negatively on those friendships than married, single, or dating people. They also found that people who are skeptical of cross-sex best friendships in general are more likely to “lash out” at their partner when they feel threatened by the partner’s best friend—as opposed to constructively communicating with their partner, or with the friend, about the situation…

The study results also suggest that relationship status can play a role in people’s level of trust in cross-sex best friendships. Gilchrist-Petty wrote to me in an email that of all their findings, she was most surprised that engaged couples were the most skeptical. Engaged couples may be particularly protective of their relationships because they’re almost across the matrimonial finish line, she posited, “and do not want anything or anyone, including a cross-sex best friend, to potentially jeopardize their upcoming marriage.” In the study, she and Bennett also note that engaged couples are in a uniquely stressful situation compared with single, dating, and married people: Not only are they transitioning to become assumed life partners, they wrote in the study, “but they are often dealing with … merging lives and planning a wedding.” As they note in the study, this can include family problems as well as financial constraints, both of which are known to place long-term stress on people and relationships.