People tend to befriend others similar to themselves, generating a pattern called homophily. However, existing studies on friendship patterns often rely on surveys that assess the perspective of relatively few participants on their friendships but do not measure actualized friendship patterns. Here, we used data from a large Slovakian online social network to assess the role of gender, age, and body mass index (BMI) in same-gender online connections among more than 400,000 users. We found that age and BMI homophily occurred in both men’s and women’s same-gender connections, but somewhat more strongly among men’s. Yet, as women diverged in BMI, their connections were less likely to be reciprocated. We discuss how the evolutionary legacy of men’s coalitional competition (e.g., warfare) and women’s mating competition or recruitment of allocare providers might contribute to these patterns in modern same-gender relationships. For example, men’s engagement in physical activities may lead to similar formidability levels among their same-gender peers. Altogether, our findings highlight the importance of trait similarity to same-gender friendship patterns.

人们倾向于与与自己相似的人交朋友，从而产生了一种称为同质性的模式。然而，现有的关于友谊模式的研究通常依赖于评估相对较少的参与者对其友谊的看法的调查，但不衡量实际的友谊模式。在这里，我们使用来自斯洛伐克大型在线社交网络的数据来评估性别、年龄和体重指数 (BMI) 在超过 400,000 名用户的同性在线联系中的作用。我们发现，男性和女性的同性关系中都存在年龄和体重指数的同质性，但男性的同质性更为强烈。然而，由于女性的体重指数存在差异，她们的关系不太可能得到回报。我们讨论男性联盟竞争（例如战争）和女性交配竞争或招募异体护理提供者的进化遗产如何促成现代同性关系中的这些模式。例如，男性参与体育活动可能会导致同性同龄人具有相似的可怕程度。总而言之，我们的研究结果强调了特征相似性对同性友谊模式的重要性。