Trends of Friends – Time dynamics of Surface- and Deep- level traits in friendship formation and maintenance

People become friends with one another primarily due to things they have in common, like shared demographic characteristics or shared interests. But on what similarities are people becoming friends at different stages of knowing one another? To study this, we use a longitudinal dataset that followed a cohort of students of one study programme at a Swiss technical university. We model how demographic traits of nationality and gender, and less observable interest-related traits of being social, a partygoer, a smart and hard-working student, contribute to friendships. Using Stochastic Actor- Oriented Models, we find a baseline level of both demographic and interest-related friendship homophily, indicating that there are differing reasons behind friendships. We also find that homophily based on demographic traits diminishes over time when students get to know those in their study cohort better. This suggest that homophily on observable traits is mainly relevant when people first meet but becomes less important over time. • People become friends with one another due to different characteristics at different stages of knowing one another. • Demographic characteristics, which are usually easy to observe, decrease in importance over time as a driver for friendships. • Friendships are often based on interests and are consistent in importance for friendships over time. • Demographic clustering seemingly decreases over time due to friendships being based more on non-demographic characteristics.