The Stability of Transient Relationships Valent´ ın Vergara Hidd ∗ EduardoL´opez † Simone Centellegher ‡ Sam Roberts § Bruno Lepri ¶ Robin Dunbar ‖ April, 2022 Abstract In contrast to stable relationships, far less is known about the temporal evolution of tran- sient relationships, although these constitute a substantial fraction of people’s communication networks. Previous literature suggests that ratings of relationship emotional intensity decay gradually until the relationship ends. Using mobile phone data from three countries (US, UK, and Italy), we demonstrate that the volume of communication between ego and its alters is stable for most of the duration of a transient relationship. Alters with longer durations receive more calls, with the duration of the relationship being predictable from call volume within the first few weeks of first contact. Relationships typically start with an early elevated period of com- munication, settling into the longer steady regime until eventually terminating abruptly. This is observed across all three countries, which include samples of egos at different life stages. These results are consistent with the suggestion that individuals initially engage frequently with a new alter so as to evaluate their potential as a tie, and then settle back into a contact frequency commensurate with the alter’s match to ego on the so-called Seven Pillars of Friendship. 1 Introduction Humans are social animals and having strong and supportive relationships with others has large effects on both physical and mental health [27, 24]. These social relationships are not static, but change over time due to two key processes. First, relationships have a natural tendency to weaken over time - to ‘decay’ [10]. Indeed, if no effort is made to maintain relationships, the level of emotional closeness between two individuals will tend to decrease [48] and the relationship will eventually drop out of the person’s social network in terms of the meaningful ties they maintain with others [10]. Long-term studies of people’s social networks (the set of relationships they maintain with their family and friends) show a degree of turnover in network members (alters), with some alters leaving the network and others joining [40, 58]. Second, specific life events such as going ∗ George Mason University. vvergara@gmu.edu † George Mason University. elopez22@gmu.edu ‡ Fondazione Bruno Kessler. centellegher@fbk.eu § Liverpool Johns Moores University. S.G.Roberts1@ljmu.ac.uk ¶ Fondazione Bruno Kessler. lepri@fbk.eu ‖ University of Oxford. robin.dunbar@psy.ox.ac.uk 1 arXiv:2204.05447v1 [physics.soc-ph] 11 Apr 2022