Individuals automatically imitate a wide range of different behaviors. Past research has shown that such imitative behavior serves important functions, as it creates a social bond between interaction partners. Although the prevalence of human imitation and its positive social consequences are well-known, the underlying mechanisms of this ubiquitous phenomenon are still poorly understood. Especially, the question of whether and how social processes modulate imitation is part of an ongoing lively debate in the literature. So far, two different theories have been proposed to explain social modulation of imitative behavior. On the one hand, motivational theories postulate that people use imitation either consciously or unconsciously as a tool to affiliate with others. As a result, individuals should imitate others more strongly when they have an affiliation goal. On the other hand, self-other overlap theories suggest that imitative tendencies are learned responses that develop as a result of self-observation and interaction with other, often similar, individuals. Consequently, individuals who are perceived as more similar should be imitated more strongly. Contributing to this literature, in the first funding period of this project, we found in line with self-other overlap theories that focusing on similarities, compared to differences, increases perceived similarities and thereby fosters automatic imitation. Going one step further, we investigated whether other social factors that are supposed to increase perceived similarities modulate automatic imitation too. The results of these studies are mixed and illustrate that some social variables modulate imitative behavior, whereas others do not. This raises the fundamental question of which social factors influence imitative behavior based on which processes. Strikingly, current social modulation theories of imitative behavior do not specify which specific social factors should modulate imitative behavior. In the proposed project, we will fill this gap within three Work Packages (WPs) that include a formalized theory of social modulation of imitative behavior. WP1 will test the degree to which different processes (i.e., similarity and affiliation goal) put forward by social modulation theories actually influence imitative behavior. In WP 2, we will test whether social variables previously assumed to modulate imitative behavior trigger perceived similarity and the goal to affiliate. Based on the insights gathered in WP 1 and 2, we will develop a mathematical formula to formalize a theoretical model that allows deriving precise predictions about how strongly different social factors will modulate imitative behavior. At the end, we will empirically test these predictions.