4 | 105 The transition to adulthood: A game changer!? is clearly an issue in sport participation, as lack of time is by far the mostmentioned reason for not practising sport (European Commission, 2014). The main constraints reported for sport participation relate to work or study, social commitments, and family obligations (Deelen et al., 2016). Not only is time scarce, but time devoted to one activity often must be traded off against time required for other pursuits. Because leisure activities like sport are relatively informal and optional, people feel pressure to relinquish these when more formal, obligatory tasks arise, like paid work or childcare (Kraaykamp et al., 2009). Second, practising sport is influenced by individuals’ social surroundings; social network contacts may encourage but also discourage sport participation (Kraaykamp et al., 2013). At the same time, sport participation provides social resources, facilitating growth and maintenance of social networks and social capital (Putnam, 1995). Social motivation theory helps us to understand why changes in social resources associated with the transition to adulthood may influence people to change the time they devote to sport participation (Van Houten et al., 2017). This theory states that people gravitate towards activities that have a high social payoff, especially when they are experiencing time constraints (Hills et al., 2000a). A major life event is likely to change the relevance of the social resources that come with practising a sport. Specifically, starting a job, entering an intimate relationship, and having a child may lead to alternative social resources, with higher social payoffs in the new social network. This increases the likelihood that existing sport practices will be traded off for new activities, possibly leading to changes in sport behaviour, like participating less frequently, in fewer sports, and in a “lighter” setting (or not at all) (Van Houten et al., 2017). The effects of major life events: Expectations Major life events that accompany the transition to adulthood – like leaving full-time education and beginning work, entering an intimate relationship, formalising a relationship through cohabitation or marriage, and becoming a parent – impose restrictions on time to practise sport (Deelen et al., 2016; Kraaykamp et al., 2009; Ruseski et al., 2011; Tiessen-Raaphorst et al., 2014; Van Houten et al., 2014, 2017). These major events, additionally, bring new roles and social responsibilities, like maintaining a professional and family network,