74 | Chapter 1 3 INTRODUCTION Why do so many young adults stop practising sport? Typically, the stability (also called “tracking”) of physical activity and sport participation is found to be quite low in young adulthood (Hirvensalo & Lintunen, 2011). Between the ages of 18 and 35 years, we see a sharp decline in individual sport participation (Engel & Nagel, 2011; Leslie et al., 2001; van Tuyckom, 2011), as well as in sport club membership (European Commission, 2014; Tiessen-Raaphorst et al., 2010; Van Tuyckom, 2011). Reasons why young adults drop out of sport are manifold, and include changing lifestyles, disappointment with selection mechanisms in sport, and insecurity over one’s physique (Coakley &Donnelly, 1999). Moreover, young adulthood is, formany, a period of key individual life events: peoplemove out and start living on their own, they find their first job, start cohabiting or get married and have children (Allender et al., 2008; Engberg et al., 2012). Such life events alter roles and responsibilities, leading to changes in daily routines and available resources (Holmes and Rahe, 1967; Hirvensalo and Lintunen, 2011). Consequently, life events influence the choices people make regarding their leisure time (Allender et al., 2008; Engberg et al., 2012; Hirvensalo & Lintunen, 2011; Van Houten et al., 2014). Some prior studies indicate that physical activity decreases when people begin to work (Brown & Trost, 2003; Zsolt et al., 2007), when they switch from being single to cohabitation or marriage (Bell & Lee, 2005; Brown et al., 2009) and when they have a child (Brown et al., 2009; Hull et al., 2010). Yet other research has found no association between physical activity and getting married or cohabiting (Hull et al., 2010; King et al., 1998) or having a child (Blum et al., 2004). Possible explanations for these contradictory findings are the investigation of different age groups, as the effects of life events may vary according to the phases of life or ages the study population represents (Engberg et al., 2012), and the use of mostly cross-sectional data. Subsequently, Bell and Lee (2005) and Tiessen-Raaphorst et al. (2010) point to the need for research to focus on specific life phases. In addition, few studies have examined multiple life events (Engberg et al., 2012). It has thus remained unclear which life events are most important (Hirvensalo & Lintunen, 2011). It seems needless to stress that sport does not equal physical activity. Not every type of physical activity can be labelled sport, and not every sport requires physical activity. Nonetheless, with ongoing processes of sportisation, the boundaries between the two do appear to be increasingly eroding (e.g.,