Examinations of the relationship between individuals’ personal characteristics and the social positions that individuals receive in everyday peer networks have often found an association between extraversion and popularity. This thesis assesses the conditions (when) and mechanisms (why) of this association.Four research questions focus on when the link between extraversion and popularity is present. The study examines whether extraversion is already associated with popularity among seven- to eight-year-olds (Study I), if extraversion is associated with popularity in a less talkative and more stereotypically introverted culture as well (i.e. in Finland; Studies I and II), whether the association is more reflective of the popularity of extraverts or the unpopularity of introverts (Studies II and III), and if the size of the surrounding social ecology is an important precondition of this association (Study III). In addition, the study considers two why research questions. Studies II and III examine if dyadic combinations of extraversion could serve as popularity particles that would explain why extraverts are ultimately more popular in the group, whereas Study I evaluates the mediating role of oral fluency between extraversion and popularity among children.The association between extraversion and popularity is found to be highly generalizable, as it is present among young and adult Finns as well as in social networks of varying sizes. In addition, the association is linear and unilateral: introverts are unpopular as much as extraverts are popular, and dyadic combinations of extraversion are not significant in explaining this phenomenon. Finally, the higher oral fluency of extraverts partially explains their popularity in middle childhood.The discussion focuses on the causality of this association and engages with the ontological status of trait extraversion throughout the thesis. The research also highlights the role of popularity and social networks in accounting for the coalescence of extraversion.