“there is nothing intrinsically natural about separating all human beings into two opposing categories on the basis of one physical attribute of their overall being-in-the-world” (Rawlins, 2009, p. 125

More extraverted people dress more stylishly, behave more confidently, and express more positivity, for instance through smiling—factors that make extraverted people attractive to others and make others like them better (Back et al., 2011

Importantly, adolescents’ friendships also serve as training models to prepare for intimate romantic relationships (Fraley, Roisman, Booth-LaForce, Owen, & Holland, 2013; Simpson, Collins, Tran, & Haydon, 2007

People differ in their networks and maintain either (1) relatively few long-term, emotionally close friends, (2) larger networks with close friends and also loose acquaintances, or (3) even social networks without friends and only family ties.

the average number of relationships in social networks ranges from 3 to 40 (Wrzus et al., 2013

the Adams-Blieszner-Ueno integrative conceptual framework for friendship research (Figure 3.1), depicts friendship patterns as dynamic and contextualized. Individual characteristics, consisting of social structural positions and psychological dispositions, which affect each other through interpretation and internalization, lead to the development

Individuals develop strategies to make contacts, to start friendships, and to maintain but also to end voluntarily these relationships. Behavioral motifs vary by gender, social status, life stage, family status, and other personal characteristics. They are highly influenced by personality traits that moderate openness to new contacts

Similarity in traits, attitudes, or interests among friends may be a byproduct of social homogamy (e.g., meeting in the neighborhood where people of similar socioeconomic status and political orientation live; McPherson et al., 2001) and further environmental factors influencing friendship formations (e.g., meeting during shared leisure activities

During the formation of friendships, people reciprocate favors and support quickly to avoid the impression of exploiting the other person (Lydon, Jamieson, & Holmes, 1997). In established friendships, however, such tit-for-tat behavior (immediate reciprocation; Axelrod & Hamilton, 1981) is detrimental as people value balanced relationships, but