Loneliness in Adolescence: Genetic origins and environmental influences

Loneliness is a negative emotional reaction to the experience of a di erence between the perceived and the desired quantity or quality of social relations (Perlman & Peplau, 1981). Because loneliness has been related to various physical (e.g., S. Cacioppo, Grippo, London, Goossens, & Cacioppo, 2015) and mental health problems (e.g., Heinrich & Gullone, 2006; Mahon, Yarcheski, Yarcheski, Cannella, & Hanks, 2006), it is of great importance to get a better understanding of factors that predict individual di erences in loneliness. As loneliness is especially prevalent during adolescence (Qualter et al., 2015), the current dissertation examined individual factors and environmental factors associated with adolescents' loneliness. More speci cally, the current dissertation examined 1) the genetics of loneliness, 2) the cognitive and behavioral characteristics associated with loneliness, and 3) the in uence of parents and peers on adolescents' loneliness.

The first part of the current dissertation examined the genetics of loneliness. It provides an updated overview of genetic studies conducted on loneliness, which also discussed the importance of this research for theories of loneliness. In addition, the association between loneliness and a specific gene was examined. Previous research indicated that this gene, coding for serotonin transporter, is related to loneliness when in interaction with low parental support. However, we were not able to replicate this gene-by-environment effect.

Moreover, the cognitive and behavioral characteristics associated with loneliness were examined. This dissertation provides a new framework to integrated seemingly unrelated empirical studies into the cognitive and behavioral characteristics of loneliness. Specific cognitive and behavioral characteristics of loneliness were also examined in more detail. It was found that some adolescents experience feelings of loneliness but not of depression, whereas other adolescents only experience feelings of depression but not loneliness. The least and most lonely individuals were found to be the most prosocial and loneliness was not related to aggressive behavior.

Lastly, the role of parents and peer is discussed. Positive parenting behavior, high friendship quality, and having more friends was related to lower levels of loneliness. Despite that parents and peers are important social context in adolescents, loneliness in relation to parents was unrelated over time to loneliness in relation to peers.

In sum, the current dissertation emphasizes the importance of various individual and environmental factors for the development of loneliness in adolescence. The proposed theoretical frameworks and the results in this dissertation raise new question that will contribute to the continuing development of loneliness research and theory.