The question whether social media use benefits or undermines adolescents' well-being is an important societal concern. Previous empirical studies have mostly established across-the-board effects among (sub)populations of adolescents. As a result, it is still an open question whether the effects are unique for each individual adolescent. We sampled adolescents' experiences six times per day for one week to quantify differences in their susceptibility to the effects of social media on their momentary affective well-being. Rigorous analyses of 2,155 real-time assessments showed that the association between social media use and affective well-being differs strongly across adolescents: While 44% did not feel better or worse after passive social media use, 46% felt better, and 10% felt worse. Our results imply that person-specific effects can no longer be ignored in research, as well as in prevention and intervention programs. Ever since the introduction of social media, such as Facebook and Instagram, researchers have been studying whether the use of such media may affect adolescents' well-being. These studies have typically reported mixed findings, yielding either small negative, small positive, or no effects of the time spent using social media on different indicators of well-being, such as life satisfaction and depressive symptoms (for recent reviews, see for example 1-5). Most of these studies have focused on between-person associations, examining whether adolescents who use social media more (or less) often than their peers experience lower (or higher) levels of well-being than these peers. While such between-person studies are valuable in their own right, several scholars 6, 7 have recently called for studies that investigate within-person associations to understand whether an increase in an adolescent's social media use is associated with an increase or decrease in that adolescent's well-being. The current study aims to respond to this call by investigating associations between social media use and well-being within single adolescents across multiple points in time 8-10. Person-specific effects. To our knowledge, four recent studies have investigated within-person associations of social media use with different indicators of adolescent well-being (i.e., life satisfaction, depression), again with mixed results 6, 11-13. Orben and colleagues 6 found a small negative reciprocal within-person association between the time spent using social media and life satisfaction. Likewise, Boers and colleagues 12 found a small within-person association between social media use and increased depressive symptoms. Finally, Coyne and colleagues 11 and Jensen and colleagues 13 did not find any evidence for within-person associations between social media use and depression. Earlier studies that investigated within-person associations of social media use with indicators of well-being have all only reported average effect sizes. However, it is possible, or even plausible, that these average withinperson effects may have been small and...

This study had three goals. First, we examined the prevalence of the participant roles of bullying in middle adolescence and possible gender differences therein. Second, we examined the behavioral and status characteristics associated with the participant roles in middle adolescence. Third, we compared two sets of criteria for assigning students to the participant roles of bullying. Participants were 1,638 adolescents (50.9% boys, M(age)  = 16.38 years, SD =.80) who completed the shortened participant role questionnaire and peer nominations for peer status and behavioral characteristics. Adolescents were assigned to the participant roles according to the relative criteria of Salmivalli, Lagerspetz, Björkqvist, Österman, and Kaukiainen (1996). Next, the students in each role were divided in two subgroups based on an additional absolute criterion: the Relative Only Criterion subgroup (nominated by less than 10% of their classmates) and the Absolute & Relative Criterion subgroup (nominated by at least 10% of their classmates). Adolescents who bullied or reinforced or assisted bullies were highly popular and disliked and scored high on peer-valued characteristics. Adolescents who were victimized held the weakest social position in the peer group. Adolescents who defended victims were liked and prosocial, but average in popularity and peer-valued characteristics. Outsiders held a socially weak position in the peer group, but were less disliked, less aggressive, and more prosocial than victims. The behavior and status profiles of adolescents in the participant roles were more extreme for the Absolute & Relative Criterion subgroup than for the Relative Only Criterion subgroup.

Eighteen earlier studies have investigated the associations between social media use (SMU) and adolescents’ self-esteem, finding weak effects and inconsistent results. A viable hypothesis for these mixed findings is that the effect of SMU differs from adolescent to adolescent. To test this hypothesis, we conducted a preregistered three-week experience sampling study among 387 adolescents (13–15 years, 54% girls). Each adolescent reported on his/her SMU and self-esteem six times per day (126 assessments per participant; 34,930 in total). Using a person-specific, N = 1 method of analysis (Dynamic Structural Equation Modeling), we found that the majority of adolescents (88%) experienced no or very small effects of SMU on self-esteem (−.10 < β < .10), whereas 4% experienced positive (.10 ≤ β ≤ .17) and 8% negative effects (−.21 ≤ β ≤ −.10). Our results suggest that person-specific effects can no longer be ignored in future media effects theories and research.

The formation and maintenance of friendship closeness is an important developmental task in adolescence. To obtain insight in real-time processes that may underly the development of friendship closeness in middle adolescence, this preregistered experience sampling study [ESM] investigated the effects of social media use on friendship closeness. The study was conducted among 387 adolescents (54% girls; M age ϭ 14.11 years; 96% Dutch) from different educational tracks (44% lower prevocational secondary education, 31% intermediate general secondary education, 26% academic preparatory education). Adolescents reported six times per day for 3 weeks on their Instagram, WhatsApp, and Snapchat use in the previous hour and their momentary experiences of friendship closeness (126 assessments; 34,930 observations). Multilevel analyses revealed positive between-person associations of friendship closeness with general WhatsApp use and Instagram use with close friends. In contrast, at the within-person level, we found small negative overall associations of general WhatsApp use and Instagram use (with and without close friends) with friendship closeness. However, there was large heterogeneity in the personspecific effect sizes of the within-person associations of social media use with friendship closeness. For example, person-specific effect sizes of the association of Instagram use with close friends with friendship closeness ranged from ␤ ϭ Ϫ.745 to ␤ ϭ .697. These results underline the importance of acknowledging person-specific effects in developmental and media effect theories.

Animal and human studies have shown that both early-life traumatic events and ongoing stress episodes affect neurodevelopment, however, it remains unclear whether and how they modulate normative adolescent neuro-maturational trajectories. We characterized effects of early-life (age 0–5) and ongoing stressors (age 14–17) on longitudinal changes (age 14 to17) in grey matter volume (GMV) of healthy adolescents (n = 37). Timing and stressor type were related to differential GMV changes. More personal early-life stressful events were associated with larger developmental reductions in GMV over anterior prefrontal cortex, amygdala and other subcortical regions; whereas ongoing stress from the adolescents’ social environment was related to smaller reductions over the orbitofrontal and anterior cingulate cortex. These findings suggest that early-life stress accelerates pubertal development, whereas an adverse adolescent social environment disturbs brain maturation with potential mental health implications: delayed anterior cingulate maturation was associated with more antisocial traits – a juvenile precursor of psychopathy.