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Ever since the launch of MySpace in 2003 there has been an incredible rise of social media platforms (Ortiz-Ospina 2019). As with most new technologies, social media has been particularly attractive for adolescents. Social media provides immediate and often rewarding feedback and easily connects adolescents with their friends (Odgers and Robb 2020). While friendships play an important role across all ages, the need to be socially connected to peers is a central aspect of young peoples’ lives (Denworth 2020). Adolescents’ specific biological and psychological changes trigger an increased need for interaction with peers and a heightened sensitivity to social stimuli (Blakemore 2008). That is, during adolescence there is a peak in activation of subcortical-affective brain regions during monetary and social rewards (Braams et al. 2015; Schreuders et al. 2018). The peak in affective brain activation could explain why adolescents make more emotional and less rational decisions and are highly motivated to engage in social and emotional experiences, as the protracted development of cognitive control areas put less weight on controlling and regulating behavior and emotions (Casey 2015; Chein et al. 2011; Crone and Dahl 2012; Somerville, Jones, and Casey 2010). Currently, it remains unclear how the social connectedness though social media affects ongoing brain development throughout adolescence, and vice versa. It has been suggested that adolescents heightened emotional sensitivity and protracted development of cognitive control may make them specifically reactive to social media (Crone and Konijn 2018). In this study, we will examine the longitudinal associations between social media use and structural brain development during adolescence. Additionally, we will explore whether heterogeneity in social media use is associated to differences in subjective mental well-being, as social media use may affect individuals differently. The heightened sensitivity to peers during adolescence has left scientists and society worried about social media platforms and social media use among adolescents (Martinotti et al. 2011). As adolescents spend more time with peers and experience increased sensitivity to peer feedback (Blakemore and Mills 2014), they show more frequent peer related stressors, and react emotionally to these peer-stressors, potentially leading to negative feelings (Brown and Larson 2009). While some researchers have suggested that social media use is related to an increase in depressive symptoms and suicide-related outcomes (Twenge, Martin, and Campbell 2018), most scientific studies on the association between social media use and well-being have reported mixed results with small effect sizes (Odgers and Jensen 2020; Orben 2020; Orben and Przybylski 2019). These mixed findings and small associations might be the result of the large heterogeneity of adolescent’s social media use (Odgers and Jensen 2020). That is, the effects of social media on well-being varies substantially between individual adolescents (Beyens et al. 2020). These results highlight the need to look at intra-individual (within-person) associations between social media and well-being, using longitudinal data (Crone and Elzinga 2015). Therefore, the first aim of this study is to examine heterogeneity in developmental trajectories of social media use across adolescence and to test whether individual differences in social media use across development are associated with baseline levels (intercept) and changes (slopes) in mental well-being, as measured by levels of depression and anxiety. Most studies to date have focused solely on the direct association between social media use and well-being, without taking into account the possible underlying brain mechanisms (Prinstein, Nesi, and Telzer 2020). Two decades of developmental neuroscience research has shown important changes in brain structure and function across the transition from childhood to adulthood. Decreases in brain grey matter volume, which can be seen as a proxy for brain maturation (Huttenlocher 1990), continue until the mid-twenties (Gogtay et al. 2004; Mills et al. 2016; Tamnes et al. 2017). Specifically, social brain regions such as the medial prefrontal cortex (MPFC), posterior superior temporal sulcus (pSTS) and temporal parietal junction (TPJ), as well as cognitive control regions such as the lateral prefrontal cortex (LPFC) show extensive decreases in grey matter volume (i.e., cortical thinning) across adolescence (Becht et al. 2020; Mills et al. 2016). Individual differences in cortical thinning have been related to behavioral outcomes. Specifically, accelerated cortical thinning of the prefrontal cortex has been associated with higher levels of depression within a non-clinical adolescent sample (Bos et al. 2018). Likewise, diminished cortical thickness of (amongst others) the ventral LPFC and TPJ across adolescence have been related to an increased preference for small fast rewards over larger delayed rewards (i.e., inability to delay gratification (Pehlivanova et al. 2018). Very little is known about the association between brain development and social media use in adolescence (Crone and Konijn 2018), but this accelerated cortical thinning could be an underlying mechanism driving both the increase in social media use as well as the increase in anxiety and depression across the adolescent population (Prinstein et al. 2020). Therefore, the second aim of this study is to examine associations between social media use and structural brain development across adolescence. We will specifically focus on regions of interest of the social brain (MPFC, TPJ, pSTS) and a region more strongly involved in cognitive control (LPFC). We will test specificity of the relations between brain change and social media use by a follow up analysis on a lower-order processing region (cuneus; see (van der Meulen et al. 2020) for similar approach). We will make use of data from the preregistered accelerated longitudinal Self-Concept Study (van der Cruijsen et al. 2018). Social media use and mental well-being are measured through self-report. A detailed description of the questionnaire measures and structural brain measures is provided below.

Youth of today grow up in a digital social world but the effects on well-being and brain development remain debated. This study tracked longitudinal associations between structural brain development, social media use and mental well-being. The study demonstrated two pathways of heterogeneity in brain development. First, adolescents who used social media more than their peers showed higher baseline cortical thickness in lateral prefrontal cortex (PFC) and medial PFC; and stronger decreases in the lateral PFC and temporal parietal junction. In contrast, adolescents with lower mental well-being showed lower baseline levels of surface area in the medial PFC and posterior superior temporal sulcus relative to their peers. Whereas the associations between structural brain development and well-being remained significant after correction for multiple testing, the results for social media use did not survive FDR correction. These findings demonstrate that although social media use and mental well-being were both associated with differential trajectories of brain development, the associations we report are distinct. These results show a nuanced perspective on the presumed relations between social media use and well-being and provide a starting point to further examine neural mechanisms that could explain which adolescents thrive by social media and which might be harmed.