Rationale --------- In 1995, near the beginning of a period of near-exponential growth in Internet pornography, Philip Elmer-Dewitt shocked parents everywhere with his Time cover story about “Cyberporn.” The cover line salaciously promised to inform interested readers about how widespread and “wild” Internet pornography had become, and explicitly framed the debate around the balance between protecting free speech and protecting children (Elmer-Dewitt, 1995). The story described soon to be discredited research that claimed that growth in online pornography was primarily driven by pedophilic and hebephilic interests (see Rimm, 1995). Upon release, both Rimm’s (1995) study and Elmer-Dewitt’s (1995) story were lambasted—with the Time article eventually becoming required reading in ethics of journalism classes as a case study in what no to do (Elmer-Dewitt, 2015). Despite criticism by many journalists and scholars, both pieces resonated with deep-seated fears among many Americans surrounding sexuality and children (Levine, 2006) which ultimately coalesced into the Communications Decency Act, the first attempt by the U.S. Congress to regulate Internet pornography. Concern about pornography and children is not new, as George Putnam asserted back in 1965, “[a] flood-tide of filth is threatening to pervert an entire generation of our American children” (Citizens for Decent Literature, 1965). However, as Elmer-Dewitt’s story illustrates, in recent decades, pornography on the Internet seems to have given new urgency to the issue, perhaps because of the increased anonymity, affordability, and accessibility (Cooper, Delmonico, & Burg, 2000; Cooper, Delmonico, Griffin-Shelley, & Mathy, 2004) that it allegedly provides (Byers, Menzies, & O’Grady, 2004). In our current age, discussions of the dangers of Internet pornography for children and adolescents have begun to revolve around public health conceptualizations, with Utah being the first American state to declare pornography a “public health crisis” (Emba, 2016), Canada commissioning a Parliamentary study on the public health effects of violent and degrading pornography (Standing Committee on Health, 2017), and the UK implementing increasingly restrictive regulation of Internet pornography despite admissions by regulatory bodies that no clear harms to children have been demonstrated (Petley, 2014). With an estimated 7% to 59% of adolescents accessing pornography (Peter & Valkenburg, 2016), there are clear reasons to be concerned about the impact of Internet pornography on adolescent development. Specific concerns have included pornography’s impact on sexual risk taking, sexual functioning, body image, sexual objectification and sexual aggression among adolescents (Collins et al., 2017; Owens, Behun, Manning, & Reid, 2012; Peter & Valkenburg, 2016; Pizzol, Bertoldo, & Foresta, 2016). According to the accumulating literature, Internet pornography may threaten many facets of adolescent development and well-being. Especially because “children and adolescents are widely considered the most vulnerable audiences to sexually explicit material” (p. 117, Owens et al., 2012; for similar claims also see Collins et al., 2017; Valkenburg & Peter, 2013). Of particular interest are the implications of potential harms of pornography on adolescent psychological wellbeing. By psychological wellbeing, we broadly mean positive and negative indicators of psychological health and wellbeing. Precise conceptualizations of both psychological health and wellbeing are still open to debate, but it is clear from the accumulating literature that these concepts are inter-related to some degree (Dodge, Daly, Huyton, & Sanders, 2012; La Placa, McNaught, & Knight, 2013). In the current paper, we are using psychological wellbeing as broad term to refer to the differentiable but related concepts of self-esteem, mental health, and subjective wellbeing. There is both direct and indirect evidence that links pornography to poor psychological wellbeing in adolescents. Indirectly, pornography use may contribute to personal insecurities about adolescents’ bodies, their appearance, or their sexual performance (Collins et al., 2017; Doornwaard et al., 2014; Morrison, Ellis, Morrison, Bearden, & Harriman, 2006); it may undermine attachment functioning, leading to relationship dysfunction, and social isolation (Mesch, 2009 as cited in Owens et al., 2012; Philaterou, Mahfouz, & Allen, 2005; Tylka, 2015); and it may promote sexual risk taking (Collins et al., 2017; Peter & Valkenburg, 2016). All of these potential harms should impair psychological wellbeing among adolescents. More directly, cross-sectional surveys have found that pornography use is related to reports of more negative affect, poor mental health and lower quality of life among adults (Tylka, 2015; Weaver et al., 2011) as well as lower life-satisfaction and self-esteem, and more symptoms of depression among adolescents (Kim, 2001, 2011; Peter & Valkenburg, 2016; Ybarra, Mitchell, Hamburger, Diener-West, & Leaf, 2011). On the basis of this evidence, the case for pornography having a negative impact on adolescent psychological wellbeing seems strong. Several studies, however, fail to support this conclusion. For example, some research has indicated that pornography use is either unrelated, or positively related, to body and genital satisfaction, and sexual esteem among adult samples (Morrison, Bearden, Harriman, Morrison, & Ellis, 2004; Morrison et al., 2006; also Duggan & McCreary, 2004, as well as Schooler & Ward, 2006, as cited in Tylka, 2015). Further, other studies have failed to observe a significant relationship between social connectedness, attachment to parents and peers, and pornography use among adolescents (Mesch & Maman, 2009 as cited in Peter & Valkenburg, 2016; Peter & Valkenburg, 2011), or have found that adult pornography users actually have more close relationships than non-users (Popovic, 2011). Finally, at least one direct assessment of psychological wellbeing failed to find a significant association between pornography use and self-esteem among adolescents (Mesch & Maman, 2009 as cited in Peter & Valkenburg, 2016), while another reported a positive relationship between the two constructs in young adult males (Morrison et al., 2004). These exceptions aside, the bulk of accumulated evidence seems to favour the hypothesis that pornography use is associated with worse psychological wellbeing among adolescents. Even so, it remains unclear if pornography use causes impairments in psychological wellbeing. The primary issue is the failure to control for potential confounding variables, or variables that may reasonably be expected to create spurious correlations between pornography use and psychological health. For example, it is unlikely that pornography use among adolescents causally contributes to impulsivity, poor family functioning, and delinquency —although all of these characteristics have been found to be associated with pornography use (Peter & Valkenburg, 2016; Wetterneck, Burgess, Short, Smith, & Cervantes, 2012) and most are likely connected to poor psychological health. A failure to control for such variables may contribute to the conflicting findings discussed above. Secondly, despite causal reasoning that underlies many theories that are employed when studying the presumed effects of pornography use (e.g., Social Cognitive theory, Sexual Scripting theories, Social Comparison theory, etc), the vast majority of research in this domain has employed cross-sectional designs. Although growing in number, there are still relatively few longitudinal studies of pornography use among adolescents which are helpful for uncovering evidence of antecedent order, and for obvious ethical reasons, no experimental studies (that we are aware of). Assuming that pornography use is responsible for impaired psychological wellbeing ignores the possibility that pornography use, as an entertainment medium, may be used intentionally to improve mood or adjust poor psychological health states. When pornography users are asked why they use pornography, aside from its obvious sexual gratification function, many people report using pornography to induce positive affect (e.g. use for entertainment) or to alleviate negative affective states such as boredom, stress, or depression (Bridges & Morokoff, 2011; Hempel, 2012; Lawrence & Herold, 1988; Perse, 1994), suggesting that negative mental states can precede, rather than follow, pornography use. Substantiating this possibility further, the only longitudinal analysis of the connection between pornography use and psychological wellbeing among adolescents reported that low life satisfaction predicted subsequent increases in pornography use over time (Peter & Valkenburg, 2011). Such evidence challenges unidirectional causal thinking in favor of transactional theories, such as the Differential Susceptibility to Media Effects model (Valkenburg & Peter, 2013), which articulate the causal interplay between pornography use and its presumed harms over time. **The Current Study** To address shortcomings in our understanding of the relationship between pornography use and psychological wellbeing among adolescents, we conducted a 6-wave longitudinal study that assessed pornography use, depression and anxiety, self-esteem, and well-being over 3 years in two independent panels of Croatian adolescents (Zagreb vs. Rijeka panels). Data from the Zagreb panel were used for exploratory analytic work that was not pre-registered before the analyses were run. These results, coupled with relevant theoretical work and other research findings were then used to pre-register specific hypotheses that were tested with the Rijeka panel.