Randomized controlled trials (RCTs) and person-level observational studies feature prominently in debates over social media harms. I highlight some under-acknowledged limitations of such evidence. Most important is that published RCTs typically identify effects of a \textit{local}, or small-scale, intervention: a person is assigned to quit social media, but her immediate peers continue using it in large numbers. Critics of social media, in contrast, focus on a \textit{global}, or large-scale, intervention: the mass adoption of social media among U.S. teenagers. Such global interventions alter both the proximal social environment and the broader culture, potentially harming teenagers who abstain from social media entirely. This paper discusses the local--global distinction at length and offers other notes on the limits of learning about social media harms from existing RCTs and person-level observational studies. I suggest that triangulating different forms of imperfect evidence may provide the deepest insights about social media's aggregate effect on teen mental health.

翻译：随机对照试验（RCTs）和个体层面观察性研究在关于社交媒体危害的辩论中占据重要地位。本文强调了此类证据中一些未被充分认识到的局限性。其中最重要的是，已发表的随机对照试验通常仅识别出局部或小规模干预的效果：即个体被要求退出社交媒体，但其身边的同龄人仍大量使用。相比之下，批评社交媒体者关注的是全局性或大规模干预——即美国青少年大规模采用社交媒体。这类全局性干预同时改变了近端社会环境与更广泛的文化，可能对完全不用社交媒体的青少年造成伤害。本文详细探讨了局部-全局差异，并就现有随机对照试验和个体层面观察性研究在理解社交媒体危害方面的局限性提出其他见解。我建议，整合不同类型不完美证据的三方验证法，或许能对社交媒体对青少年心理健康的总体影响提供最深刻的洞察。