The first one is one that I didn’t find in my own time. It correlates heavy usage of TikTok with a decreased ability to block one’s own distracting thoughts. Certainly interesting, and worth further study, but the authors appear to have equated that correlation with a causal effect. They did not satisfactorily delineate between someone who has a poor attention span and is attracted to TikTok because of it and someone with a poor attention span caused by TikTok.

The second and third studies I have already addressed in my other comments. The second study being the Chinese one that demonstrated a correlation between heavy TikTok usage and memory loss, anxiety, stress, depression, etc. Again, important findings, but crucially not causal. The third is the meta analysis that refused to make a statements regarding detrimental effects of TikTok usage.

The fourth isn’t a study, it’s an article. This article does link to several studies, however the only one the directly mentions TikTok is, again, that same study of roughly 3,000 Chinese students. The rest of the studies mentioned are targeting social media use in general.