I think class/education i.e. social capital is a major factor. Consider that the cohort in this study (linked elsewhere in this thread) are all graduate students. I expect you'd find the effect sizes of this study would significantly increase given a sample of college dropouts rather than graduates.

I'm a dropout from a great Philosophy course: I had pre-existing problems with substance abuse, and addiction vendors like Reddit and YouTube allowed me to pour away my attention in a similar way when the coursework load (and my stress) increased. Smartphones provide yet another vector for existing psychopathologies to play themselves out.

This vindictive industry damages social cohesion and changes destinies. I now sell second-hand phones in a major UK city, and my experience is that surveillance capitalism/the digital addiction industry is cleaving out a new underclass from the working class: as a child, your membership is determined by whether or not your parents are addicted to smartphones.