We have a depressing state in America where you can predict the parents’ income based on whether their kids’ school bans smartphones.

If you want to measure something for this measure happiness or strength of social circles. Good luck with that.

> our results indicate that there is an improvement in student performance of 6.41% of a standard deviation in schools that have introduced a mobile phone ban.

> Finally, we find that mobile phone bans have very different effects on different types of students. Banning mobile phones improves outcomes for the low-achieving students (14.23% of a standard deviation) the most and has no significant impact on high achievers. The results suggest that low-achieving students are more likely to be distracted by the presence of mobile phones, while high achievers can focus in the classroom regardless of whether phones are present.

https://cep.lse.ac.uk/pubs/download/dp1350.pdf

I believe OECD and Pisa results have also pointed towards banning as a net postive since their 2022 report.

I think it's fair to say that it's not a "black-and-white" thing. As the research points out, digital devices aren't the only factor in the equation. I believe OECD research has also found that using a digital device with a parent can be a benefit while using it alone will most certainly be a negative for children aged 2-6. I'm sure you can imagine why there might also be other factors that make a difference between parents who can spend time with their children and those who can't.

Aside from that there are also benefits from digital devices for students with learning disabilities like dyslexia. In most class-rooms this can be solved by computers + headphones, but for crafts people (I'm not sure what the English word for a school that teaches plumbers, carpenters etc. is), having a mobile phone in the workshop can often help a lot with insturctions, manuals and such.

So it's not clear cut, but over all, banning phones and smartwatches seem to be a great idea.

I'm not sure what you were hoping to achieve with the request for evidence, but what you're asking is not yet subject to a longitudinal study. The move has certainly been praised by educators, and that should be enough given it's the first or second year year of implementation in many cases, and what they are advocating for isn't a social taboo, nor draconian.