There are numerous studies showing this, for almost all disease categories. All psychological illness is negatively corelated with socioeconomic status.
https://en.wikipedia.org/wiki/Socioeconomic_status
I'd say it's hard to find good sources of nutrition and the time to exercise when you're part of the lower class.
edit: negatively correlated...
Also depression. It took me one minute to find three sources, could you please make some effort here? I have other things to do.
And you're right, it's not a causative factor necessarily, but I think there's pretty strong evidence at this point, it's hard to find a poor person who doesn't have some condition.
[1] https://www.sciencedirect.com/science/article/abs/pii/S00223...
[2] https://psycnet.apa.org/doiLanding?doi=10.1037/0021-843X.111...
[3] https://ajph.aphapublications.org/doi/full/10.2105/AJPH.93.1...
Why would you say that? Do you think that rich people are happy and poor people are depressed? Anecdotally, the poorest people I know are often the happiest and the richest people are often the most miserable. I was a whole lot happier when I was younger and poorer. One thing has nothing to do with the other.