> Why would you say that? Do you think that rich people are happy and poor people are depressed?

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...