Social struggle is quite different than the day-to-day struggle to survive that was the default until recently on the human scale. Not sure that needs a reference.
There has been some work on it. There is a higher prevalence of depression in our modern society. Other societies facing more struggles are less prone to depression. It makes sense, if you think about it - if you're struggling to feed yourself and your family, you're not happy, but you're also not depressed. You haven't got time to be depressed.
Of course, there's other interpretations: people with depression would have not bothered to get up to feed themselves, because what's the point? They would be early victims of whatever violence the society was under.
I've seen similar while travelling. Poor societies with better family connections are less depressed. Mentally unwell people are looked after by their families. Everyone seems happier and more content with their lives, despite being materially poorer.
Returning to Australia after a year away in Cambodia, my first impression was "why is everyone so unhappy and angry?". It's strange. We have everything, yet we're unhappy.
I’ve never heard this before, and that’s a bold claim to make so matter-of-fact without a reference. Do you have one?