There is evidence that people do worse on average across a variety of metrics in more unequal societies.

Naturally this is disputed by those on the right and implicitly believed by those on the left.

See the book "The Spirit Level" and subsequent discussions around that for more info.

Ignoring study, that would be a more graphic example: https://www.youtube.com/watch?v=meiU6TxysCg

Or historically, if this wasn't true we would be the happiest humans to ever live on earth. Doesn't seem to be true on any possible metric.