The Evidence | The Equality Trust: "Great inequality is the scourge of modern societies. We provide the evidence on each of eleven different health and social problems: physical health, mental health, drug abuse, education, imprisonment, obesity, social mobility, trust and community life, violence, teenage births, and child well-being. For all eleven of these health and social problems, outcomes are very substantially worse in more unequal societies.
We have checked the relationships wherever possible in two independent test beds: internationally among the rich countries, and then again among the 50 states of the USA. In almost every case we find the same tendency for outcomes to be much worse in more unequal societies in both settings."
http://www.equalitytrust.org.uk/why/evidence