Over half of the world’s population currently lives in cities, with that number expected to grow to 66% by the year 2050
http://www.un.org/en/development/desa/news/population/world-urbanization-prospects-2014.html

mental disorders from city living:
article- https://www.scientificamerican.com/article/does-city-life-pose-a-risk-to-mental-health/
studies: The current status of urban-rural differences in psychiatric disorders- https://www.gwern.net/docs/nature/2010-peen.pdf
Meta-analysis of the association of urbanicity with schizophrenia- https://www.ncbi.nlm.nih.gov/pubmed/23015685
Urban-rural differences in incidence rates of psychiatric disorders in Denmark- https://www.ncbi.nlm.nih.gov/pubmed/26678865
The latter two I accessed via a school account, I couldn't find a public source to list here.

Does population density make a person more liberal?
just a random person's project, but combines two legitimate sources very clearly- http://overflow.solutions/special-projects/does-population-density-make-a-city-more-liberal/

Escaping the Laboratory: The Rodent Experiments of John B. Calhoun & Their Cultural Influence
http://eprints.lse.ac.uk/22514/1/2308Ramadams.pdf

article on voting data and liberalism in cities- https://www.theatlantic.com/politics/archive/2012/11/red-state-blue-city-how-the-urban-rural-divide-is-splitting-america/265686/