Mental health is one of the key parameters of the quality of social and personal environment we can think of, and changes in the prevalence and the impact of mental health are hugely important measures of socio-economic evolution. Here is a fascinating set of data visualizations and analytical notes on the state of mental health around the world and across the recent decades: https://ourworldindata.org/mental-health.
There are thousands of interesting, and often non-intuitive, observations one can draw from these interactive charts and from the published analysis.
