“Lithium in Ground-Water and Well-Being”, 2010-10-14 (; backlinks; similar):
Lithium is a well-known mood stabilizer & suicide preventive; some research suggests lithium may be a cognitively-protective nutrient and at the population level, chronic lithium consumption (through drinking water) predicts lower levels of mental illness, violence, & suicide.
The metal lithium is a well-known mood stabilizer & suicide preventive widely used in psychiatry. It is also a trace mineral present to various levels in all drinking water and much food. A long-running but obscure vein of research speculates on whether lithium is beneficial and a nutrient, specifically, cognitively-protective. Epidemiological research has correlated chronic lithium consumption through drinking water with a number of population-level variables like rates of mental illness, violence, & suicide. If causal, lithium should be regarded as a vital nutrient for mental health and added to drinking water to substantially improve population-wide outcomes.
However, the evidence is weak. Most of this research is cross-sectional, only some is longitudinal, none offers particularly strong causal evidence using natural experiments or other designs, there are questions about confounding with autocorrelated spatial properties such as altitude, and some of the best research, using Scandinavian population registries, offers more mixed evaluations of claimed correlates.
It is unlikely that further such correlational research will resolve the debate, despite the mounting opportunity cost. I suggest that formal experimentation is required, and concerns about harms from lithium supplementation making experiments ‘unethical’ can be circumvented by instead removing lithium or looking for natural experiments with cause changes (such as changes or upgrades to water treatment plants or plumbing modify lithium concentration).