Nothing in biology makes sense except in the light of evolution.
Taking an evolutionary perspective permits a deeper appreciation of mental health and of psychiatric disorders. Are anxiety, mood swings and psychosis ever useful? If yes, when and why? It depends on timing, quantity, and fit.
Anxiety: BG (before genes are transmitted, i.e. before reproduction) keeps organism safe from harm; AG (after genes are transmitted, i.e. after reproduction) over-reacts to threats. Mood Swings: BG permit organism to strongly react short-term to abundance and penury; AG get organism into trouble long-term. Psychosis: BG permits organism to be creative; AG misfit in society.
Families with sub-normative (clinically ill members) also have supra-normative (highly successful members), which have fewer of the mutations, in different combinations, or with different environmental exposures. A little bit of mutations involved in disease (supra-normative) is better than nothing (“normal”), and better than too much (clinically ill). Family of high achievers in society contain psychiatric cases. The fortunate individuals should be grateful to, and not ashamed of, those less fortunate. They can give back by investing in and supporting prevention and mitigation strategies (see below).
A good fit with the environment maximizes organismal success. To ensure good fit with an environment, an organism can be flexible and adaptable; or it can move to a different environment that is a better fit; or the organism can actively change the environment around it to fit its needs. Usually it is a combination of the three strategies that leads to success. Understanding the characteristics and strengths of people with mental health risk factors (see above), and helping them maximize their environmental fit, is a win-win strategy for that individual and for society.
Live. Happier. Longer.