The only way you can estimate environmental influences is by measuring them.

Conventional psychiatry has emphasized the genetic roots of psychosis based on the claim that twin and other studies show that schizophrenia is 80% heritable, which means that 80% of the cause is genetic.

There is also evidence from epidemiological studies that psychotic-like experiences are much more common than has hitherto been thought (with about 10% of the population affected) and that these experiences exist on continua with healthy or 'normal' functioning: instead of the world falling into two groups (the psychotic and the non-psychotic) people vary in their disposition to psychosis and only a minority of people who have these experiences require or seek help.

Childhood trauma is not necessarily a prophecy of doom, because some children are resilient or because later experiences help to restore mental health.

There are many kinds of adult adversities that can provoke severe psychological distress, including debt and unemployment, dysfunctional marital relationships and occupational stress. The reality is that the social causes of mental ill-health are all around us.

At the population level, making the world more just and less unequal, while trying to figure out the toxic aspects of the urban environmental will probably help prevent a lot of psychosis.

Countries with the best-resourced medical services have the best outcomes for physical illness (it is better to have a heart attack in Washington or London than in rural Africa) whereas precisely the opposite is the case for mental illness (developing nations with limited psychiatric resources have better outcomes and lower suicide rates).