The dopamine hypothesis suggests that dopamine over-activity causes schizophrenia. Dopamine over-activity could happen in one of the following ways:

Nature Nurture Debate: Born Lazy? Don't like going to the gym? Hate exercise? Now there's an excuse: new studies on mice have shown that the desire to exercise may be largely determined by genetic factors. This suggests that some people may find it easy to increase their activity levels to get healthy, while others will struggle.