Predictors of several major mental disorders include strong genetic linkage that is modified by biological, psychological and social factors. Insight into the biological mechanisms underlying these interactions is growing. Several candidate genes have been identified but their predictive value is still limited for predictions in the single individual. Comorbidities are common, including combinations of different mental disorders and/or combinations of mental and physical disorders. The combination of a mental and physical disorder often indicates a worse outcome for the physical disorder.