The heritability and molecular genetics of mental disorders

People differ in their feelings, emotions, and behavior, and for centuries, scholars have been interested in the sources of this variation. Modern research in the last four decades, using family and twin designs, has shown that variation in mental health and disorders is explained by both genetic and environmental variation. The influence of genetic variation (i.e., heritability) on mental disorders differs. Neurodevelopmental disorders are among the most heritable disorders while internalizing disorders, like major depressive disorder, show lower heritability estimates. Genome-wide association studies (GWAS) are used to search the whole genome for associations between genetic variants and mental disorders. These analyses have resulted in multiple associations between particular genetic variants and mental disorders; however, genetic variants explain only a relatively small proportion of variation in mental disorders, and more research is needed to unravel their biological function. GWAS results confirmed that different mental disorders have many genetic variants in common, which may explain the co-occurrence of mental disorders and the overlap in disorder symptoms that is often observed. Finally, GWAS results can be used to obtain polygenic scores that provide an indication of the degree of genetic risk for mental disorders that may become of clinical utility in the near future.