Christian LifeSkills
For Personal & Spiritual Growth

MENTAL DISORDERS AND GENETICS: WHAT WE KNOW TODAY

Mental Illness

Mental illnesses profoundly affect an individual's ability to think, feel, and act. They are also very common, affecting as many as one in five Americans over their lifetimes, irrespective of age, gender, or race. Four percent of the Nation's population lives with severe mental illnesses. The annual cost to the United States for treatment, social service and disability payments, lost productivity, and premature mortality is more than $150 billion.¹ The diagnosis, treatment, and prevention of mental illnesses continue to be crucial to improving the quality of life for affected individuals, as well as to reduce health care costs.

Researchers and clinicians have worked for decades to reduce the suffering of those with these disabling disorders, and current treatments can alleviate symptoms for many. Unfortunately, none of these treatments offer sustained relief. Better treatments depend on discovering the causes of these disorders.

Although mental disorders were recognized as illnesses in the mid-18th century, suspicion and fear often overshadowed understanding. Gradually, trepidation has been replaced by knowledge as the fields of psychiatry, behavioral science, neuroscience, biology, and genetics have progressed. Through research conducted in each of these domains, a shared finding arises: the risk of developing an illness is increased if another family member is similarly affected, suggesting a strong hereditary component.

This finding of familial risk has been documented through twin studies, which use two types of twin pairs for exploring the role of inheritance. Identical or monozygotic twins come from the same fertilized egg and share 100 percent of their genes.² Fraternal or dizygotic twins come from two different fertilized eggs and share only 50 percent of their genes, just as any biological siblings would. To evaluate heritability, the rate of the disorder in monozygotic twins is compared with the rate in dizygotic twins. If the rate among monozygotic twins is significantly higher, then heredity is an important factor. For instance, in bipolar disorder, if one monozygotic twin is affected, then the other has a 60 to 80 percent chance of also having the disorder. In contrast, a dizygotic twin of an affected individual has only an 8 percent chance of having the disorder. Similarly, a monozygotic twin of a person with schizophrenia has a 46 percent chance of being affected, whereas a dizygotic twin has only a 14 percent chance of being affected.

Despite strong evidence for genetic susceptibility, no specific gene has been unambiguously identified for common forms of mental disorders. Many researchers believe that this is due, in part, to the critical role that the environment plays in modulating genetic susceptibility in mental disorders. Citing the twin studies above, researchers point out that monozygotic twins are not always concordant (i.e., do not share the disorder). Clearly, if twins with the same genes do not both have the disorder, there is strong evidence for the role of environmental factors.

Researchers may differ on their estimates of the amount that genes and the environment each contribute to the onset of mental disorders, but once a genetic component is reliably implicated, the search for the source and the location of the apparent genetic component underlying the mental disorder can begin. The estimate of the influence of environmental factors on the disorder provides an index of how difficult the search will be.

            Report provided by the National Institute of Mental Health