Is Schizophrenia Inherited?

We have known for some time, from family studies done in Europe in the first half of the 1900s, that schizophrenia runs in families. These studies found the risks for the parents, brothers, and sisters of people with schizophrenia to be be­tween 4% and 14%— on average about ten times as high as that for the general population. For children of those with schizophrenia, the risk was 12%, nearly 15 times the general population risk. When both parents had schizophrenia, the risk increased to about 40%. The risk to uncles and aunts, nephews and nieces, grandchildren, and half- brothers or -sisters was roughly three times the risk in the general population. This risk was much lower than the risk to the relatives in the immediate family circle. On the whole, these pioneering studies showed thatthe closer the blood relationship of a person to an individual with schizophrenia, the higher the risk for the disorder.Modern studies using morestrict research methods and reliable definitions of schizophrenia also found the disorder to run in families. However, they find risk estimates that are a bit lower than those found in earlier studies. For ex­ample, in a large family study from Iowa, Tsuang and his team reported the risk of schizophrenia to brothers and sisters of individuals with schizophreniato be about 3%. This level of risk was five times greater than the risk to relatives of persons without schizophrenia. Other modern studies found similar results. Diagnostic practises seem to play a strong role in these different estimates. The early European studies usually used a fairly broad definition of the illness while the modern studies used a strict criterion- based diagnosis developed for use in research. Indeed, modern researchers have noted that their family risk figures for schizophrenia are similar to the figures obtained by the earlier European studies when atypical cases are included in the definition of schizophrenia. In other words, the level of risk that we can pin on a family history of schizophrenia depends on how broadly we define the disorder and whether or not we include atypical cases and spectrum disorders.