Though scientists are still unsure about the causes of autism spectrum disorder, a multinational team of researchers believe that they have linked the condition with schizophrenia and bipolar disorder. The research may help scientists better understand the genetic components behind mental illness, and may prove to be a missing link.

The study was conducted by a multinational team of researchers from England, Israel, Sweden, and the United States. They examined three different studies, two undertaken in Sweden and one conducted in Israel. Researchers found that people with autism spectrum disorder are significantly more likely to have a relative with schizophrenia in the family than other people. In fact, one study found that children with autism spectrum disorder were 12 times more likely to have a sibling with schizophrenia. A similar, though weaker, link also existed for bipolar disorder.

At first glance, autism spectrum disorder and schizophrenia may seem like conditions that have nothing in common. But researchers point out that both disorders are marked by a lowered ability to function cognitively and socially, as well as increased difficulty living a normal life alone.

Indeed, autism spectrum disorder and schizophrenia were not always seen as so separate. The authors write in the study, "Historically, ASD [autism spectrum disorder] was often regarded as childhood schizophrenia because the impaired social interactions and bizarre behavior found in ASD were reminiscent of symptoms of schizophrenia. Indeed, the psychiatrist who coined the term schizophrenia counted autism (an active turning away from the external world) as an important distinguishing feature of schizophrenia." It was not until the 1980s that researchers separated the two disorders into disparate conditions.

Incidences of autism spectrum disorder have risen tenfold since then in the United State. The CDC reports that between 1 in 80 and 1 in 240 children have the disorder.

The study was published in the Archives of General Psychiatry.