A new study suggests that if schizophrenia runs in a family, there's a good chance that bipolar disorder does as well (and vice versa). The findings, published today in the journal The Lancet, suggest that the two disorders are caused by some of the same genes.

"These findings say that [schizophrenia and bipolar disorder] are related, above all, for genetic reasons," says lead study author Paul Lichtenstein, a genetic epidemiologist at the Karolinska Institutet in Stockholm, Sweden. "[Therefore] it might not be a good idea to view these disorders as separate entities."

Lichtenstein and his colleagues (researchers from both the U.S. and Sweden) scoured the entire Swedish population for anyone with schizophrenia or bipolar disorder by reviewing psychiatric discharge data from all Swedish hospitals between 1973 and 2004.

They identified 35,985 people with schizophrenia (0.40 percent of the population) and 40,487 people with bipolar disorder (0.45 percent of the population). To figure out if and how some of these patients were related, the researchers searched for these individuals in Sweden's multi-generation register, a population database that links nearly every person born in Sweden (population: around 9 million) to his or her parents. This way, they were able to identify parents, children and siblings who shared the diseases.

They discovered that if mom or dad had schizophrenia, a child was 9.9 times more likely to have the disorder (characterized by delusions, hallucinations, and social withdrawal) as well as 5.2 times more likely to have bipolar disorder than someone without a schizophrenic parent. Conversely, someone with at least one parent with bipolar disorder (marked by extreme highs and lows in mood, or periods of mania and depression) was 6.4 times more likely to be bipolar and 2.4 times more likely to have schizophrenia (than those sans a bipolar parent).

Having a brother or sister with one of the psychiatric conditions also upped a person's odds of having both disorders. Those with schizophrenic siblings were 3.7 times more likely to be bipolar and 9 times more likely to have schizophrenia; those with bipolar siblings were 3.9 times more likely to have schizophrenia and 7.9 times more likely to be bipolar.

According to Lichtenstein, this data illustrates the extent to which these two disorders are genetically related. He speculates that hundreds if not thousands of genes are at the root of each disorder, about half of which may overlap.  But, as other researchers have pointed out, the vast majority of these genes are yet to be found.

All around the world  "there are large-scale genetic studies trying to look at genes responsible for causing these disorders," says Lichtenstein, who is currently conducting one of them. "We should look for overlap between not only schizophrenia and bipolar disorders (but also for other psychiatric illnesses such as depression)."

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