Bipolar disorder is a condition marked by high and low mood swings. People with schizophrenia suffer from hallucinations and delusions.
But the conditions can have similar features -- depression and occasional bouts of psychosis or loss of contact with reality.
Paul Lichtenstein of Sweden's Karolinska Institute is the study's lead author. Lichtenstein says common symptoms have prompted a long-running debate about whether bipolar disorder and schizophrenia are related.
"It has been a debate for some one hundred years now whether there are two distinct psychotic disorders or whether they do share a common genetic etiology [i.e., origin]. And there has not been good genetic proof," he said.
Swedish researchers, led by Lichtenstein, combed through the medical records of nine million people from the country's population registry and identified nearly 36-thousand patients who had bipolar disorder between 1973 and 2004.
Investigators confirmed that people directly related to someone with the disorder were significantly more likely than the general population to develop the condition. The same was true for schizophrenia.
Researchers discovered that there was a significant overlap in the number of families who had members with both schizophrenia and bipolar disorder.
They found that in families with bipolar disorder, there was a 64 percent increased risk of developing schizophrenia.
Lichtenstein adds that finding common causes for the two diseases could help in developing treatments.
"I think it has importance for how physicians look at patients with psychic symptoms and disorders, and its [probable] impact on how we treat them in the future medically or behaviorally."
The study on the genetic link between bipolar disorder and schizophrenia was published this week in the journal The Lancet.