Adoption studies compare the rates of bipolar disorder among adoptees, their biological parents, and their adoptive parents.  These studies usually find that there is a greater risk for bipolar disorder among the biological parents of bipolar disorder adoptees compared to the risk in the adoptive parents.  The effect size of this result is somewhat lower than that seen in family studies which suggests that there is a significant environmental component.
 

Mendlewicz and Rainer (1977) researched 29 bipolar and 22 normal adoptees and 31 bipolar non-adoptees.  They found that there was a significantly greater risk of bipolar disorder in biological parents of bipolar adoptees (18%) compared with adoptive parents (7%).  The risk of biological parents of bipolar adoptees was similar to risk of relatives of bipolar non-adoptees.  This shows further evidence for a genetic component of bipolar disorder.