Further Evidence of a Cohort Effect in Bipolar Disorder: More Early Onsets and Family History of Psychiatric Illness in More Recent Epochs

OBJECTIVE: Given that a cohort effect is rarely mentioned as one of the possible contributors to the increased incidence of childhood-onset bipolar disorder in the United States, we reexamined evidence for the phenomenon within our outpatient Bipolar Collaborative Network.

METHODS: 968 outpatients (mean age, 41 years) with DSM bipolar disorder from 1995 to 2002 from 4 sites in the United States and 3 in the Netherlands and Germany (abbreviated as Europe) gave informed consent and provided detailed demographic, illness, and family history information on a patient questionnaire. Family history of bipolar disorder, depression, suicide attempt, alcohol abuse, substance abuse, and "other" illness was collected for each parent and the 4 grandparents. Age at onset and family history of illness burden were then assessed as a function of the age of the patient at network entry.

RESULTS: Data for patients aged 35 years or older (n = 613) were included in the first analysis. Compared to older patients, those who were younger when they entered the network had an earlier age at onset of their bipolar disorder (r = 0.33, P < .001). Similarly, the youngest patients at entry (representing the most recent cohorts) had parents and grandparents with more psychiatric illness than patients born in earlier cohorts (n = 968).

CONCLUSIONS: These preliminary data, taken with the substantial literature, suggest a cohort effect for earlier age at onset of bipolar disorder and greater burden of psychiatric disorders in 2 generations of direct progenitors of our patients. The resulting likely increase in severity of bipolar illness in future generations based on this earlier age at onset and increased familial loading, particularly in the United States, deserves focused clinical and public health attention and attempts at amelioration.