Clinical depression and bipolar disorder are currently classified as separate illnesses. Some researchers increasingly view them as part of an overlapping spectrum that also includes anxiety and psychosis.

The National Comorbidity Survey replication is a study concerning international and U.S. rates of bipolar spectrum disorder. There are two audio talks. The first talk is entitled "The Bipolar Spectrum: Epidemiology and Clinical Perspectives," by Kathleen Merikangas Ph.D. of the NIMH 1st talk. The second talk is entitled "Prevalence and Effects of Mood Disorders on Role Performance in the United States," by Ronald Kessler Ph.D. from Harvard Medical School 2nd talk.

According to Hagop Akiskal, M.D., at the one end of the spectrum is bipolar type schizoaffective disorder, and at the other end is unipolar depression (recurrent or not recurrent), with the anxiety disorders present across the spectrum. Also included in this view is premenstrual dysphoric disorder, postpartum depression, and postpartum psychosis. This view helps to explain why many people who have the illness do not have first-degree relatives with clear-cut "bipolar disorder", but who have family members with a history of these other disorders.

In a 2003 study, Hagop Akiskal M.D. and Lew Judd M.D. re-examined data from the landmark Epidemiologic Catchment Area study from two decades before. The original study found that 0.8 percent of the population surveyed had experienced a manic episode at least once (the diagnostic threshold for bipolar I) and 0.5 a hypomanic episode (the diagnostic threshold for bipolar II).

By tabulating survey responses to include sub-threshold diagnostic criteria, such as one or two symptoms over a short time-period, the authors arrived at an additional 5.1 percent of the population, adding up to a total of 6.4 percent of the entire population who can be thought of as having a bipolar spectrum disorder. This and similar recent studies have been interpreted by some prominent bipolar disorders researchers as evidence for a much higher prevalence of bipolar disorders in the general population than previously thought.

However these re-analyses should be interpreted cautiously because of substantive as well as methodological study limitations. Indeed, prevalence studies of bipolar disorder are carried out by lay interviewers (that is, not by expert clinicians/psychiatrists who are more costly to employ) who follow fully structured/fixed interview schemes; responses to single items from such interviews may suffer limited validity.

Furthermore, a well-known statistical problem arises when ascertaining disorders and conditions with a relatively low population prevalence or base-rate, such as bipolar disorder: even assuming that lay interviews diagnoses are highly accurate in terms of sensitivity and specificity and their corresponding area under the ROC curve (that is, AUC, or area under the receiver operating characteristic curve), a condition with a relatively low prevalence or base-rate is bound to yield high false positive rates, which exceed false negative rates; in such a circumstance a limited positive predictive value, PPV, yields high false positive rates even in presence of a specificity which is very close to 100%. To simplify, it can be said that a very small error applied over a very large number of individuals (that is, those who are *not affected* by the condition in the general population during their lifetime; for example, over 95%) produces a relevant, non-negligible number of subjects who are incorrectly classified as having the disorder or any other condition which is the object of a survey study: these subjects are the so-called false positives; such reasoning applies to the 'false positive' but not the 'false negative' problem where we have an error applied over a relatively very small number of individuals to begin with (that is, those who are *affected* by the condition in the general population; for example, less than 5%). Hence, a very high percentage of subjects who seem to have a history of bipolar disorder at the interview are false positives for such a medical condition and apparently never suffered a fully clinical syndrome (that is, bipolar disorder type I): the population prevalence of bipolar disorder type I, which includes at least a lifetime manic episode, continues to be estimated at 1%.

A different but related problem in evaluating the public health significance of psychiatric conditions has been highlighted by Robert Spitzer of Columbia University: fulfillment of diagnostic criteria and the resulting diagnosis do not necessarily imply need for treatment. As a consequence, subjects who experience bipolar symptoms but not a full-blown, impairing bipolar syndrome should not be automatically considered as patients in need of treatment.

Recent studies have indicated that at least 50% of adult sufferers report manifestation of symptoms before the age of 17. Moreover, there is a growing consensus that bipolar disorder originates in childhood. In young children the illness is now referred to as pediatric bipolar disorder. Today about 0.5% of children under 18 are believed to have the condition. For children, the main concern is that bipolar disorder needs to be diagnosed correctly and treated properly because it can look like unipolar depression, ADHD, or conduct disorder. If a child with bipolar disorder is misdiagnosed and treated with antidepressants or stimulants, the child may become violent, suicidal, homicidal, or otherwise severely destabilized. Young children, adolescents and adults each express the illness differently according to child and adolescent bipolar disorders expert Demitri Papolos M.D. and the Child and Adolescent Bipolar Foundation. There is, however, controversy about this last point.

Bipolar disorder manifests in late life as well. Some individuals with hyperthymic temperament (or "hypomanic" personality style) who experience depression in later life appear to have a form of bipolar disorder. Much more needs to be elucidated about late-life bipolar disorder.

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