The many attempts to identify genes for bipolar disorder (BD) have met with limited success, which has generally been attributed to genetic heterogeneity and small gene effects. However, it is also possible that the categorical phenotypes used in genetic studies of BD are not the most informative or biologically relevant. Although quantitative phenotypes provide an alternative to categorical phenotypes based on diagnosis, they have not been fully exploited in BD genetics due to the lack of accessible biological measures. We have explored aspects of temperament as quantitative phenotypes that might define subtypes of BD with different clinical features and courses of illness. Temperament is a heritable personality factor that establishes the baseline level of reactivity, mood, and energy of a person.
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