Many human behavioral traits have both genetic and environmental components. Bip

Variations in the diagnosis criteria for bipolar disorder may make differing degrees of the disorder difficult to distinguish.  In addition, there may be certain variants, or genes with major versus minor input, so the combination of mutations in an individual may affect the severity of their phenotype. This also may affect the age of onset variability, which can make diagnosis difficult and could be affected by environmental factors such as hormones, stress, etc. There may also be variable response to medications, which could be a function of the particular mutation or the other genes that the individual possesses that may allow them to respond better to a particular treatment.