A previous study also found a link between high test scores and a greater risk of bipolar disorder. Dr. Jari Tiihonen and colleagues at the University of Kuopio, Finland, looked at different aspects of intelligence among people who go on to develop bipolar disorder.
They analyzed test results from 195,019 apparently healthy males conscripted into the Finnish Defense Forces. In Finland, all men serve for 6, 9 or 12 months at around 20 years of age.
The 100 participants who went on to have bipolar disorder had significantly higher scores for “arithmetic reasoning.” A high score was associated with a more than 12-fold greater risk, report the researchers.
“The finding of an association between progressively increasing risk of bipolar disorder and high arithmetic intellectual performance is rather surprising,” they write. “The arithmetic test not only requires mathematical skills but also rapid information processing, since the limited amount of time for solving the tasks allows only a small percentage of subjects to finish the test.
“It is plausible to assume that subjects having the ability to rapidly process information may share the same neurobiological characteristics as subjects who develop mania, a state characterized by high alertness and psychomotor activity. It is tempting to speculate that good arithmetic or psychomotor performance may have contributed in human evolution to the persistence of bipolar disorder, which is strongly genetically transmitted and associated with a high mortality rate.”
Nevertheless, the majority of previous studies that have measured intelligence in relation to bipolar disorder have found no significant difference compared with the general population. In fact, “cognitive impairment consistent with deficits in IQ” has been reported during acute episodes of mania and depression, reports Dr. Katherine E. Burdick of the North-Shore-Long Island Jewish Health System, New York.
She writes, “There are a handful of studies that have reported impairment in current IQ performance in bipolar patients; however, when premorbid [before the illness] intellectual capacity has been evaluated, bipolar patients have consistently demonstrated performance comparable to control subjects.
“These data suggest that IQ deficits in bipolar patients are likely to reflect a decline in functioning due to the onset of the disease, and more specifically due to the onset of psychosis.”
Other studies indicate that a higher pre-illness IQ may be a protective factor against the psychotic form of bipolar disorder, whereas lower IQ is often associated with developing psychotic bipolar disorder. A great deal of research is being carried out in this area, with the aim of fully understanding how this illness is linked to intelligence.
Dr. Stanley Zammit of the University Hospital of Wales, Cardiff, UK, concludes, “Premorbid IQ is likely to be a risk factor for psychotic illnesses in general. However, it does not seem to have an effect on risk of developing bipolar disorder.”
He believes this indicates different pathways of causality for bipolar disorder to those for schizophrenia, psychoses, and severe depression.
MacCabe, J. H. et al. Excellent school performance at age 16 and risk of adult bipolar disorder: national cohort study. British Journal of Psychiatry, Vol. 196, February 2010, pp. 109-15.
Tiihonen, J. et al. Premorbid intellectual functioning in bipolar disorder and schizophrenia: results from a cohort study of male conscripts. The American Journal of Psychiatry, Vol. 162, October 2005, pp. 1904-10.
Burdick, K. E. et al. The role of general intelligence as an intermediate phenotype for neuropsychiatric disorders. Cognitive Neuropsychiatry, Vol. 14, July 2009, pp. 299-311.
Zammit, S. et al. A longitudinal study of premorbid IQ score and risk of developing schizophrenia, bipolar disorder, severe depression, and other nonaffective psychoses. Archives of General Psychiatry, Vol. 61, April 2004, pp.354-60.