Unaffected twins of patients with bipolar disorder show enhanced neurocognitive function and positive temperament traits, research shows.
These enhanced fitness-related traits may explain the persistence of the illness in the general population, say Tyrone Cannon (Yale University, New Haven, Connecticut, USA) and colleagues.
Writing in TheAmerican Journal of Psychiatry, the researchers explain that the persistence of severe mental illness is a “paradox that has long puzzled evolutionary psychologists and psychiatric geneticists.”
They therefore evaluated the presence of key fitness-related traits (temperament and neurocognitive abilities) in unaffected twins of patients with bipolar disorder, as well as schizophrenia patients and their unaffected twins as psychiatric controls.
The team used the Swedish Twin Registry to identify twin pairs discordant for bipolar disorder and schizophrenia and control twin pairs – 258 individuals in total.
Temperament analyses showed that unaffected twins of patients with bipolar disorder scored significantly higher on the positivity scale than their affected twins or controls. There was no significant difference between affected twins and controls.
By contrast, unaffected twins scored significantly lower than their affected twins on the negativity scale, but not significantly lower than controls.
When the researchers assessed neuropsychological features of the study participants they found that unaffected twins scored significantly higher on the verbal learning and fluency domain relative to controls. There were no significant differences for any other measures assessed.
Among the schizophrenia pairs, the unaffected twins of schizophrenia patients had similar results to their schizophrenic twins, although the differences relative to controls were less pronounced. They showed reduced positivity, compared with controls, and generalised reductions in neurocognitive functioning, contrasting with the specific differences observed for unaffected twins of bipolar patients.
The researchers say this suggests “segregation at the endophenotypic level between these two syndromes, at least with respect to temperament and neurocognitive functioning.”
The team concludes: “These findings contribute to mounting evidence linking creativity and enhanced cognitive functioning and risk for bipolar disorder and may inform models of mechanisms underpinning this association.
“That liability for bipolar disorder may confer temperament and neurocognitive benefits could provide one explanation for the genetic persistence of this illness.”
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