Recent investigations of the neuroanatomy of complex social behaviors suggest that the underlying brain circuits involve multiple cortical and subcortical structures. The neuroanatomic origins of agreeableness have not yet been clearly elucidated. However, frontotemporal dementia (FTD) patients can evidence dramatic alterations in agreeableness arising from frontal and temporal lobe damage. Based on previous research, we hypothesized that agreeableness would be negatively correlated with left medial orbitofrontal cortex size and positively correlated with right amygdala volume. First-degree relatives of 27 FTD patients (diagnosed according to the Lund-Manchester criteria) were asked to fill out the NEO-Five Factor Inventory to assess the patients’ current level of agreeableness, a construct comprised of the facets trust, straightforwardness, altruism, compliance, modesty, and tender-mindedness. These patients underwent T1-weighted MRI imaging, and gray matter volumes for right and left orbitofrontal lobes and amygdalas were derived via segmentation and region of interest tracing, normalizing for total intracranial volume. Regression analysis revealed that 38% of the variance in the NEO agreeableness score was predicted by a model in which right orbitofrontal volume (β = 0.731) was positively correlated with agreeableness, and left orbitofrontal lobe volume (β = –0.638) was negatively correlated with agreeableness (p < 0.01). Contrary to our hypothesis, amygdala volume did not significantly predict agreeableness. This finding partly replicates a previous study that used a different measure of social functioning, the Interpersonal Adjective Scale, to delineate a left frontal-right amygdala circuit for agreeableness. These data support the hypothesis that regulation of agreeableness arises from a balanced, mutually inhibitory circuit involving both hemispheres.
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