ABSTRACT
The results of several studies have shown that homosexual men have an increased prevalence of non-right-handedness and atypical patterns of hemispheric functional asymmetry. Non-right-handedness in men has been associated with increased size of the corpus callosum (CC), particularly of the isthmus, which is the posterior region of the callosal body connecting parietotemporal cortical regions. We hypothesized that isthmal area would be greater in homosexual men, even among right handers. Twelve homosexual and ten heterosexual healthy young men, all consistently right-handed, underwent a research-designed magnetic resonance imaging scan. We found that the isthmal area was larger in the homosexual group, adding to the body of findings of structural brain differences between homosexual and heterosexual men. This result suggests that right-handed homosexual men have less marked functional asymmetry compared to right-handed heterosexual men. The results also indicate that callosal anatomy and laterality for motoric functions are dissociated in homosexual men. A logistic regression analysis to predict sexual orientation category correctly classified 21 of the 22 men (96% correct classification) based on area of the callosal isthmus, a left-hand performance measure, water level test score, and a measure of abstraction ability. Our findings indicate that neuroanatomical structure and cognition are associated with sexual orientation in men and support the hypothesis of a neurobiological basis in the origin of sexual orientation.
Subject(s)
Corpus Callosum/anatomy & histology , Functional Laterality/physiology , Heterosexuality/physiology , Homosexuality, Male , Adult , Corpus Callosum/physiology , Humans , Logistic Models , Magnetic Resonance Imaging , Male , Organ Size , Reference ValuesABSTRACT
The factors determining the large variation seen in human corpus callosum (CC) morphology are as yet unknown. In this study heritability of CC size was assessed by comparing the concordance of CC midsagittal area in 14 monozygotic and 12 dizygotic twin pairs with a mean age of 27 years, using magnetic resonance imaging and various methods of calculating trait heritability. Heritability was high regardless of method of assessment. The application of a structural equation model resulted in the estimate that 94% of the variance in CC midsagittal size is attributable to the genome. This indicates that under normal conditions and before the effects of normal aging, there is very modest influence of the environment on CC morphology. The results suggest that correlates of CC size, such as the pattern of cerebral lateralization, cognitive abilities and neuropsychiatric dysfunction may be associated with the genetic determinants of CC morphology.