In other research, Witelson and research associate Debra Kigar, had found that left-handers have a larger region of the posterior corpus callosum -- the thick band of nerve fibres connecting the two hemispheres of the brain -- than right handers. This raised the hypothesis for the current study -- whether the anatomy of the brain of the sub-group of right-handed homosexual men is similar to that of left-handers. They found that the posterior part of the corpus callosum is larger in homosexual than heterosexual men. The size of the corpus callosum is largely inherited suggesting a genetic factor in sexual orientation, said Witelson "Our results do not mean that heredity is destiny but they do indicate that environment is not the only player in the field," she said.
The size of the corpus callosum may very well be argely inherited in the general population, but one should be cautious about extrapolating the heritability to an aytpical sample such as this. Genes which result in the normal human variation in the corpus callosum size may still have bearing on the trait variance among homosexual males. In fact, genetic variation on the loci responsible for the range in the normal population may also explain within group variation among homosexuals, who might be scaled differently because of the effect of another variable. Now, I'm not waving my hands and making the neurological features disappear. This is a real biological finding, but too often genetics has become so sexy that this subfield is recruited to "explain" any novel resut. Don't get me wrong, the concordance numbers from the twin data does suggest that something is going on which tracks genetic relationships, but the quest for the silver-bullet "gay gene" has come up with enough strikes that we should be a bit cautious about zeroing in in genetics instead of looking more broadly for other biological explanations.