Studies have highlighted structural brain differences between heterosexual and homosexual males, particularly in the anterior hypothalamus, and suprachiasmatic nuclei, which does support a biological basis for homosexuality. One study (Zhou, 1995) concluded: “This postnatal period of hypothalamic differentiation indicates that, in addition to genetic factors, a multitude of environmental and psychosocial factors may have a profound influence on the sexual differentiation of the brain.”  (http://www.glreview.org/article/evolutionary-origins-of-homosexuality/)