For decades researchers have investigated the human sex, and it seems well documented how the sex chromosome together with the sex steroid hormones effect both the human sexual phenotype, together with the prenatal sexual differentiation of the brain.
Up until now, numerous studies have searched for an explanation to which biological factors are involved in human sexual orientation and the research field has been very broad. There have been studies of both twins and families to show that same-sex sexual behavior is partly genetically influenced, and recently a genome-wide association study has been performed, in the search for genes involved in sexual orientation. In neuroscience, both postmortem studies and neuroimaging studies have investigated morphological and structural differences between men and women; and between men with different sexual preferences. Research has also been conducted in particular areas of the brain, which we know are involved in processing sensory inputs and sexual behavior, although this sort of research has mostly been performed on animals. Due to the observation, that homosexual men often have older brothers, the fraternal birth order and maternal immune hypothesis of male homosexuality has been presented. However, despite all of the above listed types of studies, there is still no meaningful tool to predict an individual’s sexual behavior.