In the search for biological correlations with homosexuality, the hypothalamus, a small lobe that hangs down at the base of the brain has been found to play a significant role. There are two different areas of the hypothalamus that have been found in rats to correspond with male typical and female typical sexual behavior. For example, the medial preoptic area (more towards the front of the hypothalamus) seems to underlie many male sexual behaviors and sexual behaviors typical to females have been linked with the ventromedial nucleus (more towards the back of the hypothalamus). (A. Soulairac and M.L. Soulairac, 1956) Critics of these studies argue that the hypothalamus plays a very insignificant role in sexuality. When the medial preoptic area is decommissioned in male rats, they are still sexual beings. The express interest in female rats, but appear to be unable to express their interest. It doesn't seem to occur to these rats to mount the female. The case is somewhat similar with primates. Male rhesus monkeys will masturbate, showing an interest in pleasurable stimulation. However, having lost the medial preoptic area, they completely lose interest in females??seemingly forgetting that females can provide a means to the same end. (Slimp, Hart and Goy, 1978)

The hypothalamuses of mammals (rats, gerbils, macaque monkeys and others) have been found to be sexually dimorphic, more specifically in the medial preoptic area. The difference in size has been directly correlated with hormone levels in utero and directly following birth. If testosterone is given to a female rat just prior to and following birth, the size of the medial preoptic area of her hypothalamus will fall within the range of that found in non-treated male rats. (R.A. Gorski, J. H. Gordon, J. E. Shryne, and A. M. Southam, 1978) When female adult rats were given testosterone, there was no change in the size of this area of their brains.

This region is also different in humans, but the research involving hormone levels has not been done on humans (for obvious reasons). However, research, upon death, has delved into the sexually dimorphic regions of the brain. In 1980, Roger Gorski found that the interstitial nuclei of the anterior hypothalamus (INAH), numbers 2 and 3 (there are four of these nuclei, were sexually dimorphic. The size differential between males and females was most apparent in INAH 3. In males, this nucleus can be from two to three times larger than it is in females. This difference spans all age groups, meaning that the differentiation must occur at some time before birth.

http://www.goshen.edu/bio/Biol410/Sr...hristiana.html