Homosexuality is often considered to be a “moral issue” in society, particularly in religious groups; whether it should be considered socially acceptable is an ongoing debate that has affected many. Recently, scientific evidence has suggested that there seem to be biological bases of homosexuality. This would mean that individuals do not have a choice regarding their sexual orientation, making it illogical for it to be considered a “sin.”. Whether this is true or not, its investigation is an important issue in the field of psychology, as it is evident that, while homosexuality greatly affects an individual’s personality and behavior, its origins are still unclear. In the study of the research question, current theories and corresponding evidence on homosexuality’s biological origins are investigated. The evidence discussed originates from a range of sources including journal articles, books, academic websites and psychological studies. These included twin studies, experiments, brain scans, observational studies, DNA linkage analysis and natural experiments. These allowed the researcher to evaluate the ways in which homosexuality can be attributed to biological factors and the way these interact with other elements. The biological factors analyzed were brain structures, the differences of the use of the brain between homosexuals and heterosexuals, and finally the fraternal birth-order effect. These biological factors are linked to the environmental factors in Bem’s exotic-becomes-erotic theory. Finally, included is an evaluation of these biological factors through the referencing of female sexuality and its apparent lack of biological causes. This paper concludes that the way in which biological factors contribute to the development of homosexuality lies in providing a predisposition that will later develop into an active sexual orientation in sexual maturity.