Why are some men attracted sexually to other men, rather than to women? Since one of the most powerful forces in nature is the drive to perpetuate the species, the origin of this perverse sexual orientation is clearly of great interest to biologists. In A Separate Creation (Hyperion, 354 pages, $ 24.95), Chandler Burr provides a riveting depiction of the latest scientific investigations of the subject. Using the tools and techniques of molecular biology and guided by a theoretical framework encompassing endocrinology, genetics, and neuroanatomy, this research is rapidly advancing against a backdrop of heated debate among scientists and fierce controversy among laymen.

As presented by Burr, these are the main points that have been determined so far: Homosexual orientation is found in a small fraction of the male population (probably somewhere between 2 and 5 percent); it is probably fixed at an early stage of fetal development by some biological mechanism dependent on the flow of hormones; that mechanism is directed in some measure by a wayward gene on the single X chromosome men inherit from their mother. Burr also points out that while homosexual orientation is virtually impossible to alter, it neither compels homosexual behavior nor precludes heterosexual activity, because human beings possess free will.