The sexual orientation of most people is heterosexual: their romantic interest is directed towards individuals of the opposite sex. However, a notable percentage of people, usually around 5-10%, have a homosexual sexual orientation. Why does this happen? What ultimately explains the phenomenon of homosexuality? The search for answers has always been obscured by prejudices and misunderstandings, e.g., that homosexuality is a disease we can cure or a lifestyle choice we make.

The first attempt at a rational approach to the phenomenon was based on the idea that sexual orientation in humans is shaped in childhood by upbringing and the social environment. This view is the most widely accepted today. However, the recent development of biology and neuroscience has allowed for a completely different approach.

Homosexual individuals, during their fetal life and immediately after birth, experienced an unusual endocrine environment, which led not only to their particular sexual orientation but also to differences in behavior and physiology. For Jacques Balthazart, the author of the book and a leading proponent of this view, homosexuality is not a matter of choice, is not a disease or a perversion, nor is it due to indifferent parents or a problematic social environment. It is yet another instance of the phenomenon of biological diversity.