Discusses scientific research into gender identity, looks at various gender systems from antiquity to contemporary cultures, and examines common myths and assumptions about trans individuals.

Introduction -- Why understanding transsexualism and transgenderism is important -- Definitions and population frequencies -- Historical and contemporary cultures -- Genetic causal factor in transsexualism and transgenderism -- Epigenetic causal factor in transsexualism and transgenderism -- The two-factor theory of transsexualism and transgenderism causation -- Childhood and transsexualism-transgenderism -- Adolescence, young adulthood, and transsexualism-transgenderism -- Neuroanatomy and neurophysiology -- Conscious choice and spirituality -- Psychopathology -- Transition procedures and outcomes -- Conclusion.