Studies on diagnostic subtypes of gender identity disorder (GID) or gender incongruence (GI), comorbidity and treatment outcome show considerable variability in results. Clinic/country specific factors may account for the contradictory results, but these factors have never been studied. This article is the first of a series reporting on a unique collaborative study of four European gender identity clinics (the European network for the investigation of gender incongruence [ENIGI]). Here, we present the diagnostic procedures of the four clinics (Amsterdam, Ghent, Hamburg, and Oslo), the standard battery of instruments, and the first results regarding applicants with GI who seek treatment. Applicants in the four clinics did not differ in living situation, employment status, sexual orientation, and age of onset of GI feelings. However, the Amsterdam and Ghent clinic were visited by a majority of natal males, whereas Hamburg and Oslo see more natal females. Male applicants were older than female applicants within each country, but female applicants in one country were sometimes older than male applicants in another country. Also, educational level differed between applicants of the four clinics. These data indicate that certain sociodemographic and/or cultural characteristics of applicants have to be taken into account in future studies.

To develop a philosophical framework to address the specification of sex/gender in humans, I will begin by summarizing what we know about the biology of sex determination in human beings. Basically, sex/gender is specified by several networks of genes anchored in the genetic interaction between the two genes, Sry and Sox9. Next, I will propose that this biological mechanism is best understood within a philosophical anthropology that embraces insights taken from systems biology to articulate a hylomorphism that explains the integrity, dynamism, and teleology of the human organism. The systems perspective described here represents one attempt to reformulate the received philosophical framework of classical Aristotelian-Thomistic hylomorphism so that it incorporates the insights of modern biology. Finally, I will use the systems perspective and key principles articulated by St. Thomas Aquinas in his philosophy of nature to identify criteria that could be used to specify the sex/gender of a particular human being: The most certain criterion for maleness would be the capacity to produce sperm while the most certain criterion for femaleness would be the complementary capacity to produce eggs. Deviations from this criterion would decrease the certitude of our judgment regarding the sex/gender of the individual.

Handedness preference was assessed in 205 boys with gender identity disorder and 205 clinical control boys referred for other reasons. Boys with gender identity disorder were significantly more likely to be left-handed than the clinical control boys (19.5% vs. 8.3%, respectively). The boys with gender identity disorder, but not the clinical control boys, also had a significantly higher rate of left-handedness compared to three independent, general population studies of nonreferred boys (11.8%; N = 14,253) by Hardyck, Goldman, and Petrinovich (1975), Calnan and Richardson (1976), and Eaton, Chipperfield, Ritchot, and Kostiuk (1996). Left-handedness appears to be a behavioral marker of an underlying neurobiological process associated with gender identity disorder in boys.