In recent years, evidence has accumulated demonstrating that endocrine disrupting chemicals (EDCs) have the potential to alter sexual development at the organizational and functional level in many species, including humans, indicating that this class of chemicals may play a role in the etiology of transsexualism. Although transsexualism has historically been attributed to social or psychological causes, little data exists to support these claims, thus requiring a closer examination of the evidence regarding changes in sexual development due to EDCs. Toward that end, this thesis considers data from studies examining hormonal signaling mechanisms and changes in sexual development observed in wildlife, laboratory animals, and humans exposed to EDCs, all providing a consistent picture that sex hormones and their receptors are highly conserved evolutionarily, finding similar effects of disruption in many species.

            In order to place the data in context, a number of historical threads are examined, including: the use of chemicals in agriculture, the use of the pesticide DDT and the pharmaceutical drug diethylstilbestrol (DES), the intertwined relationship between chemical manufacturers and the military, and the history of transsexualism since 1950. The operation and function of the endocrine system is reviewed in order to provide the background to properly interpret findings from endocrine disruptor studies, focusing particularly on the hypothalamic-pituitary-gonadal (HPG) axis. Recent physiological data regarding the vomeronasal organ (VNO) is reviewed, demonstrating that the VNO is the organ responsible for detecting pheromones, or sexually-relevant chemically-based cues, and that exposure of the VNO to extremely low levels of putative sex hormones causes numerous autonomic system responses, including alterations in endocrine function in males. It is therefore suggested that the VNO plays a central role in the circuitry involving sexual development, and a hypothetical framework for testing this concept is provided.

            Using this framework, a mechanism for the development of gender identity is proposed, suggesting that gender identity is determined via pheromones by comparing the self with others at an unconscious level. One consequence of this mechanism is that messages conveyed by pheromones can be regarded as signals that can be in contradiction from messages from society, leading to a paradoxical double bind, or a logical contradiction between messages that exist on different logical levels. Another consequence is that there may exist a class of chemicals, pheromone disruptors, that could interfere with pheromones in a manner analogous to endocrine disruptors. Further research must be performed to test this hypothesis since little data exists on pheromones in humans, but early data suggests chemicals may be found that interfere with normal pheromone function.

            The prevalence of transsexualism is examined, finding that prevalence differences reported in various countries are not well explained by social factors. Also, it is observed that existing studies have reported the prevalence of transsexuals seeking treatment over a specific time period, but this reporting method is not a measure of the number of transsexuals for each country, which is what the term implies to most people. Several recent epidemiological studies that address sexual changes from endocrine disruption are critiqued, finding that they are plagued with methodological weaknesses and contain a number of errors in interpretation. It is argued that instead of using epidemiological techniques, a more useful approach would be to perform demographic studies that map the birthplace of transsexuals in space and time to determine any patterns that may be related to environmental conditions. The lack of detailed data on transsexual demographics, especially in the United States where such data are completely lacking, has left a void where a lack of data has been interpreted incorrectly as a lack of effect.