The use of digit ratios as markers for perinatal androgen action

Many researchers are pursuing research programmes aimed at elucidating the effects of early testosterone exposure on later development outcomes, especially behavior. These include both experimental work with animal models and a large body of work in human psychology that have focused on the role of early testosterone action on behavioral sex differentiation [1]. In addition to the long-standing work on behavioral outcomes, interest has arisen more recently in the effects of early testosterone exposure on health-related outcomes in adulthood, including polycystic ovary syndrome [2,3] and reproductive cancers [4,5].The concept of "early testosterone action" is broadly meaningful in mammals because a period of high testosterone production in males during the prenatal (and possibly postnatal) periods drives the differentiation of primary reproductive tissues, and is then followed by a long period of relative testicular quiescence before the onset of puberty. Nevertheless, the particular pattern and timing of early testosterone production varies among species, and, more importantly, the pattern and timing of potential target tissue development varies substantially both between species and among tissue types. Rodents, for example, might be expected to show greater potential effects of postnatal, relative to prenatal, testosterone exposure because of their altricial pattern of development, whereas primate males (including humans) might be expected to show greater postnatal effects because they produce more testosterone in infancy [6]. This review will focus on the pattern and effects of testosterone production in human males, with comparisons to animal models where relevant.The mid-gestational peak in human male testosterone production occurs between the 10th and 18th weeks of gestation. Testosterone levels then decline, probably for a combination of reasons including an increase in hypothalamo-pituitary sensitivity to negative feedback, rising concentrations of placental