The Monks lab investigates the causes of sex differences. Sex is a term that we use for phenotypic specializations that ultimately serve reproductive functions within species, and that we recognize categorically with familiar and changing terms (e.g. female, male, intersex, non-binary). Beyond their intrinsic interest, sex differences provide a means of understanding individual differences in the nervous system and behaviour. Although the most direct behavioral manifestation of sex is sexual behaviour, sex influences many (and perhaps most) behaviours to a greater or lesser extent. Biological sex provides arguably the best understood and most highly conserved individual difference. The developmental processes that bring about sex differences, or sexual differentiation, in mammals involves a complex interplay of genes, hormones and environmental factors. Among these, we have focused primarily on hormonal influences and increasingly on environmental interactions with these hormonal influences. Our main experimental approach to these questions is to leverage the powerful methodological tools available in laboratory mice and rats to probe causal relationships by isolating and manipulating variables of interest. Notably, we have made extensive use of androgen receptor mutants, including the generation of several novel transgenic animals, in order to better understand how and where testosterone acts to promote sex differences. Our broad interest in this field is reflected in our research endeavors on a variety of topics relevant to functions of androgens in sexual differentiation, ranging from sex differences in the anatomy of neuromuscular systems, to neurodegeneration associated with androgen receptor mutations, to regulation of muscle and fat in sedentary and exercising animals, to consequences of high androgen exposure to social and sexual behaviors.