By comparing the testosterone levels of five-month old pairs of twins, both identical and non-identical, University of Montreal researchers were able to establish that testosterone levels in infancy are not inherited genetically but rather determined by environmental factors. “Testosterone is a key hormone for the development of male reproductive organs, and it is also associated with behavioural traits, such as sexual behaviour and aggression,” said lead author Dr. Richard E. Tremblay of the university's Research Unit on Children's Psychosocial Maladjustment. “Our study is the largest to be undertaken with newborns, and our results contrast with the findings gained by scientists working with adolescents and adults, indicating that testosterone levels are inherited.” The findings were presented in an article published in Psychoneuroendocrinology on May 7, 2012.