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Testosterone levels in infancy are not inherited genetically but rather determined by circumstances.

It is according to a new University of Montreal study comparing the testosterone levels of five-month old pairs of twins, both identical and non-identical.

"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 researcher Dr. Richard E. Tremblay of the university's Research Unit on Children's Psychosocial Maladjustment.

The team took saliva samples from 314 pairs of twins and measured the levels of testosterone. They then compared the similarity in testosterone levels between identical and fraternal twins to determine the contribution of genetic and environmental factors.

Results suggested that differences in levels of testosterone were due primarily to environmental factors.

The study was the largest to be undertaken with newborns, and the results contrasted with the findings gained by scientists working with adolescents and adults, indicating that testosterone levels are inherited, researchers said.

"Because our study suggests that testosterone levels in infants are determined by the circumstances in which the child develops before and after birth, further studies will be needed to find out exactly what these influencing factors are and to what extent they change from birth to puberty," Tremblay said.

The findings of the study appear in Psychoneuroendocrinology.