MONTREAL Boys will be boys, as the adage goes, with their boisterous rough-and-tumble play so stereotypically male.
Is it the doings of a primary sex hormone called testosterone, long associated with social dominance, masculinity, and strength?
Many scientists have explored the role of hormones and conduct, whether in school playgrounds, sports, war or bedrooms.
Now a Universite de Montreal study of five-month-old twins is among the first to tease out the contribution of genetic and environmental factors to circulating levels of this chemical.
The research with infant twins suggests that the environment plays a more significant role than genes when it comes to testosterone levels.
Studies in human and animal models over the past 50 years have confirmed an association between aggression, dominance, and testosterone in adolescence said lead author Richard Tremblay of UdeM's research unit on children's psychosocial maladjustment.
"The question is, when does that association start" asked Tremblay, whose team looked at newborn babies of both sexes.
Published in the online edition of Psychoneuroendocrinology, the study compared testosterone in saliva samples from a total of 314 infants identical twins with non-identical twins. Identical pairs share the same genes, while fraternal twins are like any siblings and share 50 percent of their genes.
"So if testosterone levels are genetically determined, then the identical twins would be more alike than the fraternal twins," Tremblay explained.
Researchers found that testosterone levels in infancy are not inherited genetically but instead are affected by environmental factors.
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