A new study of twins adds to evidence that prenatal testosterone levels may influence later levels of aggressive behavior in females.
Studying 13-year-old girls, Celina Cohen-Bendahan et al. compared 74 girls with twin brothers (designated as OS, or "opposite sex") to 55 girls with twin sisters (designated as SS, or "same sex"). Animal research shows that females who develop in utero with male siblings tend to exhibit more masculine patterns of behavior than females who develop in utero with female siblings, apparently because of exposure to high levels of testosterone from the males.
Cohen-Bendahan et al. found no differences in testosterone levels between the OS and SS girls. Also, testosterone levels at the time of the study did not correlate with personality traits. "However," they say, "a sex difference in aggression proneness was observed, and opposite-sex girls showed a more masculine pattern of aggression proneness than the same-sex girls." The researchers speculate that this difference could be due to greater exposure of the OS girls to testosterone during prenatal development.
"Is there an effect of prenatal testosterone on aggression and other behavioral traits? A study comparing same-sex and opposite-sex twin girls," Celina C.C. Cohen-Bendahan, Jan K. Buitelaar, Stephanie H.M. van Goozen, Jacob F. Orlebeke, and Peggy T. Cohen-Kettenis, Hormones and Behavior, Vol. 47, 2005, 230-37. Address: Celina C.C. Cohen-Bendahan, Department of Child and Adolescent Psychiatry, A01.468, University Medical Center Utrecht, P.O. Box 85500, 3508 GA Utrecht, The Netherlands, firstname.lastname@example.org.