Fetal testosterone and sex differences ☆ Rebecca Christine Knickmeyer a, ⁎ , Simon Baron-Cohen b a Department of Psychiatry, University of North Carolina-Chapel Hill, CB #7160, 7023 Neurosciences Hospital, Chapel Hill, NC 27599–7160, USA b The Autism Research Centre, Department of Psychiatry, University of Cambridge, Douglas House, 18b Trumpington Rd, Cambridge CB2 2AH, UK Abstract Experiments in animals leave no doubt that androgens, including testosterone, produced by the testes in fetal and/or neonatal life act on the brain to induce sex differences in neural structure and function. In this article, we argue that prenatal and neonatal testosterone exposure are strong candidates for having a causal role in sexual dimorphism in human behaviour, including social development. © 2006 Elsevier Ireland Ltd. All rights reserved. KEYWORDS: Testosterone; Sex differences Contents 1. Testosterone and the sexual differentiation of the brain ................................ 755 2. Human sex differences ................................................... 757 3. Measuring fetal testosterone at amniocentesis ...................................... 758 4. The Cambridge Fetal Testosterone Project ........................................ 758 Acknowledgements ........................................................ 759 References ............................................................. 759 Experiments in animals leave no doubt that androgens, including testosterone, produced by the testes in fetal and/ or neonatal life act on the brain to induce sex differences in neural structure and function. In human beings, sex differences are apparent both in brain structures and cognitive skills [1,2]. In this article, we argue that prenatal and neonatal testosterone exposure are strong candidates for having a causal role in sexual dimorphism in behaviour. 1. Testosterone and the sexual differentiation of the brain Endocrine (hormonal) systems are involved in every aspect of pregnancy, including implantation, formation of the placenta, maternal adaptation, embryonic and fetal development, parturition/birth, and fetal adaptation to life outside the ☆ The longitudinal studies of fetal testosterone and child devel- opment reviewed in this paper were carried out at the Autism Research Centre, University of Cambridge. This work was supported by the Nancy Lurie Marks Family Foundation. RCK was also supported by a British Government Overseas Research Studentship (ORS) and the Cambridge Overseas Trust. Portions of this work appeared in Knickmeyer and Baron-Cohen [42]. ⁎ Corresponding author. Tel.: +1 919 966 8990; fax: +1 919 966 8994. E-mail address: rebecca_knickmeyer@med.unc.edu (R.C. Knickmeyer). 0378-3782/$ - see front matter © 2006 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.earlhumdev.2006.09.014 available at www.sciencedirect.com www.elsevier.com/locate/earlhumdev Early Human Development (2006) 82, 755–760