The neurodevelopment of human sexual orientation

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Neuroscience and Biobehavioral Reviews 29 (2005) 1057–1066 www.elsevier.com/locate/neubiorev

Review

The neurodevelopment of human sexual orientation
Qazi Rahman*
School of Psychology, University of East London, The Green, London E15 4LZ, UK Received 29 October 2004; revised 26 January 2005; accepted 4 March 2005

Abstract One of the most enduring and controversial questions in theneuroscience of sexual behaviour surrounds the mechanisms which produce sexual attraction to either males or females. Here, evidence is reviewed which supports the proposal that sexual orientation in humans may be laid down in neural circuitry during early foetal development. Behaviour genetic investigations provide strong evidence for a heritable component to male and female sexual orientation.Linkage studies are partly suggestive of X-linked loci although candidate gene studies have produced null findings. Further evidence demonstrates a role for prenatal sex hormones which may influence the development of a putative network of sexual-orientation-related neural substrates. However, hormonal effects are often inconsistent and investigations rely heavily on ‘proxy markers’. A consistentfraternal birth order effect in male sexual orientation also provides support for a model of maternal immunization processes affecting prenatal sexual differentiation. The notion that non-heterosexual preferences may reflect generalized neurodevelopmental perturbations is not supported by available data. These current theories have left little room for learning models of sexual orientation. Futureinvestigations, across the neurosciences, should focus to elucidate the fundamental neural architecture underlying the target-specific direction of human sexual orientation, and their antecedents in developmental neurobiology. q 2005 Elsevier Ltd. All rights reserved.
Keywords: Sexual orientation; Homosexuality; Heterosexuality; Genetics; Prenatal androgens; Fraternal birth order; Developmentalinstability; Proxy markers; Maternal immunity; Hypothalamus; Developmental neurobiology; Learning

Contents 1. 2. 3. 4. 5. 6. 7. 8. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Behavioural and molecular genetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The prenatal androgen model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The fraternal birth order effect and maternal immunity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Developmental instability and sexual orientation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Neural circuitry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Is there a role for learning in the development of humansexual orientation? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion: The future of biobehavioural research on human sexual orientation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1057 1058 1058 1060 1061 1061 1062 1063 1064 1064

1. Introduction Sexual orientation refers to a dispositional sexual attraction towards persons of the opposite sex or same

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