ORIGINAL PAPER Birth Order and Ratio of Brothers to Sisters in Spanish Transsexuals Esther Go ´mez-Gil • Isabel Esteva • Rocı ´o Carrasco • M. Cruz Almaraz • Eduardo Pasaro • Manel Salamero • Antonio Guillamon Received: 27 May 2009 / Revised: 9 February 2010 / Accepted: 13 February 2010 Ó Springer Science+Business Media, LLC 2010 Abstract Three Western studies have shown that male-to- female (MF) homosexual transsexuals tend to be born later than their siblings and to come from sibships with more brothers than sisters. The objective of this study was to determine whether these variables would be replicated in 530 MF and female-to- male (FM) Spanish transsexuals according to sexual orientation. The results showed that MF homosexual transsexuals had sig- nificantly more older brothers than the non-homosexual MF group. Compared with the expected rates in the general popu- lation, birth order was significantly higher in both MF (Slater’s Index = 0.59; Fraternal Index = 0.61; Sororal Index = 0.58) and FM homosexual transsexuals (Slater’s Index = 0.65; Fra- ternal Index = 0.68; Sororal Index = 0.67), and sibling sex ratio was significantly higher than expected in homosexual MF (sex ratio = 0.55) but not in homosexual FM transsexuals. No sig- nificant differences were found in the non-homosexual sub- groups. The replication of the later birth order and sibling sex- ratio effect in MF homosexual transsexuals corroborates pre- vious findings in a variety of groups from different cultures and may suggest a common mechanism underlying the etiology of transsexualism. Keywords Birth order Á Sibling sex ratio Á Gender identity disorder Á Transsexualism Á Sexual orientation Introduction The phenomenon of transsexualism, or gender identity disor- der (GID), is present in all cultures (Go ´mez-Gil & Esteva de Antonio, 2006). The etiology is unknown, but it has been sug- gested that biological and environmental factors may contribute to gender identity variations (for a review, see Cohen-Kettenis & Gooren, 1999). The replication of biological variables in cross-cultural research may reflect common underlying causal mechanisms. Two biodemographic variables, birth order and sibling sex ratio, have been studied in homosexual men (for a review, see Blanchard, 1997; Bogaert, 2005; Slater, 1962; Zucker & Blanchard, 1994), and also in male-to-female (MF) and female-to-male (FM) patients with GID. In male samples, Western studies from Canada (Blanchard & Sheridan, 1992), The Netherlands (Blanchard, Zucker, Cohen-Kettenis, Gooren, & Bailey, 1996), and England (Green, 2000), and non-Western studies from Singapore (Tsoi, Kok, & Long, 1977), Polynesia (Poasa, Blanchard, & Zucker, 2004; VanderLaan & Vasey, 2009; Vasey & VanderLaan, 2007), and South Korea (Zucker, Blanchard, Kim, Pae, & E. Go ´mez-Gil Á M. Salamero Unidad de Identidad de Ge ´nero, Instituto Clı ´nic de Neurociencias, Servicio de Psiquiatrı ´a, Hospital Clı ´nic, Universidad de Barcelona, Barcelona, Spain I. Esteva Á M. C. Almaraz Unidad de Trastornos de Identidad de Ge ´nero, Servicio de Endocrinologı ´a, Hospital Carlos Haya, Ma ´laga, Spain R. Carrasco Unidad de Trastornos de Identidad de Ge ´nero, Servicio de Ginecologı ´a, Hospital Carlos Haya, Ma ´laga, Spain E. Pasaro Departamento de Psicobiologı ´a, Universidad de A Corun ˜a, A Corun ˜a, Spain A. Guillamon Departamento de Psicobiologı ´a, UNED, Madrid, Spain E. Go ´ mez-Gil (&) Department of Psychiatry, Institute Clinic of Neurosciences, Hospital Clı ´nic, University of Barcelona, Villarroel 170, 08036 Barcelona, Spain e-mail: esgomez@clinic.ub.es 123 Arch Sex Behav DOI 10.1007/s10508-010-9614-3