Masculinized Finger Length Patterns in Human Males and Females with Congenital Adrenal Hyperplasia Windy M. Brown,* Melissa Hines,† , ‡ Briony A. Fane,† and S. Marc Breedlove§ ,1 *Department of Psychology, University of California, Berkeley, Berkeley, California 94720; †Department of Psychology, City University, London, United Kingdom EC1V 0HB; ‡Department of Psychiatry and Biobehavioral Sciences, University of California, Los Angeles, Los Angeles, California 90095; and §Neuroscience Program, Department of Psychology and Department of Zoology, Michigan State University, East Lansing, Michigan 48824 Received December 19, 2001; revised May 13, 2002; accepted July 10, 2002 The ratio of the length of the second digit (2D) to the length of the fourth digit (4D) is greater in women than in men. Since androgens are involved in most somatic sex differences and since the sexual dimorphism in 2D:4D is stable from 2 years of age in humans, it was hypothe- sized that finger length pattern development might be affected by early androgen exposure. Human females with congenital adrenal hyperplasia (CAH) are exposed prenatally to higher than normal levels of adrenal andro- gens, providing an opportunity to test the effects of early androgen exposure on digit ratios. The 2D:4D was cal- culated for females with CAH, females without CAH, males with CAH, and males without CAH. Females with CAH had a significantly smaller 2D:4D on the right hand than did females without CAH. Males with CAH had a significantly smaller 2D:4D on the left hand than did males without CAH. A subset of six males with CAH had a significantly smaller 2D:4D on both hands compared with their male relatives without CAH. These results are consistent with the idea that prenatal androgen expo- sure reduces the 2D:4D and plays a role in the establish- ment of the sex difference in human finger length pat- terns. Finger lengths may therefore offer a retrospective marker of perinatal androgen exposure in humans. © 2002 Elsevier Science (USA) Key Words: congenital adrenal hyperplasia; prenatal hormones; androgen; finger ratios. In humans, the ratio of the index finger to the ring finger (2D:4D) is sexually dimorphic. Women have a larger 2D:4D on average than do men (George, 1930; Manning, Scott, Wilson, and Lewis-Jones, 1998). This sex difference in finger length patterns was first re- ported more than 100 years ago (Ecker, 1875). A recent replication of this sex difference in a cross-sectional sample of humans from a given population (Liver- pool, England) reported stable means of 2D:4D for males and females from 2 years of age through adult- hood (Manning et al., 1998, Manning, 2002). Since the masculinizing and defeminizing effects of androgens appear to be involved in most somatic sex differences reported thus far in the literature (Breedlove, Cooke, and Jordan, 1998), it appears likely that the 2D:4D sexual dimorphism may also be due to androgens. The 2D:4D sex difference may, however, arise from some other biological mechanism that differs between the sexes. For instance, an early report suggested that the longer index finger in women may be due to sex- influenced inheritance of a gene involved in skeletal structure that is dominant in women and recessive in men (Winchester, 1976). More recent research has em- phasized the role that the Y chromosome itself may play in sexual differentiation and resultant sex differ- ences (Arnold, 1996). It is also unclear when andro- gens are acting, if indeed they are responsible for digit length masculinization. The consistency of the 2D:4D sex difference across age in humans suggests an orga- nizational, developmentally early mode of action. Similarly, rear paw 2D:4D is greater in female than in male mice, and this sex difference is established before 1 To whom all correspondence should be addressed at Neuro- sciences Program, Psychology Research Building, Michigan State University, East Lansing, MI 48824. Fax (517) 432-1674. E-mail: breedsm@msu.edu. Hormones and Behavior 42, 380–386 (2002) doi:10.1006/hbeh.2002.1830 0018-506X/02 $35.00 © 2002 Elsevier Science (USA) All rights reserved. 380