Behav Ecol Sociobiol (2006) 60: 447–454 DOI 10.1007/s00265-006-0185-1 ORIGINAL ARTICLE Nicola Saino . Maria Romano . Paolo Innocenti Length of index and ring fingers differentially influence sexual attractiveness of men’s and women’s hands Received: 13 November 2005 / Revised: 17 February 2006 / Accepted: 20 February 2006 / Published online: 28 March 2006 # Springer-Verlag 2006 Abstract Humans show intra- and intersexual variation in second (2D) relative to fourth (4D) finger length, men having smaller 2D:4D ratio, possibly because of differ- ential exposure to sex hormones during fetal life. The relations between 2D:4D and phenotypic traits including fitness components reported by several studies may origi- nate from the organizational effects that sex hormones have on diverse organs and their concomitant effect on 2D:4D. Evolutionary theory posits that sexual preferences are adaptations whereby choosy individuals obtain direct or genetic indirect benefits by choosing a particular mate. Since sex hormones influence both fitness and 2D:4D, hand sexual attractiveness should depend on 2D:4D, a hypothesis tested only in one correlational study so far. We first presented hand computer images to under- graduates and found that opposite-sex hands with long 2D and 4D were considered more sexually attractive. When we experimentally manipulated hand images by increasing or decreasing 2D and/or 4D length, women preferred opposite-sex hands that had been masculinized by elongating 4D, whereas men avoided masculinized opposite-sex right hands with shortened 2D. Hence, con- sensus exists about which hands are attractive among different opposite-sex judges. Finger length may signal desirable sex hormone-dependent traits or genetic quality of potential mates. Psychological mechanisms mediating hand attractiveness judgments may thus reflect adaptations functioning to provide direct or indirect benefits to choosy individuals. Because the genetic mechanisms that link digit development to sex hormones may be mediated by Hox genes which are conserved in vertebrates, present results have broad implications for sexual selection studies also in nonhuman taxa. Keywords Attractiveness . Digit length . Digit ratios . Hand esthetics . Mate choice . Sexual selection Introduction In humans, males have consistently smaller length ratio between the index (2D) and the ring (4D) fingers compared to women (Manning 2002; McFadden and Shubel 2002; Peters et al. 2002; Romano et al. 2006; Saino et al. 2006). Studies of nonhuman primates and rodents have also documented sexual dimorphism in length ratios between 2D and 4D (2D:4D) (Manning et al. 2003a; McFadden and Bracht 2003; Roney et al. 2004; Leoni et al. 2005). Sex- related variation in 2D:4D has been attributed to differen- tial exposure of the two sexes to androgens and estrogens during fetal life, with high levels of androgens resulting in small 2D:4D (Manning et al. 1998; Brown et al. 2002; Okten et al. 2002; Lutchmaya et al. 2004; but see Romano et al. 2005). However, variation in the hormone environ- ment during early ontogeny may also explain within-sex variation in 2D:4D (Manning et al. 2003b; van Anders et al. 2006). 2D:4D correlates with several morphological, physio- logical, behavioral, and performance traits in humans (see Manning 2002 for a review; see also Putz et al. 2004 for references), and these relations have been attributed to the concomitant effects of sex hormones on 2D:4D and their organizational effects on diverse organ systems (Becker et al. 1992; Sisk et al. 2003). The possible positive association between prenatal and pubertal or adult androgens (e.g., Jamison et al. 1993) may also result in a correlation between 2D:4D and testosterone-dependent traits that are expressed at puberty or later. Consistent with this prediction, negative relations exist between masculiniza- tion of men’ s faces or perceived social dominance and 2D:4D (Neave et al. 2003; Bailey and Hurd 2005; but see Communicated by T. Czeschlik N. Saino (*) . M. Romano . P. Innocenti Dipartimento di Biologia, Università degli Studi di Milano, Via Celoria 26, 20133 Milano, Italy e-mail: nicola.saino@unimi.it Tel.: +39-02-50314808