Sex role identity related to the ratio of second to fourth digit length in women

Abstract

Prenatal gonadal hormones have been implicated as important factors in the development of sex-role identity. The aim of the study reported here was to examine the relationship between adult sex-role preference and the second to fourth digit ratio (2D:4D ratio) in healthy women. There is evidence that the ratio of the length of second and fourth digits associates negatively with prenatal testosterone and positively with prenatal oestrogen. In this study the 2D:4D ratio was measured on a sample of 46 female university students. The subjects completed the form of the Bem Sex Role Inventory (BSRI). It was found that the lower 2D:4D ratios associated significantly with higher, masculinized bias scores in BSRI indicating that 2D:4D ratio predicts the female or male self-reported sex-role identity in females.

Introduction

The aim of the present study was to investigate the influence of prenatal gonadal hormones on self-reported sex-role identity in women. The hormonal level of the prenatal environment was estimated indirectly using a recently suggested indicator of in utero hormone levels: the second to fourth digit ratio (2D:4D ratio).

Why should in utero levels of sex steroids be reflected in relative finger length? Manning et al. (1998) have pointed out that prenatal exposure to testosterone and estrogen may leave morphological markers (see also Manning, 2002). A fetus is exposed to prenatal testosterone from two sources: the fetal testes and adrenal glands. The main source of prenatal estrogen comes from the adrenal glands and the placenta through the aromatase conversion of testosterone (George et al., 1981). These fetal sources of steroids are highly dependent on the differentiation process of fetal gonads (Lording and De Kretser, 1972). The differentiation of the fetal gonads is controlled by Homeobox or Hox genes (Zákány and Duboule, 1999). In particular, the posterior-most Hoxd and Hoxa genes are strongly expressed in the urinogenital system, including the gonads. However, these genes are also required for the growth and differentiation of digits and toes (Kondo et al., 1997). A common control and similar developmental mechanism of the distal limbs and genital eminence can be illustrated by the progressive removal of the posterior Hox gene function: it results in a concomitant loss of digits and genital bud derivatives (Kondo et al., 1997). The hand–foot-genital syndrome (which typically involves several anomalies of distal limbs and genital buds) is also caused by Hox gene mutation (Manning and Bundred, 2000). Manning et al. (1998) suggested that this sharing of causal factors in digit and gonad differentiation raises the possibility that patterns of digit formation can reflect testis function and, therefore, prenatal sex hormone concentration.

In accordance with the ‘Manning hypothesis’, the 2D:4D ratio of the human hand is a sexually dimorphic trait. In males the fourth digits tend to be longer than the second (2D:4D≤1) but in females both digits tend to be of equal length (2D:4D≥1) (Manning et al., 2000a). There is evidence that relative digit length is fixed in utero by about the 14th week of pregnancy (Garn et al., 1975). The suggestion that sexual dimorphism in 2D:4D is established early under the influence of testosterone is indicated by the following (a) the waist:hip ratio of mothers (a positive correlate of testosterone) is negatively related to the 2D:4D of their children (Manning et al., 1999). Thus women with a propensity to produce high testosterone concentrations also have a tendency to produce children with low 2D:4D (b) children with congenital adrenal hyperplasia (CAH), a trait which is associated with high concentrations of prenatal androgens, have lower 2D:4D than sex matched controls (Brown et al., 2001) and (c) the 2D:4D ratio of mothers is positively correlated with the 2D:4D of their children, and mothers with low 2D:4D have high testosterone levels in the amniotic fluid of their fetuses (Manning, 2002).

Prenatal testosterone and 2D:4D have been implicated in the development of cognitive abilities. Thus, there is evidence that prenatal testosterone promotes the growth of the right hemisphere increasing the probability of left-handedness, better visual-spatial abilities, music and mathematics (Geshwind and Galaburda, 1987, Kimura and Hampson, 1994, Kimura, 1996, Benbow, 1988, Gotestam, 1990, Grimshaw et al., 1993). Similarly low 2D:4D ratios, indicating a higher testosterone level in utero, are also associated with left-hand preference (Manning et al., 2000b), higher object mental rotation scores (Manning and Taylor, 2001) and male membership of a symphony orchestra (Sluming and Manning, 2000). Furthermore an excess of testosterone has also been implicated in the etiology of developmental disorders including autism. A similar conclusion may be drawn from the finding that 2D:4D ratios of children with autism were significantly lower than population normative values (Manning et al., 2001).

The question of the relation between 2D:4D ratio and sexual behavior has also been investigated recently. Regarding sexual orientation, homosexual men and women have been found to have lower 2D:4D than heterosexuals (Robinson and Manning, 2000, Williams et al., 2000, Brown et al., 2002). This suggests an association between human homosexuality and high prenatal testosterone level (McFadden and Champlin, 2000).

Regarding personality aspects, findings indicate that gonadal steroids are critical in the development of a male and female typical personality (Collaer and Hines, 1995, Ehrhardt et al., 1981). Both male and female typical dimensions of personality can be defined according to scores on self-reported masculine and feminine sex-role identity scales. While males generally exhibit a more assertive and competitive behavior, females possess greater caring attitudes and sociability. These personality characters have been extensively studied in males, especially in their participation in aggressive competition (e.g. Elias, 1981, Gouzalez-Bono et al., 1998). The effects of both pre- and postnatal gonadal steroid concentrations have also been investigated on adult sex-role identities in women. One of the clearest pieces of evidence for the gonadal hormone related basis of sex-roles comes from the study of Baucom et al. (1985). They found a reliable relationship between adult level of testosterone and sex-role identity. In particular, females with high levels of masculinity had somewhat higher testosterone levels than did feminine-sex-typed females. Moreover, their results on the Adjective Checklist indicated that while females with higher level of testosterone perceived themselves as self-directed, action-oriented and resourceful individuals, the women with lower level of testosterone viewed themselves as conventional and socialized individuals. The determining role of prenatal steroids in sex-role identity also appears to be supported by studies utilizing subjects with CAH. CAH women usually showed a masculine bias in indirect aggression and on the scales of different personality inventories (e.g. Detachment and Indirect Aggression Scales, Aggression and Stress Reaction Scales, Reinish's Aggression Inventory; Collaer and Hines, 1995).

The foregoing findings from CAH women strengthen the hypothesized association between prenatal steroids and adult sex-role identity, but they provide less evidence as to hormone dependency of more typical sex-roles shown by normal subjects. The previous studies also indicated that masculinity and femininity might be present in varying degrees in both men and women.

There has been little work on the relationship of 2D:4D to the more typical dimensions of sex-dependent human personality. To our knowledge Wilson (1983) has conducted the only investigation that is of particular interest concerning 2D:4D ratio and masculinity–femininity in women. Testing the subjects, self-reported identity on three adjectives (assertive or competitive, fairly average, and feminine), he found that women with low 2D:4D ratio were more likely to describe themselves as assertive and competitive than women with high 2D:4D. This result supports the association of 2D:4D ratio and sex-role identity. However, because of the small number of gender related items further work is needed to confirm this hypothesis.

In summary, the purpose of the study reported here was to examine the relationship between self-reported sex-role identity and 2D:4D ratio. Few studies on the effects of sex steroids on sex-role identity have focused on female subjects. Therefore, the current investigation was addressed to the hormone–personality relation in females. The sex-role identity was examined on a broad range of masculinity–femininity adjectives provided by the Bem Sex Role Inventory (BSRI). Our prediction was that a low 2D:4D ratio would be related to masculinized self-reported sex role identity.