1. Introduction

Bilateral symmetry of physical traits is hypothesized to reflect overall quality of development, especially the ability to resist environmental perturbations during development (Møller & Swaddle, 1997). Hence, a symmetrical face may signal developmental homeostasis, the ability of an individual to cope with the challenges of its environment (Grammer & Thornhill, 1994). Several studies have shown a relationship between symmetry and attractiveness. If symmetry is present in the face or the body an individual is judged as being relatively attractive, and if the body is asymmetric the face is rated unattractive, even if the rater never sees the body Gangestad et al., 1994, Thornhill & Gangestad, 1993, Thornhill & Gangestad, 1994.

Facial asymmetries manifest themselves very early in human development and remain stable during one's lifetime Thornhill & Gangestad, 1996, Thornhill & Møller, 1997. These minor physical anomalies (MPAs) seem to be the result of developmental instabilities during early embryonic development. MPAs are formed in the first trimester of gestation, and fluctuating asymmetries develop throughout life. However, several studies have shown positive correlations between the frequency of MPAs and fluctuating asymmetry (see for a review, Grammer, Fink, Møller, & Thornhill, 2003).

It is well established that poor environmental conditions increase developmental instability and measures of fluctuating asymmetry (see Thornhill & Møller, 1997). Adverse environmental conditions are also known to reduce immunocompetence. It is further likely that sex hormones are one factor mediating the relationship between immune defence and developmental stability. Facial secondary sexual characteristics appear to advertise phenotypic and genetic quality because of their connection to sex hormones. Previous studies on human physical attractiveness suggest that testosterone (T) as well as estrogen (E) may negatively impact immunocompetence when present in high titers (Folstad & Karter, 1992). High titers of sex hormones are, however, necessary to produce attractive physical features such as broad jaws in males or high cheekbones in females. Consequently, attractive features such as facial symmetry, developed under the presence of sex steroids, are thought to advertise immunocompetence honestly (Grammer & Thornhill, 1994).

Males produce testosterone prenatally, particularly from around gestation week 8 to 24, and for approximately the first 6 months postnatally (Migeon & Wisniewski, 1998). Female fetuses probably produce estrogens, although it is unclear how much of a contribution the fetal ovary makes to development. There is accumulating evidence that the ratio between the length of the index finger (2D) and the ring finger (4D) is negatively related to prenatal T and positively related to prenatal E (for a review, see Manning, 2002).

The aim of the present study was to determine whether there is an association between human finger length patterns and facial asymmetry in males and females. As 2D:4D is thought to be a pointer to the early hormonal environment, we hypothesized that in males 2D:4D would be negatively related to facial asymmetry scores (as a result of T), whereas in females 2D:4D should be positively related to facial asymmetry (as a result of E). If this hypothesis were true, digit ratio could serve as identifier of prenatally elevated T and E levels as causes for an early manifestation of fluctuating asymmetries in the face. We discuss our findings within the evolutionary framework of sexual selection of human physical characteristics.

2. Materials and methods

2.1. Participants and digit measures

We measured the lengths of the second and fourth finger and the degree of facial asymmetry (as a measure of facial developmental instability) in a sample of 69 heterosexual males (mean age=21.72) and 85 females (mean age=21.98) aged 17 to 38 years. Photocopies were made of the ventral surface of the hand. Participants were asked to place their hands palm down on the centre of the glass plate of the photocopier and one photocopy per hand was made. Care was taken to ensure that details of major creases could be seen on the hands. When quality was poor, a second photocopy was made. From the photocopies, we measured the lengths of the second and fourth digits of the left and right hands from the ventral proximal crease of the digit to the tip. Where there was a band of creases at the base of the digit, we measured from the most proximal of these. In order to establish repeatabilities, the second and fourth digits were measured directly from the hand and from the photocopies. All measurements were made with digital Vernier calipers measuring to 0.01 mm. Those participants who reported injuries to the second or fourth digits (including injuries to the carpus and disruption of tendons) were discarded from the final analyses, as were participants who identified themselves as homosexual or bisexual, or left-handed, so that our final sample number was 54 heterosexual males and 68 heterosexual females.

In comparisons between 2D:4D ratios calculated from measurements made on the hand and those made from photocopies we used repeated measures ANOVA and Model II single factor ANOVA (Zar, 1984) to calculate intraclass correlation coefficients (r1) and the ratio (F) between measurement error (the differences between successive measures of 2D:4D) and between-participant differences.

where MS=mean squares. We found that between-individual differences were significantly greater than measurement error in 2D:4D (right hand r1=.68, F=5.27, P=.0001; left hand r1=.61, F=4.15, P=.0001). For 30 photocopies, the fingers were measured by two measurers. Intermeasurer reliability was high for both the left (r1=.97, F=73.64, P=.0001) and the right hand (r1=.98, F=125.85, P=.0001). We concluded that our calculated values of 2D:4D reflected real differences between individuals.

3. Results

In accordance with previous reports (Manning, 2002), males had a significantly lower 2D:4D ratio than females on the right hand but no significant sex difference was found on the left hand (2D:4D right hand: males x=0.95±0.02, females x=0.99±0.02, t=−7.82, P<.05; 2D:4D left hand, males x=0.96±0.03, females x=0.99±0.02, t=−4.92, P<.05). We found that 2D:4D ratio was significantly correlated with facial asymmetry scores in both sexes. Male right-hand 2D:4D ratio correlated significantly negatively with facial asymmetry (r=−.260; P=.029; see Fig. 1a). Female right hand 2D:4D ratio correlated significantly positively with facial asymmetry, (r=.228; P=.031, see Fig. 1b). Left hand 2D:4D and facial asymmetry were significantly negative correlated in males (r=−.319; P=.009), but there was no significant correlation in females (r=.020; P=.436).

Fig. 1. Scatterplot of right- and left-hand 2D:4D digit length ratios and facial asymmetry in men (a) and women (b). The regression lines are statistically significant except for the left hand of women.

4. Discussion

The aim of this study was to examine possible relationships between finger length ratio (2D:4D) and facial asymmetry in males and females. Male facial asymmetry was negatively correlated with 2D:4D in both hands, but only in the right hand in females. These relationships probably reflect in utero organisational effects of sex steroids on facial asymmetry rather than adult hormone effects.

There is accumulating evidence that 2D:4D is negatively related to prenatal testosterone and positively to prenatal oestrogen. Thus, (1) 2D:4D is sexually dimorphic in children and does not appear to change at puberty, (2) the waist-to-hip ratio of mothers (a positive correlate of testosterone and a negative correlate of oestrogen) is negatively related to the 2D:4D ratio, and (3) sex-dependent traits that express early embryonic development, such as handedness, autism, and Asperger's syndrome, are correlated with 2D:4D (see Manning, 2002, for a review). In addition, children with congenital adrenal hyperplasia, a trait that is characterised by high prenatal androgen, have low 2D:4D ratios Brown et al., 2002, Okten et al., 2002. In contrast to the associations between 2D:4D and prenatal sex steroids, there is little evidence that finger ratios correlate with adult sex steroids in general population samples. Manning, Scutt, Wilson, and Lewis-Jones (1998) reported a negative association between 2D:4D and testosterone and a positive correlation between 2D:4D and oestrogen. However, their sample consisted of patients from a fertility clinic, some of whom were likely to have atypical hormonal profiles. Neave, Laing, Fink, and Manning (2003) did not find an association between 2D:4D and testosterone levels in a normal sample of men.

Developmental stability, or homeostasis, facilitates the production of consistent phenotypes by buffering against stress. Fluctuating asymmetry is produced by developmental instability and is manifested as small random departures from bilateral symmetry. Bilateral symmetry reflects overall quality of development, especially the ability to resist environmental perturbations during development (e.g., Leary & Allendorf, 1989, Parson, 1990), which implies that a symmetrical face displays developmental homeostasis (Thornhill & Gangestad, 1993). Asymmetries may occur during any period in development and appear to be particularly high in children during periods of rapid growth (Wilson & Manning, 1996). The associations we have found between 2D:4D and facial asymmetry suggest that some of the variance in developmental stability of the face is determined in utero by sex steroids and persists into adulthood.