Original articles
The 2nd:4th digit ratio, sexual dimorphism, population differences, and reproductive success: evidence for sexually antagonistic genes?

https://doi.org/10.1016/S1090-5138(00)00029-5Get rights and content

Abstract

The ratio between the length of the 2nd and 4th digit (2D:4D) is sexually dimorphic, with mean male 2D:4D lower than mean female 2D:4D. It recently was suggested that 2D:4D is negatively correlated with prenatal testosterone and positively correlated with prenatal estrogen. It is argued that high prenatal testosterone and low estrogen (indicated by low 2D:4D) favors the male fetus and low prenatal testosterone and high estrogen (indicated by high 2D:4D) favors the female fetus. The patterns of expression of 2D:4D are interpreted in terms of sexually antagonistic genes.

We report data on the following. (a) reproductive success and 2D:4D from England, Germany, Spain, Hungary (ethnic Hungarians and Gypsy subjects), Poland, and Jamaica (women only). Significant negative associations were found between 2D:4D in men and reproductive success in the English and Spanish samples and significant positive relationships between 2D:4D in women and reproductive success in the English, German, and Hungarian samples. The English sample also showed that married women had higher 2D:4D ratios than unmarried women, suggesting male choice for a correlate of high ratio in women, and that a female 2D:4D ratio greater than male 2D:4D predicted high reproductive success within couples. Comparison of 2D:4D ratios of 62 father:child pairs gave a significant positive relationship. This suggested that genes inherited from the father had some influence on the formation of the 2D:4D ratio. Waist:hip ratio in a sample of English and Jamaican women was negatively related to 2D:4D. (b) Sex and population differences in mean 2D:4D in samples from England, Germany, Spain, Hungary (including ethnic Hungarians and Gypsy subjects), Poland, Jamaica, Finland, and South Africa (a Zulu sample). Significant sex and population differences in mean 2D:4D were apparent.

Section snippets

Materials and methods

The length of the 2nd and 4th digits was measured on the ventral surface of the hand from the basal crease of the digit to the tip of the digit. Vernier callipers measuring to the nearest 0.01 mm were used throughout this study. It is known that this measurement of digit length can be made with high repeatability (Manning, 1995). Subjects with injuries to the digits were not included in the study. The digit ratio was calculated by dividing the length of the 2nd digit by that of the 4th (2D:4D).

English sample

We recruited 300 subjects (117 men and 183 women) with a minimum age of 30 years from the Merseyside area. Participants were from social groups of elderly retired people and mature university students. A wide range of socioeconomic backgrounds was represented. In addition to digit measurements, participants were required to report the number and sex of their putative biological children. From the sample of 300 subjects we identified 60 couples and we recruited a further 30 (n = 90). These pairs

Study II—sex and population differences in 2D:4D

Early work on 2D:4D considered the trait as discontinously distributed into two classes, 2D < 4D and 2D > 4D, and was mainly based on measurements of subjects in the United States and Canada (for review see Phelps, 1952). In all these studies both classes are common. Fig. 1, Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7 show that in some samples (German, Hungarian, and Jamaican) the class 2D > 4D is quite rare. In order to examine this question in more detail, further samples were obtained as

Discussion

We do not argue that the 2D:4D ratio is important mechanistically or as a display trait in mate choice. Most probably it affords us a window into prenatal hormonal conditions. There is accumulating evidence for an association between gonad and digit differentiation. Testosterone and dihydroxytestosterone (DHT) are known to affect development of the epidermis and dermis of the digits. Jamison et al. (1993) found a positive association between dermatoglyphic asymmetry, which is determined in utero

Acknowledgements

We are grateful to Varpu Lindstrom and Louise Barrett for help with data collection.

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