Meta-analysis of digit ratio 2D:4D shows greater sex difference in the right hand

OBJECTIVES

Our aims are, first, to describe the sex difference in the length ratio of the second and fourth digits (2D:4D), which likely reflects prenatal testosterone levels in humans. Second, to infer the loss of reliability observed in 2D:4D based on self-measured finger lengths.

METHODS

We used random-effects meta-analysis of 2D:4D based on expert-measured finger lengths (116 samples with 13,260 females and 11,789 males).

RESULTS

We find a moderate sex difference (with lower 2D:4D for males), which shows substantial heterogeneity (which is unrelated to age). The sex difference is moderated by the type of finger length measurement and by hand. Measurement involving the distortion of soft tissue leads to a significantly larger sex difference than finger length measurement avoiding this. The sex difference in 2D:4D is larger in the right hand than in the left. The reliability of self-measured 2D:4D in the BBC internet study, by far the largest study on 2D:4D, is estimated to be 46% of that of expert-measured 2D:4D.

CONCLUSIONS

Right-hand 2D:4D might be a better indicator of prenatal androgenisation than left-hand 2D:4D. The view that 2D:4D has allometric properties (Kratochvil L, Flegr J. 2009. Differences in 2nd to 4th digit length ratio in humans reflect shifts along the common allometric line. Biol Lett 5:643-646.) is not supported. Bone lengths contribute to the sex difference in 2D:4D. In addition, there might be a sex difference in fingers' soft tissue, which should be investigated. Because of measurement unreliability, correlations between 2D:4D and variables of interest are about one-third smaller in the BBC internet study than in studies in which 2D:4D is based on expert-measured finger lengths.

Scientometric analysis and bibliography of digit ratio (2D:4D) research, 1998-2008

A scientometric analysis of modern research on the second-to-fourth digit ratio (2D:4D), a widely studied putative marker for prenatal androgen action, is presented. In early 2009, this literature totalled more than 300 publications and, since its initiation in 1998, has grown at a rate slightly faster than linear. Key findings included evidence of publication bias and citation bias, incomplete coverage and outdatedness of existing reviews, and a dearth of meta-analyses in this field. 2D:4D research clusters noticeably in terms of researchers, institutions, countries, and journals involved. Although 2D:4D is an anthropometric trait, most of the research has been conducted at psychology departments, not anthropology departments. However, 2D:4D research has not been predominantly published in core and specialized journals of psychology, but rather in more broadly scoped journals of the behavioral sciences, biomedical social sciences, and neurosciences. Total citation numbers of 2D:4D papers for the most part were not larger than their citation counts within 2D:4D research, indicating that until now, only a few 2D:4D studies have attained broader interest outside this specific field. Comparative citation analyses show that 2D:4D research presently is commensurate in size and importance to evolutionary psychological jealousy research, but has grown faster than the latter field. In contrast, it is much smaller and has spread more slowly than research about the Implicit Association Test Fifteen conjectures about anticipated trends in 2D:4D research are outlined, appendixed by a first-time bibliography of the entirety of the published 2D:4D literature.

Empathizing, systemizing and finger length ratio in a Swedish sample

The Empathy- and Systemizing Quotients (EQ and SQ, respectively; Baron-Cohen, 2003) were determined in a Swedish sample consisting mainly of university undergraduates. Females had significantly higher EQ than males, who in turn scored higher on the SQ inventory. Gender explained 12-14% of the variation. Males were strikingly overrepresented in the group defined by a high SQ/low EQ profile or by a large SQ - EQ difference; females dominated among people with a low SQ/high EQ profile or by a large EQ - SQ difference. Students majoring in the natural sciences had higher SQs than psychology majors, but in both groups the gender difference in SQ and EQ was strong. For each participant a weighted composite score was generated by multivariate processing of the EQ and SQ data (Partial Least Square Discriminant Analysis). These scores were associated in a sex-linked fashion to a biometric measure reflecting prenatal testosterone exposure, i.e. the ratio between index (2D)- and ring (4D) finger lengths. In males a high (female-typical) 2D:4D ratio predicted an enhanced tendency to empathize and a reduced tendency to systemize; in women, by contrast, the 2D:4D ratio was unrelated to these traits. The present research confirms earlier work of a gender difference in EQ and SQ. The difference appears robust as it appears as large in Sweden (a country with high cultural gender-equality) as in countries with considerably lower gender-equality.

Sex differences in directional asymmetry of digit length and its effects on sex differences in digit ratio (2D:4D)

Patterns of directional asymmetry in the length of index and ring finger (2D and 4D) may be sexually differentiated. Martin, Puts, and Breedlove (2008) found leftward bias, i.e., both a longer 2D and 4D in the left than in the right hand, more frequently in men, whereas rightward bias occurred more frequently in women. Further, rightward vs leftward bias seemed a substantial source of variation in digit ratio (2D:4D), a putative marker for prenatal androgen levels. Given the generally small effects, unreplicable results, and inconsistent findings of 2D:4D research, these findings potentially are important as the type of directional digit asymmetry could moderate or obfuscate real associations between 2D:4D and target traits. Based on six samples totaling about 3,000 individuals, the present study replicated the first finding of Martin, Puts, and Breedlove (2008): leftward bias was more frequent among men and rightward bias more frequent among women. This sex effect was small. However, the present study did not replicate the second finding of Martin, et al. (2008): relative to participants' sex the contributions of leftward vs rightward asymmetry to individual variation in 2D:4D were smaller by orders of magnitude and not significant. Implications of these findings for 2D:4D research are discussed.