title: Repeatability and interobserver error of digit ratio (2D:4D) measurements made by experts.
abstract: The second-to-fourth digit ratio (2D:4D) is sexually differentiated (lower in men than in women), a likely biomarker for organizational (permanent) effects of prenatal testosterone, and a correlate of many sex-dependent, hormonally influenced traits and phenotypes. The extent of 2D:4D measurement repeatability across different research groups is unknown. This study assessed the repeatability and interobserver error of 2D:4D measurements made by 17 experts (researchers who have contributed to the 2D:4D literature). Results indicate that 2D:4D, because it is a ratio variable, is notably less precisely measurable than finger length. Absolute-agreement intraclass correlation coefficients for these traits are about 0.75 vs. about 0.95, respectively. Associations of 2D:4D with target traits were usually of small size; measurement unreliability attenuates effects. This may explain some null findings and replication failures encountered in 2D:4D research. However, agreement levels are still sufficient to permit conclusions about findings from different research groups. One implication of this is that the marked geographical and population differences in typical 2D:4D levels, as reported in the literature, are veridical, and not due to divergent measurement habits across research groups. Some practical recommendations for 2D:4D measurement are offered.
authors.affiliation: Department of Basic Psychological Research, School of Psychology, University of Vienna, A-1010 Vienna, Austria. martin.voracek@univie.ac.at
authors.forename: Martin
authors.initials: M
authors.lastname: Voracek
authors.forename: John T
authors.initials: JT
authors.lastname: Manning
authors.forename: Stefan G
authors.initials: SG
authors.lastname: Dressler
mesh_terms: D000328:Adult; D000886:Anthropometry; D001315:Australia; D005260:Female; D005385:Fingers / Q000254:growth & development*; D006801:Humans; D008297:Male; D015588:Observer Variation; D009929:Organ Size; D015203:Reproducibility of Results; D012727:Sex Characteristics; D013739:Testosterone / Q000378:metabolism
chemical_list: D013739:Testosterone