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Repeatability and interobserver error of digit ratio (2D:4D) measurements made by experts
Martin Voracek1, John T Manning, Stefan G Dressler
1Department of Basic Psychological Research, School of Psychology, University of Vienna, A-1010 Vienna, Austria. martin.voracek@univie.ac.at
Summary
The second-to-fourth digit ratio (2D:4D) shows moderate measurement repeatability among experts, with potential implications for research findings. Despite some unreliability, 2D:4D differences across populations are considered valid.
Area of Science:
- Behavioral endocrinology
- Anthropometry
- Human sex differences
Background:
- The second-to-fourth digit ratio (2D:4D) is a sexually dimorphic trait linked to prenatal testosterone exposure.
- It is associated with various sex-dependent phenotypes, but its measurement reliability across studies is unclear.
Purpose of the Study:
- To assess the repeatability and interobserver error of 2D:4D measurements among experts in the field.
- To determine if measurement inconsistencies could explain null findings or replication failures in 2D:4D research.
Main Methods:
- 17 experts who have published on 2D:4D performed measurements.
- Repeatability was assessed using absolute-agreement intraclass correlation coefficients.
- Comparison was made with the measurement precision of simple finger length.
Main Results:
- 2D:4D measurements showed moderate repeatability (ICC ≈ 0.75), lower than finger length (ICC ≈ 0.95).
- Measurement unreliability may attenuate observed associations between 2D:4D and other traits, potentially explaining some failed replications.
- Despite this, agreement levels are sufficient to validate population-level differences in 2D:4D.
Conclusions:
- The study provides practical recommendations for improving 2D:4D measurement consistency.
- The findings support the validity of reported geographical and population variations in 2D:4D.
- Researchers should be mindful of measurement unreliability when interpreting 2D:4D associations.
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