Giving science the finger—is the second-to-fourth digit ratio (2D:4D) a biomarker of good luck? A cross sectional study
BMJ 2021; 375 doi: https://doi.org/10.1136/bmj-2021-067849 (Published 15 December 2021) Cite this as: BMJ 2021;375:e067849
- James M Smoliga, professor of physiology1,
- Lucas K Fogaca, student1,
- Jessica S Siplon, student1,
- Abigail A Goldburt, student1,
- Franziska Jakobs, student1
- Correspondence to: J M Smoliga jsmoliga{at}highpoint.edu
- Accepted 30 November 2021
Abstract
Objectives To explore whether random chance, weak research methodology, or inappropriate reporting can lead to claims of statistically significant (yet, biologically meaningless) biomarker associations, using as a model the relation between a common surrogate of prenatal testosterone exposure, second-to-fourth digit ratio (2D:4D), and a random indicator of good luck.
Design Cross sectional study.
Setting University sports performance laboratory in the United States. Data were collected from May 2015 to February 2017.
Participants 176 adults (74 women, 102 men), including university students, faculty, and staff with no history of injuries, disease, or medical conditions that would affect digit length.
Main outcome measures 2D:4D, body composition parameters potentially influenced by androgens (bone mineral content, bone mineral density, body fat percentage), and good luck (using poker hands from randomly selected playing cards as a surrogate).
Results 2D:4D significantly correlated with select body composition parameters (Spearman’s rs range −0.26 to 0.23; P<0.05), but the correlations varied by sex, participant hand measured, and the method of measuring 2D:4D (by photocopy or radiography). However, the strongest correlation observed was between right hand 2D:4D in men measured by radiograph and poker hand rank (rs=0.28, P=0.004).
Conclusions Greater prenatal exposure to testosterone, as estimated by a lower 2D:4D, significantly increases good luck in adulthood, and also modulates body composition (albeit to a lesser degree). While these findings are consistent with a wealth of research reporting that 2D:4D is related to many seemingly disparate outcomes, they are not meant to provide confirmatory evidence that 2D:4D is a universal biomarker of nearly everything. Instead, the associations between 2D:4D and good luck are simply due to chance, and provide a “handy” example of the reproducibility crisis within medical and scientific research. Biologically sound hypotheses, pre-registration of trials, strong methodological and statistical analyses, transparent reporting of negative results, and unbiased interpretation of data are all necessary for biomarker studies and other areas of clinical research.
Footnotes
Contributors: JMS conceived this idea after reviewing the digit ratio research and becoming suspicious that one biomarker could predict pretty much anything. JMS recruited LKF, JSS, AAG, and FJ to become involved in this project after convincing them that a study about fingers and playing cards was important science. JMS was the principal investigator and led the research design, statistical analysis, data visualization, and manuscript writing and revision. LKF, JSS, and AAG substantially contributed to investigation (data collection and subject recruitment), general project administration, data curation, and contributed to writing aspects of this manuscripts. FJ contributed to data curation and validation. All authors had full access to the data in the study and can take responsibility for the integrity of the data and the accuracy of the data analysis. JMS is the guarantor. The corresponding author attests that all listed authors meet authorship criteria and that no others meeting the criteria have been omitted.
Funding: This study received no specific funding.
Competing interests: All authors have completed the ICMJE uniform disclosure form at www.icmje.org/disclosure-of-interest/ and declare: no support from any organization for the submitted work; no financial relationships with any organizations that might have an interest in the submitted work in the previous three years; no other relationships or activities that could appear to have influenced the submitted work.
The lead author affirms that the manuscript is an honest, accurate, and transparent account of the study being reported; that no important aspects of the study have been omitted; and that any discrepancies from the study as planned (and, if relevant, registered) have been explained.
Dissemination to participants and related patient and public communities: The principal investigator will directly contact freelance journalists that he has worked with previously, who have written articles about his research for major media outlets; pitch a plain language summary for mainstream media websites where he has written previously (The Conversation or Scientific American blogs), as well as other similar websites; and provide a Twitter thread on the topic.
Provenance and peer review: Not commissioned; externally peer reviewed.
Data availability statement
The full dataset used for this study is available in the supplementary material.
This is an Open Access article distributed in accordance with the Creative Commons Attribution Non Commercial (CC BY-NC 4.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited and the use is non-commercial. See: http://creativecommons.org/licenses/by-nc/4.0/.