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. 2021 Dec 15:375:e067849.
doi: 10.1136/bmj-2021-067849.

Giving science the finger-is the second-to-fourth digit ratio (2D:4D) a biomarker of good luck? A cross sectional study

James M Smoliga  1 , Lucas K Fogaca  2 , Jessica S Siplon  2 , Abigail A Goldburt  2 , Franziska Jakobs  2
Affiliations

Giving science the finger-is the second-to-fourth digit ratio (2D:4D) a biomarker of good luck? A cross sectional study

James M Smoliga et al. BMJ. .

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 r s 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 (r s=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.

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Conflict of interest statement

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.

Figures

Fig 1
Fig 1
Cumulative number of PubMed indexed papers about the second-to-fourth digit ratio (2D:4D). The following search was performed to identify papers published each year, with an example of 2020: (“2D:4D” or “digit ratio”) AND ((“2020”[date-publication]: “2020”[date-publication])). This search might not capture papers that refer to prenatal testosterone or similar concepts in the abstract but use 2D:4D in the methods
Fig 2
Fig 2
Example hand images used to measure the second-to-fourth digit ratio (2D:4D). Both images are the right hand of the same participant; yellow lines represent second and fourth digit measurements. (A) Photocopy image: 2D:4D=0.966. (B) Radiographic image: 2D:4D=0.943 (radiographic image has been horizontally flipped to be in the same orientation as the photocopy)
Fig 3
Fig 3
Histograms of second-to-fourth digit ratios (2D:4D) by participant hand measured, sex, and measurement technique. Despite statistically significant mean differences between men and women for each hand and each technique, the histograms had considerable overlap. (A) Left hand, photocopy technique; (B) right hand, photocopy; (C) left hand, radiograph; (D) right hand, radiograph
Fig 4
Fig 4
Scatter plots of second-to-fourth digit ratios (2D:4D) versus bone mineral density by participant sex and hand measured. Only 2D:4D measured by radiographic technique are presented, because the ratios measured by photocopy did not show significant results. (A) left hand, women; (B) right hand, women; (C) left hand, men; (D) right hand, men
Fig 5
Fig 5
Scatter plots of second-to-fourth digit ratios (2D:4D) versus poker hand rank by participant hand measured and sex. Only 2D:4D measured by radiographic technique are presented, because the ratios measured by photocopy did not show significant correlations. (A) Left hand, women; (B) right hand, women; (C) left hand, men; (D) right hand, men
Fig 6
Fig 6
Hypothetical algorithm for identifying statistically significant relations between second-to-fourth digit ratios (2D:4D) and any outcome measure
See this image and copyright information in PMC

Comment in

  • Langer Ringfinger - mehr Glück im Spiel.
    Stiefelhagen P. Stiefelhagen P. MMW Fortschr Med. 2022 Mar;164(4):29. doi: 10.1007/s15006-022-0870-7. MMW Fortschr Med. 2022. PMID: 35211898 Review. German. No abstract available.

References

    1. Moorhead J. What do your hands say about your health? 2005. https://www.theguardian.com/society/2005/mar/08/health.lifeandhealth.
    1. Walsh F. Index finger length prostate cancer clue. 2010. https://www.bbc.com/news/health-11880415.
    1. Haelle T. What Donald Trump's Hands Might Really Say About Him–According To Science. 2016. http://www.forbes.com/sites/tarahaelle/2016/03/04/what-donald-trumps-han....
    1. Gander K. Length of Ring Finger Linked to Men's Risk of Dying from Coronavirus, Study Finds. 2020. https://www.newsweek.com/length-ring-finger-linked-mens-risk-dying-coron....
    1. Narins E. 8 Things Your Ring Finger Length Could Say About Your Personality. 2016. https://www.cosmopolitan.com/health-fitness/a6573529/things-your-finger-....