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

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.

Relationship status and gender-related differences in response to infidelity

“…Thus, this association may be simply due to chance. Smoliga et al (2021) study, published in the British Medical Journal, found a significant correlation between right 2D:4D in men and good luck (i.e., poker hand rank from randomly selected playing cards as a surrogate). Their finding was not meant to provide confirmatory evidence for the association between 2D:4D and good luck.…”

“…No previous study has examined this association. The association might be due to chance, according to Smoliga et al (2021) . However, the present study’s findings may be interpretable from an evolutionary psychological perspective, although we could not conceive of its interpretation.…”

Anthropometrics, Performance, and Psychological Outcomes in Mixed Martial Arts Athletes

“…In addition, it was also reported that athletes with lower 2D:4D present better performance in sumo wrestling [ 13 ] and Greco–Roman wrestling [ 14 ], and that more androgenized men tend to exhibit higher preferences for practicing combat sports (i.e., wrestling, judo, boxing, and kickboxing) rather than aerobics or team sports [ 11 , 12 ]. Furthermore, it was observed that a lower 2D:4D is associated with a lower body fat percentage, greater bone mineral content, and greater bone mineral density, especially in men, which could plausibly be related to prenatal androgen exposure [ 38 ]. Thus, 2D:4D might be a biomarker of one’s fate.…”

The association of prenatal amniotic sex hormones and digit ratio (2D:4D) in children aged 5 to 70 months: A longitudinal study

“…2D:4D has repeatedly been shown to correlate with various behavioral measures [ 20 ], developmental conditions [ 21 , 22 ], and even various types of cancer [ 23 , 24 ]. However, the validity of 2D:4D as a marker for prenatal testosterone exposure has been questioned [ 20 , 25 , 26 ]. Nonetheless, according to a PubMed search using the keywords “2D:4D” and “digit ratio” there were more than 100 newly published studies from 2020 till today that have investigated associations between 2D:4D and various behaviors, traits, disorders, or diseases, like muscular strength and fitness [ 5 , 27 ], parental income [ 28 ], sexual preferences [ 29 ], concentration of steroids like cortisol and vitamin D [ 30 ], thyroid disease [ 31 ], and migraine in adults [ 32 ].…”