This procedure was repeated a second time with a separate, shuffled deck of playing cards, which resulted in each participant having two separate, randomly selected, five card poker hands, drawn from two separate decks of cards. The value and suit of the cards were recorded. The participant was then requested to select any five of the face down cards and flip them over. Cards were thoroughly shuffled by an investigator and then fanned out, face down, onto a smooth, flat surface. Participants were asked to select five cards from a deck of playing cards (United States Playing Card Company, Erlanger, KY). We performed this study to demonstrate that random chance alone could produce seemingly convincing results, rather than validating the use of 2D:4D as a biomarker. We hypothesized that, by random chance alone, 2D:4D would show a statistically significant relation with good luck, similar in magnitude to body composition parameters. To put this in the context of clinically relevant outcomes, we also sought to determine whether 2D:4D was related to body composition parameters, which could plausibly be related to prenatal androgen exposure. With previous research determining that 2D:4D is related to a diversity of outcomes that seem to ultimately shape one’s decisions in life and fate, we aimed to explore the magnitude to which 2D:4D is associated with good luck (using a randomly drawn poker hand selected by each participant as a surrogate measure). 36 Thus, research involving 2D:4D could provide a prime example of the reproducibility crisis in medicine and science, and of the perpetuation of research based on weak scientific hypotheses, 37 non-rigorous methodology, and an over-dependence on confirming hypotheses from weak (potentially spurious) correlations. 35 Even if most studies produce null results, selective reporting and publication bias can create the appearance of consistent positive effect. This practice of including bonus factors without sufficient scientific justification is known to produce spurious associations in other areas of research. 33 The digit ratio is also easy to measure, facilitating its inclusion in larger studies, where attempts can be made to correlate it to many other metrics (eg, the BBC internet study 34). Digit ratio studies almost always include many comparisons, which increases the likelihood of false positive findings. 32Īssociations between 2D:4D and various outcomes are generally justified through (tenuous) biological explanations, but the possibility of spurious correlations are seldom considered. It is implausible to think that one biomarker in utero not only predicts risk of myocardial infarction and age of onset, 24 but also is associated with the likelihood of a person becoming a firefighter, 25 having musical ability, 26 27 showing pro-environmental consumption behavior, 28 having a sense of directionality, 29 being successful at Sumo wrestling, 30 being obsessed with celebrities, 31 or making a specific choice of Coca-Cola products from a vending machine. 23 However, thorough examination of the literature raises questions about the validity and reproducibility of 2D:4D research. 22 In 2020, 2D:4D was suggested to help “identify those for whom it would be advisable to exercise social distancing” to avoid contracting covid-19. 15 16 17ĭespite a lack of solid physiological justification for studying 2D:4D, an abundance of studies claim that this anthropometric remnant of the prenatal hormonal environment relates to risk of disease in adulthood (that is, cancers 18 and cardiometabolic disease 19 20 21), age of onset, prognosis, and treatment options. 11 12 However, little evidence supports this hypothesis in humans, 13 14 because prospective studies have reported a lack of consistent associations between androgen concentrations in the amniotic fluid or umbilical cord blood and 2D:4D in childhood and adulthood. 10 Variations in 2D:4D are also speculated to be rooted in genetic polymorphisms that influence testosterone metabolism and sensitivity. 6 7 8 9 These studies generally postulate that a lower 2D:4D reflects greater testosterone exposure (or greater testosterone-to-estrogen ratio), which accounts for men having lower 2D:4D than women. The digit ratio is often claimed to be a surrogate for prenatal androgen exposure, based on cross sectional human studies and experimental animal studies. The ratio between the length of the second and fourth digit, referred to as the 2D:4D digit ratio (2D:4D) has received considerable attention in the scientific community ( fig 1) and mainstream media 1 2 3 4 5 because of its apparent association with health and behavior.
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