From an intriguing new study, titled (spoiler alert!) “Digit ratio (2D:4D) is associated with traffic violations for male frequent car drivers,” by Andreas Schwerdtfeger, Regina Heimsa, and Johannes Heera, published in the current Accident Analysis and Prevention, which examines so-called “digit ratio” in the context of driving.

What’s digit ratio, you ask?

The ratio of the finger length of the index finger (2D) relative to the ring finger (4D) (2D:4D; digit ratio) represents a putative marker of prenatal hormone exposure. Specifically, the length of the fourth finger (ring finger) seems to be affected more strongly by testosterone exposure during fetal development, whereas the length of the second (index) finger seems to be more closely related to prenatal estrogen exposure (Manning, 2002). There is evidence for the hypothesis that digit ratio is related to prenatal androgens such that higher testosterone relative to estrogen exposure is associated with a lower digit ratio (e.g., [Brown et al., 2002a], [Brown et al., 2002b], [Lutchmaya et al., 2004] and [Ökten et al., 2002]). This association seems to be influenced by the action of the Homebox (so-called Hox) genes, which control differentiation of digits and the urogenital system including testes and ovaries (e.g., Kondo et al., 1997 T. Kondo, J. Zakany, J.W. Innis and D. Duboule, Of fingers, toes and penises, Nature 390 (1997), p. 29. Full Text via CrossRef | View Record in Scopus | Cited By in Scopus (125)[Kondo et al., 1997] and [Pauls et al., 2006]). Indeed, males have been found to exhibit a lower digit ratio (i.e., relatively shorter index finger length than ring finger length) than females, reflecting a preponderance of ontogenetic testosterone over estrogen exposure. This difference has been found to emerge during fetal development, to be fixed by the 13th intrauterine week, and to remain stable thereafter ([Garn et al., 1975], [Manning, 2002] and [Trivers et al., 2006]). Thus, digit ratio represents a promising variable for examining organizational effects on the developing brain (e.g., Manning, 2002).

The study looked at a group of German drivers (who worked in sales, it seems, and drove a lot).

It was found that digit ratio was a robust predictor of penalty point entries, even after controlling for other relevant demographic and personality variables. A more masculinized digit ratio (relatively longer ring finger than index finger) was related to more traffic violations. Hence, this result extends previous findings in showing that organizational effects of prenatal androgens might increase not only financial risk-taking behavior (e.g., [Apicella et al., 2008], Coates et al., 2009 J.M. Coates, M. Gurnell and A. Rustichini, Second-to-fourth digit ratio predicts success among high-frequency financial traders, Proceedings of the National Academy of Sciences USA 106 (2009), pp. 623–628. Full Text via CrossRef | View Record in Scopus | Cited By in Scopus (6)[Coates et al., 2009] and [van Honk et al., 2004]), but also traffic-related risk behavior in later life, thus enhancing the probability of being prosecuted for traffic infringements. A closer look at the data suggests that a 0.01-point decrease in digit ratio is accompanied by about the same amount of increase in penalty points as is the increase in annual mileage of approximately 20,000 km. Moreover, digit ratio was a more robust predictor of penalty points than years licensed. Taken together, the findings suggest that gonadal hormone exposure in utero might increase traffic violations in later life, in addition to already established risk factors.

Could insurance companies start offering short-finger discounts?

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