Date Published: February 22, 2019
Publisher: Public Library of Science
Author(s): Kobe Millet, Aylin Aydinli, Valerio Capraro.
The current study seeks confirmation for the hypothesis that 2D:4D (positively) predicts prosociality when people are more likely to rely on intuition than deliberation. We assess intuition and deliberation using the Cognitive Reflection Test (CRT) and measure prosociality via the validated Social Value Orientation (SVO) slider measure. Although our results do not provide collective evidence for our main proposition, we observe in the data that for low (right) 2D:4D men, the more intuitive they are, the less prosocial they become, whereas for high (right) 2D:4D men the thinking style does not affect their prosociality. Importantly, we find that two alternative measures of cognitive reflection, CRT and CRT-2, differently relate to prosocial decision making such that only CRT-2 (but not the classic CRT) positively predicts prosociality. Given that previous research on the role of cognitive reflection and 2D:4D in prosocial decision making provided inconsistent results, the present study findings are highly valuable to get a better understanding in this domain of study. Furthermore, some of our findings invite further confirmatory tests, thereby opening up multiple avenues for further research.
Second to fourth digit ratio is the ratio of the index (2D) to ring (4D) finger, shortly referred to as 2D:4D. It is a putative marker of prenatal exposure to testosterone [1, 2] with a lower ratio pointing towards exposure of higher testosterone levels during pregnancy. Direct evidence for 2D:4D as a biomarker for organizational effects of prenatal testosterone has been provided in non-human mammals like rats  and mice [4, 5] and much more (indirect) evidence in humans has been provided as well. One of the most robust effects is the sexual dimorphism in 2D:4D with men having lower digit ratios than women , in line with the observation that testosterone levels in amniotic fluid are higher for male than female fetuses [7, 8]. At least, as illustrated by the increasing number of publications in the last decades 2D:4D is commonly accepted as an indirect biomarker for fetal testosterone exposure (despite a need for further validation ).
Two hundred eighteen participants between 18 and 31 years of age took part in the study (127 men, 91 women) of which 167 in return for course credit and 51 in return for a hedonic food reward (chocolate or pringles). All subjects gave informed consent in accordance with the Declaration of Helsinki and the study was approved by the School of Business and Economics Research Ethics Review Board (Vrije Universiteit Amsterdam). The study took place in the experimental lab of the School of Business and Economics where every participant was assigned to a computer in a partially enclosed carrel in which they could not see one another and could not talk. A maximum of 14 students participated at the same time.
We first explored sex differences (see Table 1) in (a) left and right 2D:4D (b) both CRT and CRT-2 and (c) SVO angle. In line with previous literature, men have a lower 2D:4D , score higher on the CRT [34, 52] and turn out to be less pro-social than women . We further replicate the findings indicating that there is no sex difference on the CRT-2  and that the right 2D:4D sex difference is larger than the left 2D:4D sex difference . Next, we performed simple correlation analyses in men and women separately between left 2D:4D, right 2D:4D, CRT, CRT-2, and SVO angle (see Table 2a and 2b). Interestingly, CRT and CRT-2 tend to differ in their relation with the general SVO angle measurement. A correlational analysis (irrespective of gender) indicates a positive relationship between SVO angle and the CRT-2 (r = .133; p = .050), but not the CRT (r = -.031, p = .649).
The current study is of importance for the growing literature on the biological foundations of prosocial behavior. Given the mixed findings currently reported in literature on how 2D:4D relates to prosocial behavior, more systematic investigation is needed to understand the relationship between 2D:4D and prosociality. Moreover, it is at least as important to focus on theoretically plausible moderators and to set up specific studies to understand if specific associations with 2D:4D emerge under particular circumstances. Therefore, the present study tested the possible moderating effect of cognitive reflection (a marker for intuitive vs. deliberative decision making) on the relationship between 2D:4D and prosociality. Further, the study procedure also allowed us to explore the potential direct relationship between cognitive reflection (using two separate measures, CRT and CRT-2) and prosociality (using the validated SVO slider measure). Speaking towards the reliability of our findings, we replicate previous findings showing (a) a sex difference in social value orientation , (b) a sex difference in performance on the classic CRT, which is attenuated for the CRT-2  (c) a stronger sex difference in right 2D:4D than in left 2D:4D .
In general, the current study does not provide evidence for the main hypothesis that the relationship between 2D:4D and social preferences may be influenced by people’s cognitive style. At the same time we urge for a confirmatory test of our observed pattern of results in men. At least, previous findings on sex differences in (a) right vs left 2D:4D, (b) CRT vs CRT-2 and (c) SVO are replicated and therefore speak towards the reliability of the results. Given the multitude of inconsistent findings and the omission of potentially important variables in the study on the connection between 2D:4D and social preferences, it is important to report studies that focus on both direct and moderating effects, irrespective of the significance of results. Therefore, the current study is of importance to the field. Finally, the relationship between CRT-2 and prosociality deserves further attention in future research.