Correlates of 2D:4D Ratios and Fluctuating Asymmetry Patrick J. Cooper

1. Correlates of 2D:4D Ratios and Fluctuating Asymmetry • Patrick J. Cooper • The Pennsylvania State University, Altoona INTRODUCTION The 2D:4D ratio refers to the length of one’s index finger (2D) with respect to the length of their ring (4D) finger. Fluctuating asymmetry (FA) refers to deviation from perfect bilateral symmetry caused by environmental stresses, developmental instability, and genetic problems during development. It is commonly theorized that both high FA (high variability in the measures of paired traits) and lower 2D:4D ratios (greater length differences between the second and fourth digits) are a result of prenatal testosterone (T) exposure. Abnormal T exposure leads to developmental instability which results in certain personality characteristics such as greater spatial ability, decreased verbal ability, increased aggression, increased number of sexual partners, and early age of first sexual experience. Past research is conclusive in suggesting that men have lower 2D:4D ratios and higher FA than women. Since men are naturally exposed to more T, it has been suggested that prenatal T exposure results in high FA and low 2D:4D ratios. AIM To see if certain personality characteristics and mental abilities are correlated with FA and 2D:4D ratios. METHOD Participants 143 subjects were selected from a local university. Students signed up for the study and received extra credit in their respective course for participating. Males (N = 62) mean age was 23.10 years. Females (N =81) mean age was 22.86. Procedures Subjects completed a demographic questionnaire asking questions about their sexual history. Then they completed the Buss and Perry Aggression Questionnaire which measures physical and verbal aggression as well as anger and hostility. Following this, subjects completed the Vanderberg and Kuse Mental Rotations Test (MRT), which measured spatial ability, and the Thurstone Verbal Fluency Test. After the tests, the length of each ear was measured to the nearest millimeter (mm) from top to bottom using digital calipers. Each ankle’s circumference was measured using a standard tape measure exactly 6 inches above the bottom of the foot. Fingerprints of each of the participant’s fifth digit and first digit were also taken. Finally, subjects’ hands were photocopied so the digit lengths and ratios could be measured. MEASURES Digit Length Ratios (2D:4D) Calculated by dividing the length of the second digit (index finger) by the length of the fourth digit (ring finger). Ratios were calculated for both hands. Fluctuating Asymmetry (FA) Calculated by finding the difference between the measures of paired traits on the body. This includes differences in ankle circumferences, ear lengths, and the first through fourth digits. Composite FA was compiled by adding the asymmetry from each individual trait. RESULTS The results indicate that 2D:4D ratios are sexually dimorphic for the right side. Males (M = .959, SD = .032) have significantly lower right side 2D:4D ratios (R2D:4D) than females (M = .973, SD = .040, p < .023). This can be accounted for by the organizational effects of Testosterone impacting the right side of the body more than the left. Irrespective of gender, R2D:4D is significantly negatively correlated with Physical Aggression (r = -.246, p < .003). The number of sexual partners is also correlated with Physical Aggression (r = .198, p < .023) as well as Hostility (r = .210, p < .015) and Total Aggression (r = .203, p <.019). Concerning males, Spatial Ability is correlated with Physical Aggression (r = .339, p < .007) and Total Aggression (r = .283, p < .026). However, for females, these same relationships between Spatial Ability and Physical Aggression (r = -.283, p < .014) and Total Aggression (r = -.273, p < .014) were negative. FA was negatively correlated with spatial ability for men (r = -.266, p < .046), and positively correlated with Verbal Aggression for women (r = .234, p < .042). The results also indicate that there were no sex differences for number of sexual partners, verbal fluency ability, or composite FA. Relevant means and correlations can be found in Tables 1 and 2. Table 1 Summary of Relevant Mean Sex Comparisons _______________________________________________________________________ ANOVAsMeanp-value 2D/4D ratio .023 * Male .959 Female .973 _______________________________________________________________________ Verbal Ability .959 ns Male 81.70 Female 81.52 _______________________________________________________________________ Spatial Ability .0001 * Male 22.69 Female 15.51 _______________________________________________________________________ Number of Sexual Partners .688 ns Male 4.29 Female 3.93 _______________________________________________________________________ FA .589 ns Male 14.24 Female 13.55 _______________________________________________________________________ Total Aggression .0001 * Male 54.32 Female 43.64 _______________________________________________________________________ Note. * = statistically significant Figure 1. Mean aggression scores for male and female participants. PA = Physical Aggression, VA = Verbal Aggression, ANG = Anger, HOS = Hostility. * = p < .01, NS = p > .05. Table 2 Summary of Relevant Correlations __________________________________________________________________________ Pearson’s rp-value Correlations 2D:4D r = -.246 .003 * Physical Aggression __________________________________________________________________________ Total Aggression r = .203 .019 * Number of Sexual Partners __________________________________________________________________________ Physical Aggression (males) r = .339 .007 * Spatial Ability __________________________________________________________________________ Total Aggression (males) r = .283 .026 * Spatial Ability __________________________________________________________________________ Physical Aggression (females) r = -.273 .014 * Spatial Ability __________________________________________________________________________ Total Aggression (females) r = -.240 .031 * Spatial Ability __________________________________________________________________________ FA (males) r = -.266 .046 Spatial Ability __________________________________________________________________________ FA (females) r = .234 .042 Verbal Aggression __________________________________________________________________________ Note. * = statistically significant DISCUSSION The current study reiterates the evidence that 2D:4D ratios are sexually dimorphic traits. Males have lower ratios. This suggests that prenatal T exposure, which is more prominent in males, is associated with lower 2D:4D ratios. Also consistent with the literature, was the finding that males displayed more aggression and scored higher on tests of spatial ability. It is tempting to speculate that this is mediated by T exposure. Indeed, Total Aggression was positively associated with the number of sexual partners, and for males, Physical and Total Aggression were positively associated with spatial abilities – the more aggression reported, the higher the spatial scores. Interestingly, this relationship was reversed for the female participants. That is, Physical and Total Aggression were negatively associated with spatial abilities – the more aggression reported, the lower the spatial scores. This finding is paradoxical in light of the fact that low 2D:4D ratios are associated with aggression, even when examining only the female participants. More research is needed to understand the causal pathway between early testosterone, aggression, and spatial abilities, especially in female participants. However, the current results are consistent with the literature in suggesting that more male like 2D:4D ratios (early T exposure) are associated with aggression. Finally, high levels of FA were associated with poor spatial skills in men and more verbal aggression in women, further evidence for negative outcomes associated with deviations from perfect symmetry. Figure 2. Hypothetical Relationship between 2D:4D ratios, Aggression, and Spatial Scores Among the Sexes

2. Results -2D:4D sexual dimorphism is only significant for the right side of the body; males have significantly lower R2D:4D ratios. -Reason: organization effects of T dramatically influence the right side more than the left. Both Sexes -R2D:4D is negatively correlated with Physical Aggression. -Number of Sexual Partners is positively correlated with Physical Aggression. -Number of Sexual Partners is positively correlated with Hostility. Males -Spatial Ability is positively correlated with Physical Aggression. -Spatial Ability is positively correlated with Total Aggression. -FA is negatively correlated with Spatial Ability. Females -Spatial Ability is negatively correlated with Physical Aggression. -Spatial Ability is negatively correlated with Total Aggression. -FA is positively correlated with Verbal Aggression. Other Relevant Facts -No sex differences for number of sexual partners, verbal fluency ability, or composite FA. Introduction -2D:4D ratio refers to the length of one’s index finger in respect to their ring finger. -Found by dividing the length of 2D by length of 4D -FA refers to deviation from perfect bilateral symmetry. -Found by finding the difference between certain paired traits. -Prenatal T Exposure - Low 2D:4D ratios and high FA are a result of excessive prenatal T exposure. -Leads to developmental instability -Sexual Dimorphism - 2D:4D is a sexually dimorphic trait with males having lower ratios. -Aim – To find correlates of 2D:4D ratios and FA that either support or disprove past research Previous Correlates of 2D:4D -Traits previously correlated with low 2D:4D ratios include: -Increased Aggression -Greater Spatial Ability -Decreased Verbal Ability -Increased Number of Sexual Partners Correlates of 2D:4D Ratios and Fluctuating Asymmetry Discussion Evidence that Supports the Literature -Males have lower ratios. -Males are more aggressive. -Males have better spatial ability. -Males with higher FA have poor spatial ability -Females with higher FA have increased verbal aggression. New Evidence -Low 2D:4D leads to high aggression in both males in females. -Low 2D:4D leads to high spatial ability in males but low spatial ability in females. -It is thought that low 2D:4D would lead to higher spatial ability and aggression in both sexes. -Current research suggests that low 2D:4D leads to high aggression, but completely opposite results in spatial ability -More research is needed to understand the causal pathway between T, aggression, and spatial skills for females. Methods -Buss and Perry Aggression Questionnaire – measures aggression -Vanderberg and Kuse Mental Rotations Test – measures spatial ability -Thurstone Verbal Fluency Test – measures verbal ability -Demographics Questionnaire – sexual history -Measures of FA include ankle circumferences, ear lengths, and digit lengths Statistical Methods -One way between subjects ANOVA measured sex differences. -Pearson’s correlations measured specific correlations between variables.