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Findings regarding within-sex associations of digit ratio (2D:4D), a putative pointer to long-lasting effects of prenatal androgen action, and sexually differentiated personality traits have generally been inconsistent or unreplicable, suggesting that effects in this domain, if any, are likely small. In contrast to evidence from Wilson's important 1983 study, a forerunner of modern 2D:4D research, two recent studies in 2005 and 2008 by Freeman, et al. and Hampson, et a!. showed assertiveness, a presumably male-typed personality trait, was not associated with 2D:4D; however, these studies were clearly statistically underpowered. Hence this study examined this question anew, based on a large sample of 491 men and 627 women. Assertiveness was only modestly sexually differentiated, favoring men, and a positive correlate of age and education and a negative correlate of weight and Body Mass Index among women, but not men. Replicating the two prior studies, 2D:4D was throughout unrelated to assertiveness scores. This null finding was preserved with controls for correlates of assertiveness, also in nonparametric analysis and with tests for curvilinear relations. Discussed are implications of this specific null finding, now replicated in a large sample, for studies of 2D:4D and personality in general and novel research approaches to proceed in this field. [ABSTRACT FROM AUTHOR]

Preliminary evidence suggests that within-sex individual variation in the length ratio of the index finger to the ring finger (2D:4D), a putative marker of prenatal androgen levels, may be more strongly or consistently related to implicit measures (Implicit Association Test, IAT) than to corresponding explicit (self-report) measures of candidate personality traits. The underlying logic is that the non-introspective TAT may reflect earlier and inaccessible experiences, whereas introspective self-reports reflect more recent and necessarily accessible experiences. Associations of 2D:4D and absolute finger length (a marker of pubertal-adolescent androgen levels) with implicit versus explicit aggression measures (TAT vs. Buss-Perry Aggression Questionnaire and a feeling thermometer) were examined in two samples with identical procedures (Ns= 244 and 233). Attesting to procedural validity and data typicality, several experiments of related research were replicated in both samples, including sex differences in 2D:4D and theory compliant interrelations among psychometric measures. However, no theory compliant, reliable, or replicable associations of 2D:4D or finger length with implicit or explicit aggression measures resulted. These nil findings cast doubt on hypothesized advantages of implicitly (over explicitly) measured target traits for 2D:4D research. More generally, they add to a growing number of replication failures in this area. [ABSTRACT FROM AUTHOR]

The second-to-fourth digit ratio (2D:4D) is sexually dimorphic in humans, such that men on average have a lower 2D:4D than women. This somatic trait has been proposed as a biomarker for the organizational (permanent) effects of prenatal testosterone on the brain and behavior. Over the past few years, an accumulating research program has shown 2D:4D to be related to a multitude of sex-dependent, hormonally influenced psychological and behavioral traits. The present study investigated the 2D:4D ratio of 44 men and 70 women from 36 identical and 21 fraternal twin pairs. Both basic and advanced approaches for estimating heritability concordantly suggested that the trait is substantially heritable. The best-fitting Structural equation model indicated that the contributions to individual differences in 2D:4D are 81% additively genetic, 19% nonshared environmental, and 0% shared environmental. Supplemental analyses showed that, consistent with a prediction from sex-hormone transfer theory, women from opposite-sex fraternal twin pairs had significantly lower (more male-typical) 2D:4D than women from same-sex fraternal twin pairs. Directions for research are discussed, such as investigating possible influences of the sex chromosomes on the expression of 2D:4D. Further family studies will be needed to test whether the transmission mode of 2D:4D is consistent with X-linked or Y-linked inheritance. The study of sex chromosome aberrations should indicate whether the presence of additional X or Y chromosomes is associated with 2D:4D levels. [ABSTRACT FROM AUTHOR]