Lack of correlation between digit ratio (2D:4D) and Baron-Cohen’s “Reading the Mind in the Eyes” test, empathy, systemising, and autism-spectrum quotients in a general population sample

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Abstract

The second-to-fourth digit ratio (2D:4D) is sexually differentiated and is a likely biomarker for the organisational (permanent) effects of prenatal testosterone on the human brain. Recent research has highlighted a possible role of prenatal testosterone levels in both the etiology of autism-spectrum disorders and in sex and individual differences in cognitive styles of the normal mind (Baron-Cohen’s Extreme Male Brain Theory of Autism and Empathising/Systemising Theory). Importantly, autistic children present lower (hypermasculinised) 2D:4D than healthy controls. Based on these accounts, we investigated the relation of 2D:4D with Baron-Cohen’s measures of empathising (“Reading the Mind in the Eyes” test, RMET; Empathy Quotient, EQ), systemising (Systemising Quotient, SQ), and autistic-like traits (Autism-Spectrum Quotient, AQ) in the general population (N = 423 Austrian adults). Whereas sex differences into the expected direction and of expected size were obtained for all variables and internal scale consistencies tallied to retrievable reference values, 2D:4D was unrelated to RMET, EQ, SQ, and AQ scores. Candidate explanations for this lack of correlation might be possible developmental timing differences in the expression of 2D:4D and empathising/systemising, qualitative (as opposed to quantitative) functional differences between the normal and the autistic mind, or the suboptimal psychometric properties of the measures.

Introduction

It has been known for well over a century that the ratio of the second digit (2D; index finger) to the fourth digit (4D; ring finger), hereafter digit ratio (2D:4D), is sexually differentiated (for a historical review, see Peters, Mackenzie, & Bryden, 2002). In humans, males on average have lower 2D:4D than females (i.e., present a longer 4D relative to their 2D). Only recently it has been suggested (Manning, Scutt, Wilson, & Lewis-Jones, 1998) that 2D:4D may be a promising biomarker for the degree of prenatal androgen exposure and sensitivity and the associated organisational (permanent) effects entailed by these on the structure and function of the body and the morphology and neural organisation of the brain. Apparently, prenatal testosterone promotes the growth of 4D, whilst prenatal estrogen the growth of 2D (Manning, 2002).

Since then, this suggestion (the “Manning hypothesis”) has generated much research interest, including the fields of differential and personality psychology (Austin, Manning, McInroy, & Matthews, 2002), and, as is now discernible, has stimulated an expanding interdisciplinary research program. Up to now, 2D:4D has been shown to be a correlate of a suite of sex-dependent, hormonally influenced variables, including behavioural, cognitive, personality, and somatic traits, adult-onset diseases, and measures related to fertility and sexuality (Manning, 2002).

Supportive evidence for the Manning hypothesis, that 2D:4D is a convenient somatic marker which reflects the configuration of the prenatal sex-hormonal environment in the sense of a retrospective window, has cumulated over the past few years (Manning, 2002) and is now fairly strong. Latest findings include the following lines of evidence: (1) The prenatally determined individual and sex differences in 2D:4D appear sufficiently stable during subsequent growth and puberty (McIntyre et al., 2005, Trivers et al., 2006). (2) Genetic polymorphisms in the androgen receptor gene, effectuating higher tissue sensitivity for testosterone, are related to lower (masculinised) 2D:4D in men and conversely (Manning, Bundred, Newton, & Flanagan, 2003). (3) Females from opposite-sex dizygotic twin pairs present lower (masculinised) 2D:4D compared to females from same-sex dizygotic twin pairs (van Anders, Vernon, & Wilbur, 2006). (4) Volumetric side differences in the hippocampal formation are related to 2D:4D, such that smaller left-side volumes of the posterior subregion (a male-typed pattern) concur with low (male-typed) 2D:4D in females (Kállai et al., 2005). (5) There is now direct evidence from an amniocentesis study, in which prenatal hormone levels were assayed in the amniotic fluid: high prenatal testosterone relative to estradiol levels were associated to a low postnatal 2D:4D, measured at two years of age (Lutchmaya, Baron-Cohen, Raggatt, Knickmeyer, & Manning, 2004).

Children with autism or Asperger syndrome (high-functioning autism), along with their first-degree relatives (unaffected siblings, mothers, and fathers), have markedly lower (hypermasculinised) 2D:4D than healthy local population controls (Manning, Baron-Cohen, Wheelwright, & Sanders, 2001). This constitutes an informative finding (for replications, see Milne et al., 2006, Osawa et al., 2005), since the autism-spectrum disorders show markedly biased sex ratios (male preponderance) and one contemporary model of the etiology of autism, coined the Extreme Male Brain Theory of Autism (EMB), links these conditions to high prenatal testosterone levels, although direct evidence on this point is still lacking (for overviews, see Baron-Cohen, 2002, Baron-Cohen et al., 2005).

Importantly, EMB theory is a dimensional, quantitative approach to autism, embedded into the broader Empathising/Systemising (E/S) theory of sex differences in the normal mind (Baron-Cohen, 2003). The gist of this conjoined theoretical framework is as follows: one main biological, sex-differentiated factor (assumingly, prenatal testosterone levels, differing markedly between the sexes) both effectuates normal mind-related differences (“brain types”) seen between males and females and (at very high levels) gives rise to the “extreme male brain type” (postulated as being identical with the autism-spectrum disorders).

The central hypothesis of E/S theory is that there are biologically based sex differences in the mind, such that the female brain is predominantly hard-wired for empathising (E) and the male brain for systemising (S). E is the drive to identify, care about, and react to others’ feelings and thoughts, i.e., the natural way to understand a person. S is the drive to analyse, explore, understand, construct, or invent systems, i.e., the natural way to predict events and objects. By system, anything governed by rules specifying input-operation-output relationships is understood.

Individual variation in E and S gives rise to different brain types. Brain type E (with E > S, preponderant in females) is tuned into emotional relationships, preoccupied with people, showing an empathising cognitive style. Its mirror image, brain type S (with E < S, preponderant in males), is tuned into systems, preoccupied with things, showing a systemising cognitive style. As for one of the distributional margins of the E/S dimensional space, the extreme male brain (E << S) is very low in E, coupled with a talented S ability (hypersystemising), resulting in “mindblindness”. EMB theory proposes this pattern to be at the biological core of the autism-spectrum conditions.

As a consequence of the quantitative character of E/S and EMB theory, sex and individual differences in the emergence of mild forms of autistic-like traits (i.e., the extended/broader autism phenotype) are expected within healthy populations. In order to measure autistic-like traits as well as empathising and systemising, several psychometric instruments have been devised recently (Baron-Cohen, 2003, pp. 187–222). Further, supportive evidence for E/S theory has come from an amniotic fluid study, which found lower prenatal testosterone levels, as seen more commonly in females, linked to empathising traits (better language levels, communication skills, eye contact, and social skills), whereas higher levels, more commonly seen in males, linked to good systemising abilities in toddlers (Baron-Cohen, Lutchmaya, & Knickmeyer, 2004).

Based on the previous findings of a hypermasculinised 2D:4D in autistic children (consistent with EMB theory), the aim of the present study was to investigate whether 2D:4D is related to empathising, systemising, and autistic-like traits in the general population (expected by E/S theory; Baron-Cohen, 2003, pp. 153–154).

According to the “Austin-Manning hypothesis” (Austin et al., 2002, p. 1117), 2D:4D should correlate negatively with male-typed traits and positively with female-typed traits. This pattern, coupled with sex-linked (or sex-limited) associations, has been repeatedly found in digit ratio research: e.g., spatial ability, a male-typed trait, is negatively related to 2D:4D in males only (Manning, 2002), whereas reactive aggression, a female-typed trait, is positively related to 2D:4D in females only (Benderlioglu & Nelson, 2004). Further, for reasons not yet well understood, most studies have found more pronounced target-trait associations with 2D:4D of the right hand (R2D:4D) relative to the left (L2D:4D).

The present research hypotheses were thus as follows: 2D:4D should be negatively related to measures of systemising and autistic-like traits for males (but not females), while positively related to measures of empathising for females (but not males), and these sex-linked associations were expected to be stronger for R2D:4D relative to L2D:4D.

Section snippets

Sample

The sample comprised 423 native, heterosexual Austrians (206 men, 217 women) from the general population, aged 15–60 years (M = 29.7, Mdn = 24.5, SD = 11.4 years). Participants were in their younger or mid adult years, but predominantly above college age (lower and upper quartiles of the age distribution were 21 and 38 years, respectively). 18% had tertiary, 59% secondary, and 23% primary-level education; 37% were currently unmarried, but partnered, 33% unmarried and unpartnered, and 24% married (the

Finger-length measurement repeatabilities

Intraobserver measurement repeatabilities were as follows (all ps < 0.001, df1 = df2 = 422 for the F ratios): ICC = 0.989 for right 2D (F = 182.7), 0.994 for right 4D (F = 336.3), 0.992 for left 2D (F = 255.9), 0.993 for left 4D (F = 298.9), 0.936 for R2D:4D (F = 30.2), and 0.922 for L2D:4D (F = 24.7). Measurement concordance was high and interindividual differences statistically significantly and by orders of magnitude larger than measurement error. The measurements therefore reflected real differences between

Discussion

This study found individual differences in 2D:4D to be unrelated to individual differences in empathising, systemising, and autistic-like traits in the general population. Owing to the fairly large sample (one of the largest within 2D:4D research so far), a type II error (false-negative finding) appears unlikely: correlations of 0.137 (for males) and 0.134 (for females) would have been statistically reliable (p < 0.05, two-tailed), but none achieved even these modest levels. Expectable

Acknowledgements

The experimental psychology class students (winter-term 2003) of M.V. are gratefully acknowledged for participation in data collection.

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