The ratio of the second to fourth finger digits (hereafter referred to as “2D:4D ratio” or just “ratio”) refers to the length of the second finger digit (index finger) divided by the length of the fourth finger digit (ring finger). The ratio has been suggested to serve as a biomarker for prenatal androgen activity for a variety of conditions, including autism spectrum disorder (ASD), congenital adrenal hyperplasia, and Klinefelter syndrome (Manning et al. 2014
). Indeed, Lutchmaya et al. (2004
) explored the 2D:4D ratio in human fetuses and found that higher levels of testosterone compared with estrogen were associated with lower ratios, while lower levels of testosterone compared with estrogen were associated with higher ratios. Since low 2D:4D ratios are related to higher fetal testosterone and lower fetal estrogen exposure, lower ratios are more common in males and higher ratios in females (Galis et al. 2010
; Hampson et al. 2008
; Malas et al. 2006
; Manning et al. 2002
; Voracek and Dressler 2007
The 2D:4D ratio in relationship to ASD has been examined, in part, due to the extreme male brain theory (Baron-Cohen et al. 2005
), which purports an influence of fetal testosterone exposure on the development of ASD. The majority of the studies have shown lower 2D:4D ratios in individuals with ASD (Al-Zaid et al. 2015
; de Bruin et al. 2009
; Honekopp 2012
; Manning et al. 2001
; Teatero and Netley 2013
), although a recent study with nearly 6000 children failed to find a relationship between a lower ratio and ASD or autistic traits (Guyatt et al. 2015
). Mixed results have been shown for the association of the ratio with autism symptoms, autistic traits, as well as other autism related endophenotypes. de Bruin et al. (2009
) explored the 2D:4D ratio in ASD, other neurodevelopmental disorders (NDDs) and psychiatric disorders and found a negative association for the left-hand ratio with autism symptoms on the Autism Diagnostic Observation Schedule—Generic in a small subsample of girls. Empathizing traits (assumed to be higher in females) and systemizing traits (assumed to be higher in males) have been explored previously in studies on ASD, but a review by Honekopp (2012
) on the association between the digit ratio these traits found no evidence for a link in typically developing adults, which is in alignment to earlier findings by Voracek and Dressler (2006
Although the sex ratio is skewed in most NDDs (American Psychiatric Association 2013
), the exploration of the 2D:4D ratio is limited in other NDDs, such as attention-deficit/hyperactivity disorder (ADHD), intellectual disability (ID), communication disorders, specific learning disorders, and motor disorders or broader defined psychiatric groups. However, one study (de Bruin et al. 2006
) showed that males with ASD and ADHD had lower 2D:4D ratios in comparison with males with anxiety disorders or typical development.
A previous study investigating the 2D:4D ratio in twins suggests that the ratio is highly heritable, with an estimate around 80% (Voracek and Dressler 2007
). The study included typically developing twins only, not clinical samples with NDDs. Twin studies provide insight into the proportion of genetic and environmental factors influencing phenotypes like the ratio. The main premise of the twin design is that monozygotic twins share nearly identical genetic information and therefore, differences in outcomes like the ratio can be attributed with high likelihood to environmental factors (Martin et al. 1997
; Willfors et al. 2017
Technically, studies exploring digit ratios have primarily used either scanned or photocopied images of the palmar surface of hands, along with calipers, to indirectly measure digit length (Al-Zaid et al. 2015
; de Bruin et al. 2009
; Guyatt et al. 2015
; Manning et al. 2001
). Indirect measurements using photocopies of hands have been shown to be highly repeatable, but may result in lower digit ratios compared with direct measurements. This lower ratio is speculated to be due to changes that occur in the fat pad and finger curvature when taking a photocopy of the hand versus direct measurement (Manning et al. 2005
). On the contrary, a previous study by Manning et al. (2000
) found no difference between direct and indirect measurements and similarly, a meta-analytic review by Honekopp (2012
) found the method of measurement of the 2D:4D ratio used in the studies reviewed for individuals with ASD did not have an effect on the ratio. Medical photographs can be taken of hands so that the fingers are not compressed on a photocopier or scanner. Additionally, the use of medical photography allows for easy use of a digital measurement programs to assess digit length. Digital measurement programs are now freely available and can quickly measure finger lengths in images, with the possibility for automation in the future to measure finger lengths in a large number of images quickly and precisely.
Because there is substantial overlap among NDDs and research shows higher rates of ASD and ADHD in males (Ramtekkar et al. 2010
; Werling and Geschwind 2013
), further studies are desirable to examine the association between NDDs, neurodevelopmental traits, and the ratio across sexes. Furthermore, to the authors’ best knowledge, no study has yet examined the 2D:4D ratio in a sample of twins with NDDs, which could provide insight into genetic and/or environmental influences on the ratio. Thus, this study sought to investigate the 2D:4D ratio in a carefully characterized, rare sample of monozygotic (MZ) and dizygotic (DZ) twins concordant or discordant for ASD, ADHD and other NDDs as well as typically developing (TD) control pairs using digital measurement of high-quality medical photographs of hands. We aimed to examine the association between the 2D:4D ratio for (i) NDDs as a whole, ASD and ADHD separately, and autistic traits, (ii) sex, and (iii) zygosity. We predicted lower 2D:4D ratios in individuals with categorical diagnoses of NDD and males compared to TD and females, respectively, a negative correlation between the ratio and autistic traits, and higher correlations of the ratio in MZ versus DZ twins.
This study examined the association between the 2D:4D ratio and NDD diagnoses, as well as autistic traits. Findings replicate previous observations of sexual dimorphism in the ratio with a lower overall 2D:4D ratio in males compared to females (Galis et al. 2010
; Malas et al. 2006
; Manning et al. 2004
). As expected, 2D:4D ratios were highly correlated in MZ twins, who share all or nearly all their genetic information, and also in DZ twins, who share 50% of their genetic profiles. An association was found between the 2D:4D ratio and the presence of any NDD and ADHD diagnoses in males in the between-pairs model and any NDD in females in the within-pairs model. For males, the finding of a significant relationship between the ratio and any NDD or ADHD diagnosis in the between-pairs model may suggest the influence of genetic factors in the development of the ratio, resulting in a lower 2D:4D ratio for those males with a NDD in general or an ADHD diagnosis in particular. For females, the relationship between the ratio and NDDs may be masked by confounding factors between twin pairs, such as genetics and shared environment, and therefore, the link is only observed when controlling for these factors as occurs in the within-pairs model. In contrast to published meta-analyses, which reported ratios lowered between .011 and .077 in individuals with ASD versus those without (Honekopp 2012
; Teatero and Netley 2013
), we did not find a relationship between the ratio and a diagnosis of ASD, consistent with results from a large recent study (Guyatt et al. 2015
Descriptively, male pairs concordant for ASD had the lowest overall 2D:4D ratio compared with females with concordant ASD. These results are consistent with Guyatt et al. (2015
), who demonstrated males with ASD to have lower overall 2D:4D ratios compared with females with ASD. In contrast to our hypotheses, the between-pairs model demonstrated a positive association between the ratio and autistic traits for females only, indicating that as the ratio increased, there was a tendency for autistic traits to also increase in females. It is unclear why this finding occurred and the results need to be interpreted with caution.
This study had several strengths, including the use of two, blind raters conducting the digit measurements and a sample of both and female twins with well-characterized NDDs and twins with TD. The twin design adjusts for genetic and environmental factors shared between twins in pairs, thereby excluding confounders like genetics. Potential limitations to this study include participants coming from a limited geographical region, the subjectivity of the measurements, even with a digital measurement system, and the broad age range of included twins for some analyses. The limited geographical region may have resulted in measurements that were more characteristic of a Northern European population as our digit ratios were much higher than what has been reported in previous studies with samples from other regions. For example, our study found that individuals with a concordant diagnosis of ASD had an overall hand 2D:4D ratio that was Md
= 1.014 for females and Md
= .970 for males. In contrast, Guyatt et al. (2015
) studied a population with ASD in the United Kingdom and found mean ratios of 0.969 and 0.959 for females and males, respectively. Our study used a digital measurement system (i.e., Image J) where two raters demonstrated high interrater reliability for ratio assessment. Final limitations are the generalizability of study findings to populations that do not include twins, as well as potential for chance findings due to the relatively large number of statistical tests performed. Although we present some significant findings for males and females in regards to lower digit ratios and the presence of any NDDs, and ADHD specifically for males, we might have missed associations with other single NDD diagnoses, potentially owing to smaller sample sizes in the NDD subgroups.
Influential autism research paradigms like the extreme male brain theory have purported the 2D:4D ratio as a potential avenue to explore in terms of ASD etiology. The primary findings from this study, which included participants with not only ASD, but also other NDDs, suggest that the effect of testosterone on the diagnoses of ASD may not be as strong as previously thought or perhaps rather affects neurodevelopment in a broader fashion. While associations for NDD subgroups other than ADHD, specifically for ASD, might have been missed due to subgroup sample size, other minor physical features might be more closely related to altered neurodevelopment in general and ASD in particular (Myers et al. 2017
) and may warrant further study.