Twin studies provide insight into the proportion of genetic and environmental factors influencing phenotypes like the ratio. The study included typically developing twins only, not clinical samples with NDDs. 2006) showed that males with ASD and ADHD had lower 2D:4D ratios in comparison with males with anxiety disorders or typical development.Ī 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). 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).Īlthough 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. ( 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. Mixed results have been shown for the association of the ratio with autism symptoms, autistic traits, as well as other autism related endophenotypes. 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. The majority of the studies have shown lower 2D:4D ratios in individuals with ASD (Al-Zaid et al.

2005), which purports an influence of fetal testosterone exposure on the development of ASD. The 2D:4D ratio in relationship to ASD has been examined, in part, due to the extreme male brain theory (Baron-Cohen et al.

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. ( 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. 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. 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).

2d 4d ratio study pdf