Digit ratio 2D:4D in relation to autism spectrum disorders, empathizing, and systemizing: a quantitative review.
Finger-length ratio is a faint autism marker, yet it does not explain empathizing or systemizing differences.
01Research in Context
What this study did
Hönekopp (2012) pooled every paper that linked finger-length ratio to autism, empathy, or systemizing. The team compared 2D:4D scores from people with autism and neurotypical controls.
They also checked whether lower 2D:4D (a marker thought to reflect prenatal testosterone) tracks with higher systemizing or lower empathizing in typical adults.
What they found
Across studies, autistic groups had slightly lower 2D:4D than controls. The difference was real but small.
Yet, in typical adults, 2D:4D barely correlated with empathizing or systemizing scores. The hormone-linked marker did not explain sex differences in these traits.
How this fits with other research
Orsmond et al. (2009) extends the finding to girls. They saw a strong link between low 2D:4D and autistic features in girls from a clinic, showing the marker may flag risk earlier and more clearly in females.
Lawson et al. (2004) is a predecessor that first supported the extreme-male-brain view: adults with Asperger's scored low on empathizing and high on systemizing. Hönekopp (2012) now supersedes that simple story by showing 2D:4D does not predict those traits in typical adults.
Pan et al. (2022) replicates the null result in children. They found empathizing, not systemizing, tied to autistic traits, matching the meta-analysis's conclusion that systemizing alone is not the key driver.
Why it matters
If you assess autism, measure 2D:4D only as a weak red flag, not a diagnostic tool. Do not assume kids with lower ratios will automatically show high systemizing or low empathizing. Instead, probe real-world empathy and social skills directly. Shift your interviews and interventions toward empathizing gaps rather than systemizing strengths.
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02At a glance
03Original abstract
Prenatal testosterone (PT) effects have been proposed to increase systemizing (the drive to understand lawful input-output relationships), to decrease empathizing (the drive to understand others), and to cause autism via hypermasculinization of the brain. Digit ratio 2D:4D is a putative marker of PT effects in humans. An online study (n = 1896) into the relationship between the Reading the Mind in the Eyes Test (a widely used measure of empathizing) and self-measured 2D:4D in a nonclinical sample is reported. No evidence for a link between empathizing and 2D:4D in either females or males emerged. Further, three meta-analyses are presented that look into the relationships of 2D:4D with autism spectrum disorder (ASD), systemizing, and empathizing. 2D:4D was substantially lower (more masculine) in ASD-affected individuals than in normal controls (d = -0.58, P < 0.001). However, 2D:4D was found to be virtually unrelated to systemizing and empathizing in normal adults. The results support the idea that high PT is a risk factor for autism, but they challenge the view that PT substantially contributes to sex differences in systemizing and empathizing. Possibly, this pattern reflects an interaction effect, whereby PT drives ASD characteristic changes only in brains with a specific damage.
Autism research : official journal of the International Society for Autism Research, 2012 · doi:10.1002/aur.1230