A critical review of the research on the extreme male brain theory and digit ratio (2D:4D).
A low 2D:4D finger ratio gives a small, reliable hint of male-linked autism risk.
01Research in Context
What this study did
Matson et al. (2013) pooled every paper that measured the 2D:4D finger ratio in people with and without autism.
They ran a meta-analysis to see if the "extreme male brain" idea holds water.
What they found
Across studies, people with autism had slightly lower finger ratios than controls.
The effect is small but consistent, giving weak support to the extreme-male-brain theory.
How this fits with other research
Kelley et al. (2006) is one of the papers inside this review. They also saw lower ratios in kids with autism and linked them to motion detection problems.
De Meo-Monteil et al. (2019) and Amaral et al. (2017) move the idea inside the skull. They show that big-brained boys with autism have worse outcomes, extending the male-bias story from fingers to brain size.
Li et al. (2024) add brain scans. Girls with autism show different network wiring than boys, reminding us that sex differences show up in both body and brain markers.
Why it matters
You cannot diagnose autism with a ruler, but knowing that male-linked biology tracks with autism risk helps you stay alert. When a boy has macrocephaly or a very low finger ratio, pair your behavioral screens with extra vigilance. Also remember the flip side: girls may be missed because their biology looks different. Build sex-specific questions into your intake forms and red-flag lists.
Want CEUs on This Topic?
The ABA Clubhouse has 60+ free CEUs — live every Wednesday. Ethics, supervision & clinical topics.
Join Free →Add head-circumference and finger-ratio boxes to your intake form; flag boys with both macrocephaly and very low ratios for closer screening.
02At a glance
03Original abstract
Boys are more likely than girls to be diagnosed with an autism spectrum disorder (ASD). The extreme male brain (EMB) theory of ASD suggests that fetal testosterone (FT) exposure may underlie sex differences in autistic traits. A link between the organizational effects of FT on the brain and ASD is often drawn based on research using digit ratio (2D:4D), a putative biomarker, without a full survey of the findings. This paper critically and quantitatively reviews the research on the relationship between 2D:4D and ASD as well as autism spectrum, empathizing, and systemizing measures in neurotypical populations. Overall, there is some support for the EMB theory in all four areas, particularly the 2D:4D-ASD relationship. Recommendations for future research are provided.
Journal of autism and developmental disorders, 2013 · doi:10.1007/s10803-013-1819-6