Testing the extreme male brain theory of autism spectrum disorder in a familial design.
Finger length, not finger ratio, may hint at familial autism risk, yet it tells us nothing about prenatal testosterone or cognitive style.
01Research in Context
What this study did
The team measured fingers in the families. Each family had one child with autism. They also measured 50 control families.
They checked two things. First, the 2D:4D finger ratio. This is the ring finger length divided by the pointer finger. Second, total finger length compared to body height.
The goal was to test the "extreme male brain" theory. This theory says high prenatal testosterone may shape autism traits.
What they found
Kids with autism had longer fingers for their height than typical kids. Their parents and siblings showed the same pattern.
Surprise: the 2D:4D ratio did not differ between groups. That ratio is the usual marker for prenatal testosterone.
Cognition tests also showed no male-female split in autism families. So the finger data did not line up with thinking styles.
How this fits with other research
O'Reilly et al. (2009) saw another testosterone clue. Fathers of autistic kids did not prefer low waist-hip ratios like most men do. Both papers hunt for prenatal testosterone signs, yet each uses a different ruler.
Lehnhardt et al. (2016) looked at sex-specific thinking in late-diagnosed adults. They found real cognitive sex gaps. Ingeborg did not find such gaps. The difference is age and method: adult test scores versus child finger length.
McIntyre et al. (2002) and Geurts et al. (2008) already showed eye-movement and social-attention quirks in autism parents. Ingeborg adds finger length to that list of subtle family traits.
Why it matters
You now have one more soft sign to notice. Long fingers for body size may flag familial autism risk, but it is not a diagnostic tool. Ignore the 2D:4D hype; it did not separate groups here. When you assess, keep looking at behavior, not fingers.
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Join Free →Skip the 2D:4D ruler; instead, note if the child’s fingers look long for height and share that this minor trait sometimes runs in autism families.
02At a glance
03Original abstract
Autism Spectrum Disorder (ASD) may be an extreme manifestation of some male-typical traits in both neuroanatomy and cognition. Using the ratio of the second to fourth digit (2D:4D) and digit length as biomarkers of (pre- and postnatal) testosterone levels, examined was whether hypermasculinized digit ratios and/or lengths were familial traits in ASD and investigated their relation to sexually dimorphic cognitive abilities. 2D:4D ratios and digit lengths of 216 children with ASD, 202 unaffected siblings, and 360 parents were compared with those of 174 control children and their 146 parents. Generalized Estimation Equations, Generalized Linear Models, and Linear Mixed Models were used to investigate parent-offspring relationships and group differences. In ASD probands and their relatives alike, digit length relative to overall height was significantly increased in comparison to controls. No significant group differences were found between affected and unaffected subjects, or between males and females. Additionally, 2D:4D ratios increased with age. No (consistent) associations were found between 2D:4D ratio or digit lengths and systemizing and empathizing skills. The findings emphasize the role of familially based elevated pre- and postnatal testosterone levels in the liability for ASD, but challenge the use of 2D:4D ratio as a proxy of prenatal testosterone exposure solely. Given that many genes influence digit length, the exact mechanisms underlying a familial predisposition toward increased digit length in ASD are as yet unknown and needs to be explored in future studies.
Autism research : official journal of the International Society for Autism Research, 2014 · doi:10.1002/aur.1384