Children with autism tend to have
unusually long ring fingers compared to their index fingers, researchers say.
A high level of testosterone in the womb is associated with this physical feature. The study shows that even the perfectly healthy siblings and parents of autistic volunteers had finger length ratios that differed significantly from the normal controls.
"High testosterone seems to run in these families," says John Manning at Liverpool University. The study underscores the importance of genes in these kind of illnesses.
The finding also supports the "extreme male brain" theory of autism.
According to this theory, an overabundance of testosterone during early foetal development may magnify normal male traits,such as difficulties with language and empathy. These are features of autism, which affects about one in 500 children.
Manning and his colleagues studied 49 children with full-blown autism and 23 with a milder variant of the condition, known as Asperger's syndrome.
They compared the ratio of the length of their index fingers to the length of their ring fingers with ratios found in 34 healthy siblings, 88 fathers, 88 mothers and controls that were matched for sex and age.
Relative finger length is determined in utero by about 14 weeks' gestation and does not change as we age.
Various studies have shown that finger length ratios are a robust indicator of how much testosterone a baby is exposed to in the womb.
In general, men have longer ring fingers than index fingers, whereas in women those digits tend to be of equal length.
Manning's team found that children with autism had extremely long ring fingers compared to their index fingers and children with Asperger's were not far behind.
Source: Developmental Medicine and Child Neurology (vol 43, p 160) - New Scientist