Children with autism have a relatively shorter index finger (2D) compared with their ring finger (4D). It is often presumed that the 2D:4D ratio is associated with fetal testosterone levels and that high fetal testosterone levels could play a role in the aetiology of autism. It is unknown whether this effect is specific to autism. In this study, 2D:4D ratios of 144 males aged 6 to 14 years (mean age 9y 1 mo [SD 1y 11 mo]) with psychiatric disorders were compared with those of 96 males aged 6 to 13 years from the general population (mean age 9y 1 mo [SD 1y 10 mo]). Psychiatric disorders were divided into autism/Asperger syndrome (n=24), pervasive developmental disorder-not otherwise specified (PDD-NOS; n=26), attention-deficit-hyperactivity disorder (ADHD)/oppositional defiant disorder (ODD; n=68), and anxiety disorders (n=26). Males with autism/Asperger syndrome (p<0.05) and ADHD/ODD (p<0.05) had significantly lower (though not significantly; p=0.52) ratios than males with an anxiety disorder, and males with autism/Asperger syndrome had lower ratios than those in the comparison group. These results indicated that higher fetal testosterone levels may play a role, not only in the origin of autism, but also in the aetiology of PDD-NOS and of ADHD/ODD. Males with anxiety disorders might have been exposed to lower prenatal testosterone levels. (de Bruin, E. I., Verheij, F., Wiegman, T. & Ferdinand, R. F. (2006). Differences in finger length ratio between males with autism, pervasive developmental disorder-not otherwise specified, ADHD, and anxiety disorders. Developmental Medicine and Child Neurology, 48(12), 962-5.)
I’m not clear why it is the case that testosterone levels are associated with handedness (left handedness correlated with low testosterone in males), left handedness is assocated with maturational delay, but testosterone is rarely approached in the context of maturational delay. It would seem that this would be important in such studies as noted above.
I explore the connection between autism, testosterone levels and maturation rates and timing. If maturation rates and timing are integral to understanding how we evolved (See Gould’s Ontogeny and Phylogeny), then understanding autism, and related conditions, may be central to understanding what being human exactly is.