New York: Female infants have larger volumes of grey matter around the temporal-parietal junction of the brain than males at the time of birth, a new study has found.

The temporal-parietal junction, or TP, which is found under the temporal bones near the ears, integrates the processing of social information as expressed in others’ faces and voices, a function that is impaired in those with autism spectrum disorders.

Sex differences in this area of the brain may be a clue as to why males are at higher risk for certain forms of autism spectrum disorders, researchers said.

Many early-onset neurodevelopmental disorders, including autism spectrum disorders, are more common in males than females.

Rebecca Knickmeyer and colleagues at the University of North Carolina-Chapel Hill have been characterizing sex differences in brain development in a group of over 800 normal newborns, who are assessed until they are two years old.

Another main finding in Knickmeyer’s work is that by the age of two, myelination of long fibre tracks in the brain is more developed in males than in females.

Myelination is the development of an insulating myelin sheath around nerves so that they are able to transmit information more quickly.

Knickmeyer has also shown that a genetic disorder that only occurs in females – Turner Syndrome – also involves a significant decrease in brain volume in inferior parietal lobes.

This suggests that inferior parietal lobe volume can be influenced, at least in females, by sex chromosomes.

The findings were presented recently at the annual meeting of the American College of Neuropsychopharmacology.