Seizures may develop, particularly after puberty in those autistic individuals with moderate to severe mental retardation. Other neurotransmitter systems have also been implicated, but have not been consistently replicated. Together, the MRI and autopsy findings suggest abnormal neural development in the limbic cortex and cerebellar hemispheres. However, an accurate measure of the expected level of random chance co-occurrence is somewhat difficult to decide because of the frequent overlap between autism and mental retardation. Currently, untreated cases of PKU are rarely seen, so that the association can no longer be studied in developed countries.
Similar difficulties surround the reported association with congenital rubella.
Numerous studies have been carried out to investigate the relationship between autism and prenatal and perinatal problems. Compared with IQ-matched controls, autistic individuals have a very small rate of untoward events. Because autistic children are often only children, primiparity is common. Among the vast majority of cases with unknown cause, there is clear evidence for a genetic etiology. Twin pairs are said to be concordant when both are affected with the disorder or trait under study.