WASHINGTON (AFP) - Abnormal brain growth starting at four months of age occurs in a type of autism in which toddlers lose language and social skills they once had, according to a US study published Monday. The brains of boys with regressive autism grew six percent larger than typically developing counterparts and toddlers who showed signs of autism early in life, a form called early onset autism. The research, involving 180 subjects and described as the largest study of brain development in preschoolers with autism to date, also found no evidence of a brain growth spurt in girls with autism. This adds to the growing evidence that there are multiple biological subtypes of autism, with different neurobiological underpinnings, said co-author David Amaral, research director of the MIND Institute at University of California, Davis. Autism includes a wide spectrum of developmental differences and may range from mild social awkwardness to complete inability to communicate, repetitive movements, sensitivity to certain lights and sounds, and behavioral problems. As many as one in 110 children is diagnosed with autism, though its cause remains a mystery. The disorder is more common in boys than girls by a factor of four to one. Previous studies have suggested that clinical signs of autism tend to coincide with a period of abnormal brain and head growth that becomes apparent between the ninth and 18th month of life. However this study, published in the Proceedings of the National Academy of Sciences issue of November 29, is the first to show a difference in brain size between toddler boys with regressive versus early onset autism.