The findings, published in the February issue of the journal Archives of Pediatrics & Adolescent Medicine, suggest that recent increases in autism diagnoses in many parts of the world may be part of an overall trend in childhood mental illness, said researchers at the University of Aarhus in Denmark.

Children with autism have social and language abnormalities and repetitive patterns of behavior, while those with hyperkinetic disorder are hyperactive and tend to move from o­ne activity to another without completing any o­ne task. Tourette syndrome is characterized by uncontrollable vocalization or movement, according to background information in the article.

For this study, the researchers analyzed trends in the these and a fourth condition -- obsessive-compulsive disorder -- among all 669,995 children born in Denmark between 1990 and 1999. Through 2004, 4,376 of the children were diagnosed with at least o­ne of the four conditions.

The study authors found significant increases in rates of autism, hyperkinetic disorder and Tourette syndrome, but no increase in obsessive-compulsive disorder. It's not clear why that condition showed a different pattern than the other three, the researchers said.

"Although the reasons for the observed common pattern of change in reported cumulative incidence in Tourette syndrome, hyperkinetic disorder and autism spectrum disorder cannot be addressed with these data, it is clear that the number of children with neuropsychiatric disorders and their families in need of support and services has been growing in recent years," the study authors wrote.

"Furthermore, while the search for causes should proceed unabated, the ultimate value of these data are their contribution to the growing awareness of child neurodevelopment problems in general and understanding of the resources needed to ensure optimal development for all children," the researchers said.