Leo Kanner described autism in 1943, and Hans Asperger described the syndrome in 1944. The term Pervasive Developmental Disorders (PDD) was first used in the 1980s to describe a class of disorders that include (1) Autistic disorder, (2) Rett disorder or syndrome, (3) Childhood Disintegrative Disorder, (4) Asperger's disorder or syndrome, and (5) Pervasive Developmental Disorder Not Otherwise Specified, or PDDNOS. Autism prevalence studies published before 1985 showed prevalence rates of 4 to 5 per 10,000 children for the broader autism spectrum, and about 2 per 10,000 for the classic autism definition. Since 1985 there have been higher rates of autism reported from several countries. From the UK a prevalence rate of 16.8 per 10,000 children for autistic disorder was reported, and 62.6 per 10,000 for the entire autistic spectrum disorders. Sweden reported a prevalence of 36 per 10,000 for Asperger and 35 per 10,000 for social impairment, or a total prevalence of 71 per 10,000 for suspected and possible cases. From the US, 40 per 10,000 in three to ten year old children for autistic disorder and 67 per 10,000 children for the entire autism spectrum was reported. From the north region in Israel for children born between 1989-93 in the Haifa area, an incidence rate of 10 per 10,000 was found for autism. In recent years concern has been shown about the possible increase in the prevalence of autistic spectrum disorders. Studies have shown an increase, but during these last twenty years diagnostic criteria and definition have also changed. Although many factors are at play, it is evident that there has been an increase.