The prevalence of the disease is one of the topics London addressed in his Town Hall talk. Studies give very different estimates as to how common the condition is, but all seem to show the rate is going up, he said. The best estimates seem to show a flat rate of about 0.4 cases per 1,000 from 1965 to 1985, he said. Between 1985 and 1995, that figure inched toward and surpassed 1 per 1,000, and now seems to be at about 1.2 per 1,000, although some studies have suggested it may be as high as 3 or even 6 in 1,000 people, he said. That's pretty powerful.
Nobody can argue with that.
But not enough studies have been done to show a strong trend in incidence rate or to isolate possible factors like geography or environmental factors. London worked on a 1998 U.S. Centers for Disease Control-sponsored study of children between ages 3 and 10 in Brick Township, N.J. that identified children by reports from parents, schools and service agencies and then medically diagnosed them. A rate of 4 per 1,000 was found, he said. Major, large-scale studies are under way in Georgia, West Virginia, Staten Island, N.Y., and in Denmark, he said. We just don't know.