The usual medical view is that most autism is due to a genetic disorder, and with good reason. The existence of so many families where autism and related spectral disorders appear again and again is compelling evidence.

However, this is probably not the whole story. Even the strongest proponents of genetic research are now beginning to talk in terms of 'genetic fragility' or 'genetic predisposition'.

Genetic susceptibility is not a black or white, yes or no, issue. Taking the population as a whole, the genetic susceptibility to any form of illness is more likely to be in the shape of a

normal distribution curve. A few people will be immensely susceptible, the vast majority will be moderately susceptible and some will be immune whatever the size of the burden.

Incidence skyrocketing

Increasing reported levels of autism have been noted from many parts of the world, but this may reflect no more than an increasing awareness of the disorder and changing diagnostic criteria (Br J Psychiatr, 1998; 158: 403-9; BMJ, 1996; 312: 327-8).

However, two recent reports on autism and vaccination would suggest otherwise. One paper, by Taylor et al, showed a whopping 1700 per cent increase of reported incidence between 1979 and 1992 (Lancet, 1999; 353: 2026-9).

Another 1999 study report showed a 273 per cent increase in autism for the state of California over a similar period. Studies from the past 12 months indicate a continuing rise in the reported incidence (Changes in the Population of Persons With Autism and Pervasive Developmental Disorders in California's Developmental Services System: 1987 Through 1998, report to the State Legislature from the California Health and Human Services Agency, March 1, 1999).

Informal and unpublished data from many parts of the UK (Thrower D, Evidence presented to HM Government, 1999) and other parts of the world are showing similar increases. Although not supported by official publications, parallel, staggering increases in incidence apparently are occurring in similar and related disorders such as dyslexia and attention deficit disorder (ADD), which may be combined with hyperactivity (ADHD). Indeed, reports from the US and Australia suggest that 10 per cent of school age children are currently taking Ritalin to ameliorate the symptoms of ADHD.

In other circumstances, such increases would be regarded as an epidemic and worthy of concern and considerable research. So far, environmental factors have been completely and utterly ignored by government funded agencies within the UK and throughout the world. If these increases are indeed genuine, then there must be factors, additional to the purely genetic, which trigger the problem.

Although there may be genetic elements or 'fragilities' involved in all of these situations, the fact that the increases have been so dramatic point to environmental factors.

Environmental agents

Numerous environmental factors have changed over the past 20 years in the UK. The use of lead paint has decreased dramatically, as has the use of mercury in pesticides and dental fillings. Asbestos has been largely removed from the environment, but there are increasing levels of aluminium in water and potentially toxic fumes and radiation from TV sets and computers.