In 2007 the U.S. Centers for Disease Control (CDC) estimated that 1 in 150 eight-year-old children in the United States has an autism spectrum disorder (ASD). This shocking life and health statistic spurred a barrage of debates: are the elevated numbers simply the result of improved diagnosis or is something else�"perhaps something society is doing�"causing an increase in ASDs?

Autism spectrum disorders are listed as "pervasive developmental disorders" in a widely accepted authoritative reference tool, Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (Text Revision), or DSM-IV-TR, published by the American Psychiatric Association. The causes of ASDs are undetermined, but evidence points to a strong genetic component.

Many parents and families of children with ASDs are looking beyond the genetics and are expressing grave concerns about a possible connection between immunization�"in particular the standard combined measles, mumps and rubella vaccine that is routinely administered to children around the world�"and autism. The main ingredient in question is thimerosal (called thiomersal in some countries), a preservative that is 49 percent ethylmercury.

Not all vaccines under debate incorporate thimerosal, which has now been phased out of most childhood vaccines. It remains in many influenza vaccines, however. The CDC maintains that "there is no convincing evidence of harm caused by the low doses of thimerosal in vaccines, except for minor reactions." Even so, the number of people claiming to have been harmed by vaccinations prompted the U.S. Court of Federal Claims to establish the National Vaccine Injury Compensation Program "as a no-fault compensation scheme whereby persons allegedly suffering injury or death as a result of the administration of certain compulsory childhood vaccines may petition the federal government for monetary damages." As of June 2007, about 5,000 claimants with autism or similar conditions have filed for compensation.

Although long-term studies on the subject are ongoing, the official stance of government organizations in many countries is that there is no connection between vaccinations and autism. The World Health Organization states that "there is no reason on grounds of safety to change current immunization practices with thiomersal-containing vaccines, as the risks are unproven." The position of the U.S. National Institute of Child Health and Human Development is similar: "To date there is no definite, scientific proof that any vaccine or combination of vaccines can cause autism."

Like ASD, the causes of schizophrenia, another mental disorder listed in the DSM-IV-TR, are undetermined, but evidence points to a strong genetic connection. This is largely based on studies of siblings and twins, particularly monozygotic (genetically identical) twins. When one twin develops schizophrenia, the other has a 50 percent likelihood of developing the disorder as well. An important factor that seems to be overlooked in some studies is that two-thirds of monozygotic twins share more than genetics: their shared placenta is a strong environmental factor.

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