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Some of our major presidential candidates have sounded recently like they think they're scientists. Specifically, Barack Obama and John McCain -- allies on few topics -- have both asserted a link between autism rates -- an 805 percent increase between 1992 and 2003 -- and vaccines containing mercury. The Washington Post Fact Checker quotes both men:

"We've seen just a skyrocketing autism rate. Some people are suspicious that it's connected to the vaccines. This person included. The science right now is inconclusive, but we have to research it." -- Barack Obama, Pennsylvania rally, April 21, 2008.

"It's indisputable that (autism) is on the rise among children, the question is what's causing it. And we go back and forth and there's strong evidence that indicates it's got to do with a preservative in vaccines."  --John McCain, Texas town hall meeting, February 29, 2008.

The debate over the alleged link between autism rates and mercury-bearing vaccines was further inflamed in March, when it was revealed that the family of Hannah Poling, a 9-year-old autistic girl, was compensated by the Department of Health and Human Services' Division of Vaccine Injury Compensation. The details of the payoff were not revealed, but it seemed to be a concession by the federal government in the long-running controversy.

And, of course, there was the autism storyline in the opening episode of the ABC series "Eli Stone."

So is there a link between vaccines and autism? Multiple studies -- from the Centers for Disease Control, American Academy of Pediatrics, Institute of Medicine and the New England Journal of Medicine -- have found no such link.

First, some background. Why is there mercury in vaccines in the first place? According to the CDC, it was an ingredient in the the chemical thimerosal, which was used starting in the 1930s to give vaccines a longer shelf life before use. But as the controversy over thimerosal has intensified, its use has dropped:

Since 2001, with the exception of some influenza (flu) vaccines, thimerosal is not used as a preservative in routinely recommended childhood vaccines.

But a January study in the journal Archives of General Psychiatry noted that, despite the reduced use of thimerosal, autism cases in California had continued to climb during the 21st century, indicating some other cause for the rise.

The DDS data do not show any recent decrease in autism in California despite the exclusion of more than trace levels of thimerosal from nearly all childhood vaccines. The DDS data do not support the hypothesis that exposure to thimerosal during childhood is a primary cause of autism.

The new study confirmed what other researchers have found over the years:

* In Quebec, where thimerosal was discontinued in 1996, autism rates have also continued to rise, according to 2006 study of 28,000 children:

"There is no relationship between the level of exposure to MMR vaccines and thimerosal-containing vaccines and rates of autism," says Dr. Eric Fombonne, Director of Pediatric Psychiatry at the Montreal Children's Hospital of the MUHC and lead investigator of the new study. Thimerosal was used to prevent bacterial and fungal contamination in the manufacture of various vaccines until its elimination from vaccine formulas in 1996 in Quebec. "According to our data, the incidence of autism was higher in children who were vaccinated after thimerosal was eliminated from vaccines," says Dr. Fombonne.

* The Institute of Medicine examined the body of research in 2004 and rejected links between autism and the vaccines:

The committee concludes that the body of epidemiological evidence favors rejection of a causal relationship between the MMR vaccine and autism. The committee also concludes that the body of epidemiological evidence favors rejection of a causal relationship between thimerosal-containing vaccines and autism.

* And in 2002, the New England Journal of Medicine published a study examining autism rates of all children born in Denmark from January 1991 through December 1998. The results:

This study provides three strong arguments against a causal relation between MMR vaccination and autism. First, the risk of autism was similar in vaccinated and unvaccinated children, in both age-adjusted and fully adjusted analyses. Second, there was no temporal clustering of cases of autism at any time after immunization. Third, neither autistic disorder nor other autistic-spectrum disorders were associated with MMR vaccination.

The findings have been consistent. Yet the vaccine-autism connection persists, for parents and politicians. Why? Probably because influential people and organizations continue to make the case for the link. Robert F. Kennedy Jr. wrote in support of the connection in a famous -- and widely debunked --  2003 Rolling Stone article. The National Autism Association also suggests a link by citing similarities between autism and mercury poisoning, and by pointing out previous government failures to properly respond to health threats -- but cites the greatly criticized work of Mark Geier as proof of such a link.

So: Autism is caused by mercury-bearing vaccines. Truth or not?