About half of Americans agree with at least one medical conspiracy theory, a new study suggests.

The study surveyed more than 1,300 Americans to see whether they agreed with six popular medical conspiracy theories — such as the discredited link between vaccines and autism, or the belief that water fluoridation is a cover-up to allow companies to dump dangerous chemicals into the environment.

Nearly half, or 49 percent, of those surveyed agreed with at least one medical conspiracy theory, and 18 percent agreed with three or more theories. [Top Ten Conspiracy Theories]

The most commonly endorsed theory was the belief that the Food and Drug Administration is "deliberately preventing the public from getting natural cures for cancer and other diseases because of pressure from drug companies." More than a third of Americans, or 37 percent, agreed with this statement.

Twenty percent agreed with the statement: "Health officials know that cell phones cause cancer but are doing nothing to stop it because large corporations won’t let them." The vaccine-autism link was supported by 20 percent of participants.

Study researcher Eric Oliver, a professor of political science at the University of Chicago, said he was not surprised by the findings. Studies of American's belief in political conspiracy theorieshave yielded similar results.

"We see that Americans have conspiracy theories about a lot of things, not just about politics, but also about health and medicine as well," Oliver said.

Why we believe

Belief in conspiracy theories is not necessarily a sign of a psychological condition such as paranoia, Oliver said. Rather, in cases of uncertainty, people have a natural tendency to assume that malevolent forces are behind the "unknown," Oliver said.

"These narratives seem like very compelling explanations for complicated situations," Oliver said.

Humans may have evolved to think this way, Oliver said. "If you hear a noise in the bush, it's much more adaptive to believe that there's a predator there than not," he said.

Public health implications

However, this widespread belief in medical conspiracy theories may have implications for pubic health.

Participants who supported medical conspiracy theories were less likely to get flu shotsand use sunscreen, and more likely to say they got health information from celebrity doctors, than those who did not endorse these theories.

Doctors should be aware that patients who endorse medical conspiracy theories may be reluctant to follow medical advice or comply with medical treatments, Oliver said.

Oliver suspects that overturning such beliefs would be difficult.

"People are attaching themselves to these narratives for psychological reasons, these narratives are providing them with feelings of certainty," Oliver said.

Oliver noted that science embraces a lot of uncertainty, which may not be intuitive to some people. But improving education, particularly about science and medicine, may help people better understand scientific information, he said.

The study is published in the March 17 issue of the journal JAMA Internal Medicine.

Follow Rachael Rettner @RachaelRettner. Follow Live Science @livescience, Facebook & Google+. Original article on Live Science.