Americans ages 18 to 29 are more likely to oppose mandatory childhood vaccination and say vaccines can cause autism, according to a new survey.

A survey of 1,000 US adults by YouGov, conducted between January 26 and January 28, found 21 percent of young adults believe the long-debunked idea that vaccines cause autism, while just 13 percent of all US adults agree.

The empirical research on this issue is clear: there is no connection between vaccines and autism. The Lancet, a British medical journal, in 2010 retracted a high-profile study from 1998 that falsely claimed to find a link between vaccines and autism, discrediting the one major piece of evidence vaccine skeptics cited in the past.

The YouGov survey also found 57 percent of Americans support mandatory childhood vaccinations, while 32 percent say parents should decide. Younger adults were much more likely to say childhood vaccination should be optional.

Many schools and states require vaccination to support what's known as herd or community immunity, a coverage threshold that effectively prevents an illness from spreading. Vaccinated people essentially act as barriers to outbreaks, since diseases can't pass through them and infect other people. This barrier helps protect some of the most vulnerable populations: infants under 12 months of age, who can't get vaccinated and are more susceptible to infection, and the elderly, who have a higher risk of death if they contract these illnesses.

Vox's Dylan Matthews explains why it's so important to vaccinate