Faced with risking autism or measles, some parents thought the answer was obvious. Most had never seen measles, mumps or rubella because vaccines had nearly eliminated them. But they believed they knew autism.
And most people are notoriously poor at assessing risk, say experts in medical decision-making.
Many stumble on omission bias: “We would rather not do something and have something bad happen, than do something and have something bad happen,” explained Alison M. Buttenheim, an associate professor of nursing and health policy at the University of Pennsylvania School of Nursing.
People are flummoxed by numerical risk. “We pay more attention to numerators, such as ‘16 adverse events,’ than we do to denominators, such as ‘per million vaccine doses,’ ” Dr. Buttenheim said.
A concept called “ambiguity aversion” is also involved, she added. “Parents would like to be told that vaccines are 100 percent safe,” she said. “But that’s not a standard we hold any medical treatment to.”
Relatively few people are absolutists about refusing all vaccines. “But if you’re uncertain about a decision, you’ll find those who confirm your bias and cement what you think,” said Rupali J. Limaye, a social scientist who studies vaccine behaviors at the Johns Hopkins Bloomberg School of Public Health.
Nowhere is that reinforcement more clamorous than on social media, Dr. Limaye added. “You may only see your pediatrician a few times a year but you can spend all day on the internet,” she said.
People tend to believe an individual’s anecdotal narrative over abstract numbers. By 2007, when Ms. McCarthy, the actress, insisted that vaccines caused her son’s autism, thousands found her to be more persuasive than data showing otherwise. A nascent movement took hold.
At the same time that these powerful attacks on vaccine confidence were underway, a constellation of trends was emerging.
The definition of a good parent was becoming fraught with the responsibility for overseeing every aspect of a child’s life.
“As we adopted a culture of individualistic parenting, public health became a hard sell,” Dr. Reich said.The primary reason for healthy people to get the flu shot is to protect those with compromised immune systems, like infants and older adults, from getting sick. But altruism isn’t a great motivator for parents, Dr. Buttenheim said. “They are much more concerned about protecting their own child at all costs,” she said.