Dr. Kristin Hendrix, a professor of pediatrics at the Indiana University School of Medicine, researches how parents make decisions about their children’s healthcare, including vaccinations. “It’s a combination of pretty complex psychological factors,” Hendrix says. “Some folks are very predisposed to trust information about others’ personal experience.” She emphasizes that a story is even more likely to trump scientific data when the story comes from a friend or family member.

“Even if the situation that a person hears about didn’t actually happen to their friend or family member, but is being relayed by them, they trust that more than a face-to face conversation with a physician,” Hendrix says. “That information, anecdote, narrative, personal account, rare instance that may or may not be true, tends to carry more gravity and weight when it comes from someone they know.”

Hendrix says she thinks this preference for story over evidence may be caused by a general human tendency to misunderstand numbers, especially for risk. She says people overinflate the likelihood of something bad happening. And sometimes parents fear a negative event from a vaccination more than they fear the actual disease. She understands their perspective in some ways, she says, because illnesses like polio have been eradicated in this country. “It’s easier to believe there are no effects to not vaccinating,” Hendrix says.