Why are parents lying to investigators about their children's vaccinations?

"Prevent Measles" poster displayed by Public Health Department, Minneapolis, MN. February 14, 1939 Credit: City of Minneapolis Archive/CC BY 2.0.

Public health officials in California are trying to stop the measles outbreak, by tightening restrictions on vaccine exemptions. However, some parents are telling health investigators that their children are vaccinated when they’re not. Some parents are confused why government workers are seeking their medical history. Others are influenced by the anti-vaccine movement.