The CDC recently announced that measles cases in the US have passed the 150 mark. Faced with the growing number of cases, why do some parents still choose not to vaccinate?

On both sides of the debate are parents who simply want to protect their children. Those who chose not to vaccinate are fearful of autism or other complications; signs of autism start to appear around the same time many children receive their MMR vaccinations, so some parents believe one causes the other even though that link has been disproved.

Jessica Kendorski, a psychologist at Philadelphia College of Osteopathic Medicine, says this is the same reason why baseball players might continuously wear the same pair of socks after winning a game. Even though these things happen together, they are not related.

Kendorski says this tendency to equate causality with timing has long been shown in behavioral science research, such as with B.F. Skinner’s experiments with pigeons. She suggests that in order to increase the number of parents who vaccinate, simply showing them the science isn't enough; it’s also important to explain why these two aren't related, and that just because two things occur together does not mean one is the cause of the other.