Why Is It So Hard to Make New Habits?

In 1905, a man argued that he shouldn't have to get vaccinated because vaccines could cause him harm. The case went before the Supreme Court and when we come back, we'll find out what the decision was. Why is it so hard to make new habits? Are pets really that great? Would you rather be given an award or a bonus? Answers to these questions and more every week on Hidden Brain from NPR.