What is the No. 1 reason people don’t meet their goals? Willpower. The good news, though, is that this power can be studied as a science, according to health psychologist and author Kelly McGonigal. McGonigal’s new book, The Willpower Instinct, heavily based on research studies, explores willpower challenges: dieting, addiction and exercise to name a few. She refers to biology and neuroscience and the origins of human thought, spelling out complex scientific findings in layman’s terms. McGonigal’s own research led her to create “The Science of Willpower,” a class offered to the public through Stanford University’s Continuing Studies Program and that helps explain how to break old habits and create new, healthy ones.