More than 65 years ago in Massachusetts, doctors began a longitudinal study that would transform our understanding of heart disease. The Framingham Heart Study, which started with more than 5,000 people and continues to this day, has become a data source for not just heart disease, but also for insights about weight loss (adjusting your social network helps people lose weight), genetics (inheritance patterns), and even happiness (living within a mile of a happy friend has a 25% chance of making you happier).