Dr. Nicholas Christakis is a sociologist and physician known for his research on social networks and on the socioeconomic, biosocial, and evolutionary determinants of behavior, health, and longevity. Follow Dr. Christakis on Twitter @NAChristakis or go to www.humannaturelab.net to learn more. Purple Mattress: Get a free pillow by texting DREW to 474747 TrueCar: Visit TrueCar to enjoy a more confident car-buying experience