The Willpower Instinct: How Self-Control Works, Why It Matters, and What You Can Do To Get More of It

THE SPREAD OF AN EPIDEMIC
The Centers for Disease Control and Prevention are well known for tracking outbreaks like the H1N1
virus and the early AIDS epidemic. But they also keep track of long-term changes to our national
health, including the obesity rates in every state in the country. In 1990, no state in the nation had an
obesity rate equal to or higher than 15 percent. By 1999, eighteen states had a rate between 20 and 24
percent, but still no state had a rate equal to or higher than 25 percent. By 2009, only one state
(Colorado) and the District of Columbia had a rate lower than 20 percent, and thirty-three states had a
rate of 25 percent or higher.
Two scientists, Nicholas Christakis at Harvard Medical School and James Fowler at the University
of California, San Diego, were struck by the language being used by health officials and the media to
describe this trend: an obesity 
epidemic
. They wondered whether weight gain could spread from
person to person in much the same way as other contagious outbreaks, like the flu. To find out, they
gained access to data from the Framingham Heart Study, which has tracked more than 12,000
residents of Framingham, Massachusetts, for thirty-two years. It began in 1948 with 5,200
participants, and added new generations in 1971 and 2002. The members of this community have
reported the intimate details of their lives for decades, including weight changes and their social
connections to everyone else in the study.
When Christakis and Fowler looked at participants’ weight over time, they saw what looked like a
real epidemic. Obesity was infectious, spreading within families and from friend to friend. When a
friend became obese, a person’s own future risk of becoming obese increased by 171 percent. A
woman whose sister became obese had a 67 percent increased risk, and a man whose brother became
obese had a 45 percent increased risk.
Obesity wasn’t the only thing going around the Framingham community. When one person started
drinking more, the bar tabs and hangovers spread throughout the social network too. But they also
found evidence for the contagiousness of self-control. When one person gave up cigarettes, it
increased the odds that their friends and family would quit too. Christakis and Fowler have found the
same pattern of contagion in other communities, and for willpower challenges as diverse as drug use,
sleep deprivation, and depression. As unsettling as it may be, the implication is clear: Both bad
habits and positive change can spread from person to person like germs, and nobody is completely
immune.