Obesity Is ‘Socially Contagious,’ Study Finds
friends making you fat? Or keeping you slender? According to new
research fromHarvard and the University of California, San Diego, the
short answer on both counts is "yes."
the July 26 issue of the New England Journal of Medicine, a study
coauthored by Nicholas Christakis of Harvard Medical School and James
Fowler of UC San Diego suggests that obesity is "socially contagious,"
spreading from person to person in a social network.
– the first to examine this phenomenon – finds that if one
person becomes obese, those closely connected to them have a greater
chance of becoming obese themselves. Surprisingly, the greatest effect
is seen not among people sharing the same genes or the same household
but among friends.
If a person
you consider a friend becomes obese, the researchers found, your own
chances of becoming obese go up 57 percent. Among mutual friends, the
effect is even stronger, with chances increasing 171 percent.
and Fowler also looked at the influence of siblings, spouses and
neighbors. Among siblings, if one becomes obese, the likelihood for the
other to become obese increases 40 percent; among spouses, 37 percent.
There was no effect among neighbors, unless they were also friends.
researchers analyzed data over a period of 32 years for 12,067 adults,
who underwent repeated medical assessments as part of the Framingham
Heart Study. They were able to map a densely interconnected social
network of the study’s subjects by using the tracking sheets
(which had previously been archived in a basement) that recorded not
only the subjects’ family members but also unrelated friends who
could be expected to find them in a few years.
map took two years to assemble and includes information on the
participants’ body-mass index. Among the first things the
researchers noticed was that, consistent with other studies finding an
obesity epidemic in the U.S., the whole network grew heavier over time.
immediately apparent were distinct clusters of thin and heavy
individuals. Statistical analysis revealed that this clustering could
not be attributed solely to the selective formation of ties among
people of comparable weights.
not that obese or non-obese people simply find other similar people to
hang out with," said Christakis, a physician and a professor in Harvard
Medical School’s department of health care policy. "Rather, there
is a direct, causal relationship
analysis also suggested that people’s influence on each
other’s obesity status could not be put down just to similarities
in lifestyle and environment, to, for example, people eating the same
foods together or engaging in the same physical activities. Not only do
siblings and spouses have less influence than friends, but also
geography doesn’t play a role. The striking impact of friends
seems to be independent of whether or not the friends live in the same
looked at the effect of distance, we found that your friend who’s
500 miles away has just as much impact on your obesity as [one] next
door," said Fowler, an associate professor of political science at UC
San Diego and an expert in social networks.
because the study also identifies a larger effect among people of the
same sex, the researchers believe that people affect not only each
other’s behaviors but also, more subtly, norms.
to be happening is that a person becoming obese most likely causes a
change of norms about what counts as an appropriate body size. People
come to think that it is okay to be bigger since those around them are
bigger, and this sensibility spreads," said Christakis.
about people's ideas about their bodies and their health," Fowler said.
"Consciously or unconsciously, people look to others when they are
deciding how much to eat, how much to exercise and how much weight is
effects, I think, are much stronger than people before realized.
There’s been an intensive effort to find genes that are
responsible for obesity and physical processes that are responsible for
obesity and what our paper suggests is that you really should spend
time looking at the social side of life as well," said Fowler.
implications of the study, the researchers say, are profound. The
social-network effects extend three degrees of separation – to
your friends’ friends’ friends – so any
public-health intervention aimed at reducing obesity should consider
this in its cost-benefit analysis.
"When we help
one person lose weight, we’re not just helping one person,
we’re helping many," Fowler said. "And that needs to be taken
into account by policy analysts and also by politicians who are trying
to decide what the best measures are for making society healthier."
important to remember," Fowler said, "that we’ve not only shown
that obesity is contagious but that thinness is contagious."
This research was funded by the U.S. National Institutes of Health/National Institute on Aging.
For more information on the University of California at San Diego, visit www.ucsd.edu.