Dynamic spread of happiness in a large social network:
longitudinal analysis over 20 years in the Framingham
James H Fowler, associate professor,1Nicholas A Christakis, professor2
Objectives To evaluate whether happiness can spread
from person to person and whether niches of happiness
form within social networks.
Design Longitudinal social network analysis.
Setting Framingham Heart Study social network.
Participants 4739 individuals followed from 1983 to
Main outcome measures Happiness measured with
validated four item scale; broad array of attributes of
social networks and diverse social ties.
in the network, and the relationship between people’s
happiness extends up to three degrees of separation (for
example, to the friends of one’s friends’ friends). People
in the future. Longitudinal statistical models suggest that
and not just a tendency for people to associate with
that a person is happy by 25% (95% confidence interval
1% to 57%). Similar effects are seen in coresident
spouses (8%, 0.2% to 16%), siblings who live within a
mile (14%, 1% to 28%), and next door neighbours (34%,
7% to 70%). Effects are not seen between coworkers.The
Conclusions People’s happiness depends on the
happiness of others with whom they are connected. This
provides further justification for seeing happiness, like
health, as a collective phenomenon.
Happiness is a fundamental object of human
existence,1so much so that the World Health
Organization is increasingly emphasising happiness
as a component of health.2Happiness is determined
by a complex set of voluntary and involuntary
factors. Researchers in medicine,3economics,1 4 5
psychology,6 7neuroscience,8and evolutionary
biology9have identified a broad range of stimuli to
happiness (or unhappiness),1including lottery wins,10
possibly key determinant of human happiness: the
happiness of others.
individual to another by mimicry and “emotional
contagion,”17perhaps by the copying of emotionally
seen in others.18-20People can “catch” emotional states
they observe in others over time frames ranging from
seconds to weeks.1721-23For example, students ran-
domly assigned to a mildly depressed room-mate
became increasingly depressed over a three month
period,24and the possibility of emotional contagion
between strangers, even those in ephemeral contact,
has been documented by the effects of “service with a
smile” on customer satisfaction and tipping.2526
spread over short periods from person to person, little
in social networks. As diverse phenomena can spread
in social networks,27-35we investigated whether happi-
ness also does so. We were particularly interested in
whether the spread of happiness pertains not just to
relationships (such as friends of friends) and whether
there are geographical or temporal constraints on the
spread of happiness through a social network.
The Framingham Heart Study was initiated in 1948,
when 5209 people in Framingham, Massachusetts,
were enrolled into the “original cohort.”36In 1971, the
the original cohort, and their spouses, was enrolled.37
This cohort of 5124 people has had almost no loss to
follow-up other than death (only 10 people dropped
out). Enrolment of the so called “third generation
cohort,” consisting of 4095 children of the offspring
cohort, began in 2002. The Framingham Heart Study
also involves certain other smaller cohorts (such as a
1Department of Political Science,
University of California, San Diego,
2Department of Health Care
Policy, Harvard Medical School,
and Department of Sociology,
Harvard University, Cambridge,
Correspondence to: N A Christakis
Cite this as: BMJ 2008;337:a2338
BMJ | ONLINE FIRST | bmj.compage 1 of 9
minority over-sample called the OMNI cohort,
enrolled in 1995). At regular intervals participants in
all these cohorts come to a central facility for detailed
examinations and collection of survey data.
in this cohort is connected to other people via
friendship, family, spousal, neighbour, and coworker
relationships. Each relationship is a “social tie.” Each
“alter.” For example, one ego in the offspring cohort
had 18 alters: a mother, a father, a sister, two brothers,
three children, two friends, five neighbours, and three
coworkers. We wanted to know how each of these
alters influences an ego. Many of the alters also
happened to be members of a studied cohort in
Framingham, which means that we had access to
detailed information about them as well. Overall,
within the entire Framingham Heart Study social
network, composed of both the egos and any detected
alters in any Framingham Heart Study cohort, there
were 12067 individuals who were connected at some
point in 1971-2003.
To create the network dataset, we computerised
information about the offspring cohort from archived
handwritten administrative tracking sheets that had
been used since 1971 to identify people close to
participants for the purpose of follow-up. These
documents contain valuable social network informa-
tion because participants were asked to identify their
relatives, “close friends,” place of residence, and place
such procedures for identifying social ties between
individuals are known as “name generators.”38
The ascertainment of social ties in the Framingham
Heart Study was wide and systematic. The study
recorded complete information about all first order
relatives (parents,spouses, siblings,children),whether
on home address was also captured at each time point,
which we geocoded to determine neighbour relation-
at each wave allowed us to identify ties to coworkers
within the network.
Our dataset identifies the network links among
participants longitudinally, an unusual and advanta-
geous feature. Over the course of follow-up, the
participants spread out across the United States but
continued to participate in the Framingham Heart
Study. As a person’s family changed because of birth,
death, marriage, or divorce, and as their contacts
changed because of residential moves, new places of
employment, or new friendships, this information was
only one mutually exclusive category—that is, spouse,
sibling, friend, coworker, or neighbour.
There were 53228 observed social ties between the
5124 egos and any other alters in any of the
of 10.4 ties to family, friends, and coworkers over the
also ascertained, based on information about place of
residence, but they are not included in the foregoing
“neighbour” is defined (for example, whether we
restrict the definition to immediate, next door neigh-
25 or 100 metres, etc).
Given the compactnatureof theFraminghamsocial
network in the period 1971-2007, many of the
nominated contacts were also participants in one or
another Framingham Heart Study cohort3234so we
have detailed survey and physical examination infor-
mation about both the ego and the alter. For example,
83% of egos’ spouses were directly and repeatedly
Study. For 39% of the egos, at least one coworker
participated in the study. For 10% of the egos, an
immediate neighbour was also in the Framingham
Importantly, 45% of the 5124 egos were connected
via friendship to another person in the study; there
were 3604 unique observed friendships for an average
of 0.7 friendship ties per ego. There was substantial
variation from person to person, ranging from several
people with no friends to one person who was
nominated as a friend by eight different Framingham
Heart Study participants. Because friendship identifi-
cations are directional, we can study three different
types. An “ego perceived friend” means the ego
? Ego: the focal individual; this is the person whose behaviour is being analysed
behaviourof the ego
? Node: an object that may or may not be connected to other objects in a network; here,
these are people in the Framingham Heart Study cohorts
? Tie: a connection between two nodes that can be either one way (directed) or two way
(bilateral, or mutual); here, all family and spouse ties are bilateral (sibling, coworker,
who does not name them in return
? Homophily: thetendency for people to choose relationships with people who have
connected by at least one path to every other node in the same component
each node is connected by at least one path via nodes of the same type to every other
nodein thesamegroup—forexample, aclusterofhappy peopleconnectedby atleast
one path via other happy people to all theother people in their cluster
connected to the alters but not the ego are degree 2 (alters’ alters). Nodes that are
as the “geodesic distance”
page 2 of 9
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Accepted: 10 September 2008
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