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●IF YOU live in the northern
hemisphere, this is probably not your
favourite month. January tends to
dispirit people more than any other. We all
know why: foul weather, post-Christmas
debt, the long wait before your next holiday,
quarterly bills, dark evenings and dark
mornings. At least, that is the way it seems. For
while all these things might contribute to the
way you feel, there is one crucial factor you
probably have not accounted for: the state
of mind of your friends and relatives. Recent
research shows that our moods are far more
strongly influenced by those around us than
we tend to think. Not only that, we are also
beholden to the moods of friends of friends,
and of friends of friends of friends – people
three degrees of separation away from us who
we have never met, but whose disposition can
pass through our social network like a virus.
Indeed, it is becoming clear that a whole
range of phenomena are transmitted through
networks of friends in ways that are not
entirely understood: happiness and
depression, obesity, drinking and smoking
habits, ill-health , the inclination to turn out
and vote in elections , a taste for certain music
or food, a preference for online privacy , even
24 | NewScientist | 3 January 2009 www.newscientist.com
Cover story |
Three degrees of contagion
Your behaviour is influenced by others far more strongly than you might think, even by people you’ve never met, says Michael Bond
the tendency to attempt or think about
suicide . They ripple through networks “like
pebbles thrown into a pond”, says Nicholas
Christakis , a medical sociologist at Harvard
Medical School in Boston, who has pioneered
much of the new work.
At first sight, the idea that we can catch
the moods, habits and state of health not
only of those around us, but also those we
do not even know seems alarming. It implies
that rather than being in charge of where we
are going in life, we are little more than back-
seat drivers, since most social influence
operates at a subconscious level.
But we need not be alarmed, says Duncan
Watts , a sociologist at Columbia University,
New York. “Social influence is mostly a good
thing. We should embrace the fact that we’re
inherently social creatures and that much of
who we are and what we do is determined by
forces that are outside the little circle we draw
around ourselves.” What’s more, by being
aware of the effects of social contagion we may
be able to find ways to counter it, or use it to our
own benefit. “There’s no doubt people can have
some control over their networks and that this
in turn can affect their lives,” says Christakis.
To get an idea of what is going on, take
Christakis’s findings on the spread of
happiness, which were published last month.
His team looked at a network of several
thousand friends, relatives, neighbours
and work colleagues who form part of the
Framingham Heart Study , an ongoing multi-
generational epidemiological survey that
has tracked risk factors in cardiovascular
disease among residents of Framingham,
Massachusetts, since 1948. They found that
happy people tend to be clustered together,
not because they naturally orientate towards
each other, but because of the way happiness
spreads through social contact over time,
regardless of people’s conscious choice of
friends ( BMJ, DOI: 10.1136/bmj.a2338 ).
Christakis also found that a person’s
happiness is dependent not only on the
happiness of an immediate friend but – to
a lesser degree – on the happiness of their
friend’s friend, and their friend’s friend’s
friend. Furthermore, someone’s chances of
being happy increase the better connected
they are to happy people, and for that matter
the better connected their friends and family.
“Most people will not be surprised that people
with more friends are happier, but what really
matters is whether those friends are happy,”
says Christakis.
Happiness is nearThey also discovered that the effect is not
the same with everyone you know. How
susceptible you are to someone else’s
happiness depends on the nature of your
relationship with them. For example, if a good
friend who lives within a couple of kilometres
of you suddenly becomes happy, that
increases the chances of you becoming happy
by more than 60 per cent. In contrast, for a
next-door neighbour the figure drops to about
www.newscientist.com 3 January 2009 | NewScientist | 25
half that, and for a nearby sibling about half
again. Surprisingly, a cohabiting partner
makes a difference of less than 10 per cent,
which coincides with another peculiar
observation about some social epidemics:
that they spread far more effectively via
friends of the same gender.
All this poses a key question: how can
something like happiness be contagious?
Some researchers think one of the most
likely mechanisms is empathetic mimicry.
Psychologists have shown that people
unconsciously copy the facial expressions,
manner of speech, posture, body language
and other behaviours of those around them,
often with remarkable speed and accuracy.
This then causes them, through a kind of
neural feedback, to actually experience the
emotions associated with the particular
behaviour they are mimicking.
Barbara Wild and her colleagues at the
University of Tübingen, Germany, have found
that the stronger the facial expression, the
stronger the emotion experienced by the
person observing it ( Psychiatry Research, vol
102, p 109 ). She believes this process is hard-
wired, since it acts so rapidly and automatically.
Others have suggested it works through the
action of mirror neurons, a type of brain cell
thought to fire both when we perform an
action and when we watch someone else
doing it, though it is not clear whether the
mimicking would cause the neurons to fire
or whether their firing would trigger the
mimicry. What is clear is that unconscious
imitation allows people to “feel a pale
reflection of their companions’ actual
emotions” and even “feel themselves into the
emotional lives of others”, says Elaine Hatfield
at the University of Hawaii, Honolulu, whose
review of the latest research will appear next
April in The Social Neuroscience of Empathy.
“ Actions and feelings can be as contagious as a virus “
gain weight, this changes my idea of what
an acceptable weight is. One similarity with
happiness is that friends and relatives have
a far greater influence if they are of the same
gender. While it is not evident why that
should matter for emotional contagion,
norms of body size are clearly gender-specific:
“Women look at other women, men look at
other men,” says Christakis. This could also
help explain the epidemics of eating disorders
reported among groups of schoolgirls in
recent decades.
The spread of a social norm appears to
account for another of Christakis’s findings:
that when people stop smoking, they usually
do so along with whole clusters of friends,
relatives and social contacts. As more people
quit, it becomes the socially acceptable thing
to do, and those who choose to continue
smoking are pushed to the periphery of the
network. In this case, people are most strongly
influenced by those closest to them – if your
spouse quits, it is 67 per cent more likely that
you will too. Your work colleagues can also
have an effect, particularly if you are in a
small, close-knit workplace; and more highly
educated friends influence one another more
than less educated ( The New England Journal
of Medicine, vol 358, p 2249 ).
Happiness, obesity, smoking habits –
activities that we traditionally think of as
shaped by individual circumstances, turn
out to be ruled to a large degree by social
forces. Many other day-to-day phenomena
fit a similar pattern, often counter-intuitively.
Take autism: Peter Bearman at the Institute
for Social and Economic Research and
Policy at Columbia University who in 2004
uncovered a link between suicidal behaviour
and certain friendship patterns (American
Journal of Public Health, vol 94, p 89), is
looking at whether the recent rise in the
What is also unclear – because it has never
been properly tested – is the extent to which
emotions can propagate through virtual
networks, where the opportunity for
physiological mimicry is much reduced.
So much for emotions – what about other
phenomena that we unwittingly pick up, and
pass on, through our social networks? In 2007,
Christakis’s team, again tracking members
of the Framingham Heart Study, found that
obesity is transmitted in a similar way to
happiness. Your risk of gaining weight
increases significantly when your friends gain
weight, and it is also affected by the weight of
people beyond your social horizon. “Obesity
appears to spread through social ties,”
Christakis says. Again, how likely you are to
catch it depends on who you are interacting
with: after controlling for factors such as
difference in socioeconomic status, the
researchers found that an individual’s chances
of becoming obese increased by 57 per cent if
one of their friends became obese, 40 per cent
if a sibling did and 37 per cent if their spouse
did, irrespective of age ( The New England
Journal of Medicine, vol 357, p 370 ).
However, neighbours have no influence,
and how far away you live from a friend
counts for little, which implies that obesity
spreads via a different mechanism to
happiness. Rather than behavioural mimicry,
the key appears to be the adoption of social
norms. In other words, as I see my friends
There is plenty of evidence for emotional
contagion outside the lab. In 2000, Peter
Totterdell at the University of Sheffield, UK,
found a significant association between the
happiness of professional cricketers during
a match and the average happiness of their
teammates, regardless of other factors such
as whether the match was going in the team’s
favour (Journal of Applied Psychology, vol 85,
p 848). He found a similar effect among nurses
and office workers. It has also been shown
that if a college student suffers from mild
depression their roommate will become
progressively more depressed the longer they
live with them, and that emotional displays by
bank employees have a direct impact on the
moods of their customers.
We can see, then, how a phenomenon such
as happiness might pass quickly through a
social network and infect clusters of friends
and relatives. What none of these studies
explains, however, is why the strength of the
infection varies according to who is passing it
to whom. Why are we so much more strongly
affected by the happiness of a nearby friend
than a nearby sibling? Why does a next-door
neighbour have a significant impact, yet
someone living a few tens of metres away
on the same block have none?
The power of strangersTwo factors appear crucial: the frequency
of social contact, and the strength of the
relationship. This is not too surprising: we
know that emotional contagion requires
physical proximity. It is also likely that
the closer we feel to someone, the more
empathetic we are towards them, and the
more likely we are to catch their emotional
state. However, how these two factors play
out in day-to-day interactions is uncertain.
26 | NewScientist | 3 January 2009 www.newscientist.com
Five tips for a healthier social network1. Choose your friends carefully.2. Choose which of your existing friends you spend the most time with. For example, hang out with people who are upbeat, or avoid couch potatoes.3. Join a club whose members you would like to emulate (running, healthy cooking), and socialise with them.4. If you are with people whose emotional state or behaviours you could do without, try to avoid the natural inclination to mimic their facial expressions and postures.5. Be aware at all times of your susceptibility to social influence – and remember that being a social animal is mostly a good thing.
“ Happiness, obesity, smoking habits – all turn out to be ruled to a large degree by social forces”
diagnosis of autism is in any way socially
determined. His study is ongoing , but he
says his findings could be “explosive”. “It
is likely that if you have an autistic child in
your community the probability of your
child being diagnosed with autism is
significantly higher.”
Why three degrees?While the mechanism of social contagion
varies depending on the phenomenon being
spread, in many cases the dynamics are very
similar. For example, Christakis has found
that with happiness, obesity and smoking
habits, the effect of other people’s behaviour
carries to three degrees of separation and no
further. He speculates that this could be the
case with most or perhaps all transmissible
traits. Why three degrees? One theory is that
friendship networks are inherently unstable
because peripheral friends tend to drop away.
“While your friends are likely to be the same
a year from now, your friends of friends of
friends of friends are likely to be entirely
different people,” says Christakis.
This poses the question: what shapes the
architecture of our social networks and our
position in them? Clearly, many factors
contribute: where we live, where we work,
family size, education, religion, income,
our interests, and our tendency to gravitate
towards people similar to us. New research by
Christakis’s team, due to be published in the
next few weeks, suggests there is also a strong
genetic component. The study compared the
social networks of identical and fraternal
twins, and found that identical twins had
significantly more similar social networks
than fraternal twins, suggesting the structure
of your social network is influenced by your
genes. That may not sound remarkable, since
personality traits such as gregariousness and
shyness clearly play a role in determining how
connected we are. But there is much more to
it, says Christakis. “It’s not just about having a
genetic predilection to be friends with a lot of
people, it’s about having a genetic predilection
to be friends with a lot of popular people.
That’s mysterious: how could our genes
determine our actual location in this socio-
topological space?”
Answering that should help us
understand more about the “collective
intelligence” of social networks, which some
researchers liken to the flocking of birds – the
decision to quit smoking, for example,
is no more an isolated move than the decision
by a bird in a flock to fly to the left.
Sociologists and others are using
mathematical models to test these dynamics
to try to understand better what triggers
the spread of behaviours. Duncan Watts at
Columbia University has shown that seeding
localised social groups with certain ideas or
behaviours can lead to the ideas cascading
across entire global networks. This contradicts
the notion – promoted by the author Malcolm
Gladwell in The Tipping Point and others –
that social epidemics depend on a few key
influential individuals from whom everyone
else takes their cue. It doesn’t ring true, argues
Watts, because such “influentials” typically
interact with only a few people. The key for
the spread of anything, he says, from
happiness to the preference for a particular
song, is a critical mass of interconnected
individuals who influence one another.
Is there any way to mitigate the effects
of such powerful and pervasive social forces?
It is unlikely we can ever escape social
influence entirely, even if we wanted to.
“Even when you’re aware of it, you’re probably
susceptible,” says Watts. Still, being aware can
help, especially when we are seeking to avoid
undesirable behaviours or adopt positive
habits. We can be choosy about new friends,
seeking out people whose lifestyles we aspire
to: if you want to lose weight, for example,
join a running club and – most importantly –
socialise with its members.
Actually cutting ties with old friends might
be a bit drastic, though perhaps spending less
time with those whose traits we do not wish
to share would be a good idea – lazy people,
perhaps, or those inclined to negative
thinking. And beware those who hang out
with such people even if they do not display
their views or behaviours – remember the
three degrees of contagion rule. Finally, if you
really cannot avoid spending time with certain
people whose behaviours or emotional state
you would rather not take on board (certain
relatives at family gatherings, perhaps), you
could always try repressing your natural
inclination to mimic their body language and
facial expressions, and so limit the contagion
effect – though be prepared for them to
instinctively cool towards you as a result.
What this game plan amounts to is a kind
of subtle social reorientation. We will always
be vulnerable to what those around us are
doing, so as far as possible make sure you are
with the right people. Remember the new
adage: we are who we hang out with. ●
Read previous issues of New Scientist at www.newscientist.com/issues/current
www.newscientist.com 3 January 2009 | NewScientist | 27
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