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Social Contagion: “I’ll Have What She’s Having”

When Harry Met SallyBuying, laughing, yawning and graffiti are all socially contagious. Now research says obesity is too. This has nothing to do with the power of suggestion or keeping up with the Joneses. To be influenced by others is genetically programmed in us and is an evolutionary hangover.

- "People are DNA's way of making more DNA." - Edward O. Wilson, 1975
- “A hen is only an egg's way of making another egg.” - Samuel Butler, 1877

YawnYawning is contagious.  No-one really knows why. Emotions, like crying and happiness are catching as well … and so too is anger. No-one really knows why.

When females reside in the same house, their menstrual cycles get into alignment.  Again, there are lots of theories but no-one really knows why.

ObesityNow, according to the latest research, obesity is socially contagious and again, no-one really knows why.  Simply having a chunky spouse or sibling raises your risk of obesity by over a third and if you have a close friend who is obese, it raises your risk even more dramatically.1

Why are we so incredibly susceptible to the influence of others?

Let me spare you looking for a rational explanation based in the here and now.  The answer seems to be in our evolutionary heritage.  Only when you take that into account, does any of this start to make sense – and then it’s a stretch.

Mimicry is Innate

Here’s a fact. Mimicry is innate. “I’ll have what she’s having” is observed in newborn babies.

When they are just a few minutes old, babies will stick out their tongues at adults doing the same thing. We clearly have an instinct for imitation. (Mirror neurons in the brain, discovered recently, are involved. They allow us to grasp the minds of others through using a kind of ‘cerebral simulation’ working on feeling rather than thinking. More about them in a future column.)

Social contagion has variously been attributed to the power of suggestion, word-of-mouth or keeping up with the Joneses.  But clearly it is much more than that. We are now pretty sure that it is an evolutionary hangover and that it continues to affect us in various ways, many of which we are often oblivious.

Evolutionary Hangover

evolutionWe mimic the behavior of others as a result of our evolutionary heritage. In the wild it was an adaptive thing to do. Creatures good at reading changes in others’ behaviour, must have had a major advantage.  Rita Carter put it this way: “If you react to your neighbour’s reaction to a rustle in the bushes rather than wait to hear the rustle yourself, you speed up the process of fleeing from a potential predator.”2

Fleeing when you see others take flight is the primitive equivalent of ‘keeping up with the Joneses’.  It enabled your ancestors to survive and without it, neither you nor I would exist. This is part of our makeup that has been genetically selected by survival of the fittest.

Macaque monkey mimics tongue pokeThis provides an intuitively plausible reason why popularity is so contagious. If we see a crowd look up, can we resist doing the same? When everybody’s talking about the latest TV show, how long before we tune in too?


In my column ‘False Alarm Theory: How Humorous Ads Work –Aug 2005’, laughter turned out to have its roots in evolution also.  

According to Ramachandran, the main purpose of laughter evolved out of alerting others in the social group to danger by a warning cry.3   If you have a warning cry, you also need another signal to 'cancel that cry' when the animal realizes there is no real threat to the social group. Laughter evolved out of that second signal - the false alarm signal.

In the wild, echoing the laughter of others was functional because it amplified the signal and dispersed it to the whole social group, spread out over a considerable area. Hence the socially ‘infectious’ quality of laughter and why we find ourselves laughing along with the laugh tracks.


Not just laughter, but language is socially contagious. Have you ever had the experience of hearing somebody use an expression like ‘to die for’ or ‘get over it’ and then surprised yourself by uttering the same expression, even though you hadn’t thought about it and really had no intention to use it?  Sometimes it just pops out.

It is not just new words and expressions.  Existing ones get used more often after we hear others using them. When we reach into our minds to articulate a thought, the resultant process is an internal agenda (see Agenda Setting4). We can only think of one expression at a time, so we reach for the first one that comes to mind. If it is satisfactory we go with it.
Yet we are barely aware of how infectious this effect is.

Content-analysis studies in various media support this. The more recently and the more often a word appears in a newspaper, the greater the probability it will re-appear in today’s issue.5  This type of contagion has also been found in studies of peoples’ conversations as well as their emails. The human memory is like a cache.  The more something is encountered the more available it becomes in memory in the future and hence the more it finds its way into our communications.


It is the same when we reach into our minds to decide things like ‘what am I going to have for lunch today?’ we think of one alternative at a time. So if the first one that comes to mind is satisfactory we go with it. Low-involvement brand choice works on that principle. Only if the first one that comes to mind is unsatisfactory, do we bother to reach further down in our minds to the next, and the next, on the agenda.

Partly because they compete for top-of mind, brands just like words are socially contagious.  Success breeds success. Higher market share not only ensures a brand more familiarity and hence more availability in mind but it also assures it of more shelf space and wider distribution, thus giving it more ‘presence’ in the market place. As a brand gets more ‘presence’ and becomes more available in the market place, so too does it become more infectious and more instantly available in the mind as well. The process is reciprocal as well as contagious (see From Ad Spin to Brand Spiral). I have argued elsewhere that product placement is very powerful for precisely this reason (see Product Placement Accelerating on a Slippery Slope).


Conformity has been genetically selected in us by survival of the fittest.  We still don’t know many of the transmission mechanisms but it is clear that the social environment that we live in, infects us and we in turn spread it. This influences what we buy, what we watch and how we react in social situations.  It represents an evolutionary hangover that plausibly accounts for why our minds seem to work on the principle that 100,000 lemmings can’t be wrong! 

Paul Marsden, a noted researcher in this area, sums it up this way:
“Whilst we may like to believe that we consciously and rationally decide on how to respond to situations, social contagion evidence suggests that some of the time this is simply not the case. Rather than generating and `having' beliefs, emotions and behaviors, social contagion research suggests that, in some very real sense, those beliefs, emotions and behaviors `have' us.

 1. Christakis, N. A. and J. H. Fowler (2007). "The spread of obesity in a large social network over 32 years." New England Journal of Medicine 357(July 26): 370-379.

2. Carter. R. (2000). Mapping the Mind. London, Phoenix. P225

3. Ramachandran, V. S. and S. Blakeslee (1998). Phantoms in the Brain. New York, Quill William Morrow.

4. M. Sutherland & J. Galloway, 'The implications of agenda setting for advertising research', Journal of Advertising Research, 1981, Sept. 1983, pp. 52-6

5. John R. Anderson, Learning and Memory 2nd Ed. Wiley N.Y. 2000, p233

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