The Rules of Contagion: Why Things Spread - and Why They Stop

by Adam Kucharski

One is social contagion: perhaps you behave in a certain way because your friends have influenced you over time.

Alternatively, it may be the other way around: you may have chosen to become friends because you already shared certain characteristics. This is known as ‘homophily’, the...

You may just happen to share the same environment, which influe...

It can be tough to work out which of the three explanations – social contagion, homophily or a shared environment – is the correct one.

Again, this is generally not feasible: who wants to reshuffle their entire friendship network for a research project?

Think about social experiments you can perform during this pandemic

This has led to some suggestions that epidemiology is closer to journalism than science, because it just reports on the situation as it happens, instead of running experiments.[37]

First on his list was the strength of correlation between the proposed cause and effect.

Then there was timing: did the cause come before the effect?

Another indicator was whether the disease was specific to a certain type of behaviour

In some cases, Bradford Hill said it’s possible to relate the level of exposure to the risk of disease.

What’s more, it may be possible to draw an analogy with a similar cause and effect, such as another chemical that causes cancer.

Finally, Bradford Hill suggested it’s worth checking to see whether the cause is biologically plausible and fits with w...

Establishing such causes and effects is inherently difficult.

The next step was to estimate whether obesity was being transmitted through the network. This meant tackling the reflection problem, separating potential contagion from homophily or environmental factors.

Christakis and Fowler found evidence that it did matter, suggesting that obesity could be contagious.

Commenting on the debate about Christakis and Fowler’s early papers, statistician Tom Snijders suggested that the studies had limitations, but were still important because they’d found an innovative way to put social contagion on scientists’ agenda.

Several other research groups have also shown that things like obesity, smoking, and happiness can be contagious.

In the 1970s, sociologist Mark Granovetter suggested that information could spread further through acquaintances than through close friends.

He referred to the importance of acquaintances as the ‘strength of weak ties’: if you want access to new information, you may be more likely to get it through a casual contact than a close friend.[46]

There is a long-standing paradox in medicine: people who have a heart attack or stroke while surrounded by relatives take longer to get medical care.

There’s evidence that close-knit groups of relatives tend to prefer a wait-and-see approach after witnessing a mild stroke, with nobody willing to contradict the dominant view. In contrast, ‘weak ties’ – like co-workers or non-relatives – can bring a more diverse set of perspectives, so flag up symptoms faster and call for help sooner.[47]

Things aren’t always so simple for social behaviour. We might only start doing something after we’ve seen multiple other people doing it, in which case there is no single clear route of transmission. These behaviours are known as ‘complex contagions’, because transmission requires multiple exposures.

Researchers have also identified complex contagion in behaviours ranging from exercise and health habits to the uptake of innovations and political activism.

Whereas a pathogen like hiv can spread through a single long-range contact, complex contagions need multiple people to transmit them, so can’t pass through single links.

Why do complex contagions occur?

First, there can be benefits to joining something that has existing participants.

Second, multiple exposures can generate credibility: people are more likely to believe in something if they get confirmation from several sources.

Third, ideas can depend on social legitimacy: knowing about something isn’t the same as seeing others acting – or not acting – on it.

Finally, we have the process of emotional amplification. People may be more likely to adopt certain ideas or behaviours amid the intensity of a social gathering: just think about the collective emotion that comes with something like a wedding or a music concert.

For complex contagions to spread, interactions need to be clustered together in a way that allows social reinforcement of ideas; people may be more likely to adopt a new behaviour if they repeatedly see everyone in their team doing it.

Over a five-year period, the behaviour of less active runners tended to influence more active runners, but not the other way around. This implies that keen runners don’t want to be outdone by their less energetic friends.

If you start with a firm belief, you’ll generally need strong evidence to overcome it; if you are unsure at first, it might not take much for you to change your opinion.

Experiments had tried to persuade people of one thing, only for them to end up believing something else.[56]

The media has a strong appetite for concise yet counter-intuitive insights.

In reality, it’s very difficult to find simple laws that apply in all situations. If we have a promising theory, we therefore need to seek out examples that don’t fit.

They found that although it can be tough to convince people they’re wrong, an attempted correction doesn’t necessarily make their existing belief stronger.

‘By and large, citizens heed factual information, even when such information challenges their partisan and ideological commitments,’ they concluded.

From a Bayesian point of view, we are generally better at judging the effect of arguments that we disagree with.[67]

Arguing on familiar ground might have been a common strategy, but it wasn’t an effective one; people were far more persuasive when they tailored their argument to the moral values of their opponent.

First, the delivery method can matter. There’s evidence that people are much more likely to complete a survey if asked in person rather than by e-mail,[69] for example.

‘If you get a correction, you might reduce your belief initially, but as time goes on you’re going to re-believe in the initial misconception,’ Swire-Thompson said.

It seems repetition matters: new beliefs survived longer if people were reminded of the truth several times, rather than just given one correction.[72]

Thinking about the moral position of others. Having face-to-face interactions. Finding ways to encourage long-term change. All of the...

Comparing deep canvassing with results from a control group, they’d found that a ten-minute conversation about transgender rights could noticeably reduce prejudice.

There are several similarities between outbreaks of disease and violence. One is the lag between exposure and symptoms. Just like an infection, violence can have an incubation period; we might not see symptoms straight away.

Watts has noted that violence can follow what’s known in medicine as a ‘dose-response effect’.

If a man or woman has a history involving violence, it increases the chance of domestic violence in their future relationships.

Researchers at Columbia University noted a 10 per cent rise in suicides in the months following the death of comedian Robin Williams.[14]

There can be a similar effect with mass shootings; one study estimated that for every ten US mass shootings, there are two additional shootings as a result of social contagion.[15]

Having identified the shootings that were likely due to contagion, the team carefully reconstructed the chains of transmission between one shooting and the next. They estimated that for every 100 people who were shot, contagion would result in 63 follow-up attacks.