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

by Adam Kucharski

It can be tough to work out which of the three explanations – social contagion, homophily or a shared environment – is the correct one. Do you like a certain activity because your friend does, or are you friends because you both like that activity? Did you skip your running session because your friend did, or did you both abandon the idea because it was raining?

Much of epidemiology relies on observational analysis: in general, reseachers can’t deliberately start outbreaks or give people severe illnesses to understand how they work. 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. For example, smokers were much more likely to get lung cancer than non-smokers. Bradford Hill said this pattern should be consistent, cropping up in different places across multiple studies.

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 (although this isn’t always helpful because ...

In some cases, Bradford Hill said it’s possible to relate the level of exposure to the risk of disease. For instance, the more cigarettes a person smokes, the more likely they are to die from them.

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]

These long distance links have become a central part of network science. As we’ve seen, ‘small-world’ connections can help biological and financial contagion jump from one part of a network to another.

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.

Why do complex contagions occur? Damon Centola and his colleague Michael Macy have proposed four processes that might explain what’s happening.

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.

In many cases, we can’t deliberately change people’s behaviour, so we have to rely on observational data, as Christakis and Fowler did with the Framingham study. However, there is another approach emerging. Researchers are increasingly turning to ‘natural experiments’ to examine social contagion.[51]

If people are rational, we might expect them to update their beliefs when presented with new information. In scientific research this approach is known as ‘Bayesian reasoning’.

Your belief after exposure to new information therefore depends on two things: the strength of your initial belief and the strength of the new evidence.[54] This concept is at the heart of Bayesian reasoning – and much of modern statistics. Yet there are suggestions that people don’t absorb information in this way, especially if it goes against their existing views.

The media has a strong appetite for concise yet counter-intuitive insights. This encourages researchers to publicise results that show how ‘one simple idea’ can explain everything.

Antonio García Martínez, who spent two years working in Facebook’s ads team, recalled such a situation in his book Chaos Monkeys. Martínez tells the story of a senior manager who built a reputation with pithy, memorable insights about social influence. Unfortunately for the manager, these claims were undermined by research from his company’s own data science team, whose rigorous analysis had shown something different.

‘The backfire effect finding got a lot of attention because it was so surprising,’ Nyhan later said.[63] ‘Encouragingly, it seems to be quite rare.’

A lot rides on how we structure and present our arguments.

That’s if we even think about different arguments. A few years ago, social psychologists Matthew Feinberg and Robb Willer asked people to come up with arguments that would persuade someone with an opposing political view. They found that many people used arguments that matched their own moral position, rather than the position of the person they were trying to persuade. Liberals tried to appeal to values like equality and social justice, while conservatives based their argument on things like loyalty and respect for authority.

people were far more persuasive when they tailored their argument to the moral values of their opponent.

Good point

researchers are increasingly thinking about how ideas wane. ‘It’s this concept that once you change someone’s mind, it doesn’t stick permanently.’

‘The clustering seen in maps of killings in US cities resembles maps of cholera in Bangladesh,’ he later wrote.

Although biology has moved on from the idea of bad air, debate around crime still focuses on bad people. Slutkin thinks this is in part because contagious violence is less intuitive than disease.

remember an epiphany when I asked someone “what’s the greatest determinant of violence? What’s the greatest predictor?” And the answer was “a preceding violent event”.’

For some diseases, the risk of illness can depend on the dose of pathogen a person is exposed to, with a small dose less likely to cause severe illness.

We can find similar patterns when looking at violent acts. For centuries, people have reported localised clusters of self-harm and suicide: in schools, in prisons, in communities.[10]

Clustering is common with other types of violence too. In 2015, a quarter of US gun murders were concentrated in neighborhoods that made up less than 2 per cent of the country’s overall population.[17]

One year after the programme started, shootings in West Garfield Park had dropped by about two thirds. The change had been rapid, with interrupters breaking the chains of violence from one person to another. So what is it about these transmission chains that makes interruption possible?