Once the 2009 flu pandemic arrived in a country, however, long travel distance seemed to be less important for transmission. In the US, the virus spread like a ripple, gradually travelling from the southeast outwards. It took about three months to move 2,000 kilometres across the eastern US, which works out at a speed of just under 1 km/h. On average, you could have outwalked it.[21]
Although long-distance flight connections are important for introducing viruses to new countries, travel within the US is dominated by local movements. The same is true of many other countries.[22] To simulate these local movements, researchers often use what’s known as a ‘gravity model’. The idea is that we are drawn to places depending on how close and populous they are, much like larger, denser planets have a stronger gravitational pull. If you live in a village, you might visit a nearby town more often than a city further away; if you live in a city, you’ll probably spend little time in the surrounding towns.
This might seem like an obvious way to think about interactions and movements, but historically people have thought otherwise. In the mid-1840s, at the peak of Britain’s railway bubble, engineers assumed that most traffic would come from long-distance travel between big cities. Unfortunately, few bothered to question this assumption. There were some studies on the continent, though. To work out how people might actually travel, Belgian engineer Henri-Guillaume Desart designed the first ever gravity model in 1846. His analysis showed that there would be a lot of demand for local trips, an idea that was ignored by rail operators on the other side of the channel. The British railway network would probably have been far more efficient had it not been for this oversight.[23]
It can be easy to underestimate the importance of social ties. When Ronald Ross and Hilda Hudson wrote those papers on the ‘theory of happenings’ in the early twentieth century, they suggested it could apply to things like accidents, divorce and chronic diseases. In their minds, these things were independent happenings: if something happened to one person, it didn’t affect the chances of it happening to someone else. There was no element of contagion from one person to another. At the start of the twenty-first century, researchers started to question whether this was really the case. In 2007, physician Nicholas Christakis and social scientist James Fowler published a paper titled ‘The Spread of Obesity in a Large Social Network over 32 Years’. They had studied health data from participants in the long-running Framingham Heart Study, based in the city of Framingham, Massachusetts. As well as suggesting that obesity could spread between friends, they proposed that there could be a knock-on effect further into the network, potentially influencing friends-of-friends and friends-of-friends-of-friends.
The pair subsequently looked at several other forms of social contagion in the same network, including smoking, happiness, divorce, and loneliness.[24] It might seem odd that loneliness could spread through social contacts, but the researchers pointed to what might be happening at the edge of a friendship network. ‘On the periphery, people have fewer friends, which makes them lonely, but it also drives them to cut the few ties that they have left. But before they do, they tend to transmit the same feeling of loneliness to their remaining friends, starting the cycle anew.’
These papers have been hugely influential. In the decade after it was published, the obesity study alone was cited over 4,000 times, with many seeing the research as evidence that such traits can spread. But it’s also come under fire. Soon after the obesity and smoking studies were published, a paper in the British Medical Journal suggested that Christakis and Fowler’s analysis might have flagged up effects that weren’t really there.[25] Then mathematician Russell Lyons wrote a paper arguing that the researchers had made ‘fundamental errors’ and that ‘their major claims are unfounded’.[26] So where does that leave us? Do things like obesity actually spread? How do we even work out if behaviour is contagious?
One of the most familiar examples of social contagion is yawning, and it’s also one of the easiest forms of contagion to study. Because it’s common, easy to spot, and the delay from one person’s yawn to another is relatively short, researchers can look at transmission in detail.
By setting up lab experiments, several studies have analysed what makes yawns spread. The nature of social relationships seems to be particularly important for transmission: the better we know someone, the more likely it is that we’ll catch their yawn.[27] The transmission process is also faster, with a smaller delay between yawns among family members than among acquaintances. Yawn in front of a stranger and there’s a less than 10 per cent chance it will spread; yawn near a family member and they’ll catch it in about half the time. It’s not just humans who are more likely to pick up yawns from individuals they care about. Similar social yawning can occur among animals, from monkeys to wolves.[28] However, it can take a while for us to become susceptible to a yawn. Although infants and toddlers sometimes yawn, they don’t seem to catch them from their parents. Experiments suggest yawning doesn’t become contagious until children reach about four years old.[29]
As well as yawning, researchers have looked at the spread of other short-term behaviours, like itching, laughter, and emotional reactions. These social responses can manifest on very fast timescales: in experiments looking at teamwork, leaders were able to spread a positive or negative mood to their team in a matter of minutes.[30]
If researchers want to study yawning or mood, they can use laboratory set-ups to control what people see, and avoid distractions that could skew results. This is feasible for things that spread quickly, but what
