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 about behaviours and ideas that take much longer to
propagate through a population? It’s much harder to study social
contagion outside a laboratory. This isn’t just a challenge for human
populations. Among birds, great tits have a long-standing reputation
for innovation. In the 1940s, British ecologists noted that they had
worked out how to peck through the foil of milk bottles to get at the
cream. The tactic would persist for decades, but it wasn’t clear how
such innovations spread through bird populations.[31]
Although several studies have looked at the spread of animal
behaviour in captivity, it has been difficult to do the same in wild
populations. Given great tits’ reputation for innovation, zoologist Lucy
Aplin and her colleagues set out to see how these ideas propagated.
First they needed a new innovation. The team headed out into
Wytham Woods, near Oxford, and set up a puzzle box containing
mealworms. If the birds wanted to get the food inside, they’d need to
move a sliding door in a certain direction. To see how the birds