With the Red Balloon Challenge, the team recruited more than 5, 000 people, with some referral chains containing as many as 16 people.

ECONOMIST: Monitor

One of the first examples was the Red Balloon Challenge, staged in 2009 by DARPA, the research arm of the American Department of Defence.

ECONOMIST: Monitor

Compared with the Red Balloon Challenge this was much more difficult, says Dr Rahwan, who is now at the Masdar Institute in the United Arab Emirates.

ECONOMIST: Monitor

The difference, however, was that the Red Balloon Challenge was not a crisis, so participants had to find another way to motivate others to report sightings of the balloons.

ECONOMIST: Monitor