When it comes to travel, Americans move more like albatrosses than like dust particles. That's the conclusion from a new study of the movement of dollar bills, which may help scientists better predict the spread of disease.
Most epidemics are passed from person to person, so knowing people's travel behavior is crucial for predicting the speed of an outbreak. As a shortcut, epidemiologists often assume that human travel behavior is similar to the diffusion of dust particles on water, with random local contacts between people spreading disease steadily outward from the point of exposure. But today's planes, trains, and automobiles may render that assumption a bit unrealistic.
To get a handle on modern travel habits, a team led by Dirk Brockmann, a physicist at the Max Planck Institute for Dynamics and Self-Organization in Göttingen, Germany, turned to the Internet. A site called wheresgeorge.com has tracked the movement of 70 million $1 bills since 1998. (Volunteers e-mail their location and the serial numbers of their bills to find out where the bills have been and where they go next.) Because the bills are usually passed hand to hand, the researchers argue, the movement of money is a good proxy for the movement of people. The researchers extracted a random sample of 500,000 bills from the database and calculated the time it took for each bill to change hands and the distance it traveled.
If people traveled like dust, the dollars would rarely trek large distances without being exchanged. But, in fact, the chances of a bill's hopping 100 kilometers without being swapped was nearly the same as its leaping 500 or even 1000 km without a transaction, the team reports online 25 January in Nature. That means diseases could quickly jump from one city to the next without having to stop at several points in between. Although intuitive, the researchers say this is the first time this theory has been backed up with hard data.
Surprisingly, most bills waited around in one locality for months or even years before making a trek across the country. The best living example of this kind of travel behavior is the albatross, which spends most of its time making short trips in search of food before soaring great distances to the next locality, says Brockmann.
"The beauty of this study is that it follows an everyday item that people carry," says Angela McLean, an epidemiologist at the University of Oxford, U.K., ensuring that both short and long trips within a country are captured. But McLean would like to see the results replicated with something besides money to make sure the long waiting times between big trips aren't the result of "these dollar bills getting left in coat pockets whilst their owners carry on traveling all summer."