Researchers follow the money to show complicated ways people connect

November 18, 2010

What are borders these days? When travel was local, borders and communities were easy to define, but now our connectivity is more complex. It's time to think of borders differently, according to Northwestern University researchers.

To reflect today's reality, they have taken a look at and redrawn the borders within the United States, showing areas of the country that are most connected. Some of the borders in this new map are familiar, but many are not.

The research team, led by professor Dirk Brockmann, used the wealth of data generated by Wheres's George? (, a popular website that tracks one dollar bills spent across the country. The many millions of bank notes, having been passed from one person to another, represent links between geographic places.

An award-winning video produced by Christian Thiemann and Daniel Grady, doctoral students and members of Brockmann's research group, illustrates the work:

This video is not supported by your browser at this time.

Some divisions were expected, but many were a surprise. Some of the most significant borders split states, such as Illinois, Wisconsin and Pennsylvania. This initially puzzled the researchers, but people who have seen the research and live in those areas say the borders reflect cultural segmentation and the pull of certain large cities.

"We also thought that the strong, long-range relationships, such as between Los Angeles and New York, would overshadow the local, short-distance travel, but it did not," said Brockmann, associate professor of engineering sciences and applied mathematics at the McCormick School of Engineering and Applied Science. "The short distance travel has a stronger impact."

Some community divisions fall along more familiar lines, such as purely political boundaries (state lines), political boundaries that reflect geography (the Mississippi River) and others caused entirely by a geographical feature (the Appalachian Mountains cutting through West Virginia).

The study will be published Nov. 18 in the journal PLoS ONE.

The work already has attracted the attention of diverse individuals and groups wanting to know more about the "mobility neighborhoods." The Federal Reserve System is interested. Commuters to Chicago from northwest Indiana are interested. And a leading American linguist is interested.

"The well-defined geographic areas we found show us how people really interact and are connected," said Brockmann, who also studies how infectious disease spreads. "These results open up pathways for investigation in a variety of areas."

Where's George allows anyone to record a bill's serial number and then track its journeys as other people spend it across the country. Every time a dollar is spent in a new place, it means someone moved it there. Research has shown that this flux of money data is a good proxy for human mobility. The very large database is truly multiscale: links span thousands of miles, hundreds of miles and just a few miles.

"Think of these links as an attractive force connecting two places," said Brockmann, a member of the Northwestern Institute on Complex Systems. "The social and mobility ties represent a gravitational pull of sorts."

The information could be valuable to the Federal Reserve for optimizing the transportation of money among the 12 regional Reserve Banks across the United States. A professor of linguistics from the University of Pennsylvania wants to use the travel boundaries in his studies of regional dialect boundaries. The findings support those in northwest Indiana counties who feel more connected to Chicago and favor being in the same time zone. And anyone involved with planning large-scale infrastructure, such as high-speed rail, who needs to know where people want to go, can benefit from the human mobility map, Brockmann said.

Using supercomputers, Brockmann and his team applied a random algorithm to the Where's George? data, which collects counties into groups. The traffic within each group is very high -- very connected -- compared to the traffic between the groups. They ran the algorithm thousands of times, with each run producing a slightly different map. Then they took the results, overlaid all the maps and produced one single map, showing the strongest boundaries.

Explore further: Political borders, health-care issues complicate pandemic planning

More information: The title of the PLoS ONE paper is "The Structure of Borders in a Small World."

Related Stories

Obama vows to fast track high speed rail

April 16, 2009

US President Barack Obama Thursday called for a US high speed rail service to rival the express trains of France, Japan, Spain and China, highlighting a 13 billion dollar government funding boost.

Human behavior is 93 percent predictable, research shows

February 23, 2010

( -- Human behavior is 93 percent predictable, a group of leading Northeastern University network scientists recently found. Distinguished Professor of Physics Albert-László Barabási and his ...

Eyjafjallajokull's Global Fallout

April 23, 2010

( -- Eyjafjallajokull and its massive cloud of volcanic ash clearly have had an enormous impact on Europe and its airports, disrupting the mobility of millions and costing airlines more than a billion euros. But ...

Doing the math on where people go

September 15, 2010

Network scientists at Northeastern University have created a mathematical model that can simulate human mobility over the course of several months or even years.

Recommended for you

Who you gonna trust? How power affects our faith in others

October 6, 2015

One of the ongoing themes of the current presidential campaign is that Americans are becoming increasingly distrustful of those who walk the corridors of power – Exhibit A being the Republican presidential primary, in which ...

The hand and foot of Homo naledi

October 6, 2015

The second set of papers related to the remarkable discovery of Homo naledi, a new species of human relative, have been published in scientific journal, Nature Communications, on Tuesday, 6 October 2015.

Ancient genome from Africa sequenced for the first time

October 8, 2015

The first ancient human genome from Africa to be sequenced has revealed that a wave of migration back into Africa from Western Eurasia around 3,000 years ago was up to twice as significant as previously thought, and affected ...

From a very old skeleton, new insights on ancient migrations

October 9, 2015

Three years ago, a group of researchers found a cave in Ethiopia with a secret: it held the 4,500-year-old remains of a man, with his head resting on a rock pillow, his hands folded under his face, and stone flake tools surrounding ...

Mexican site yields new details of sacrifice of Spaniards

October 9, 2015

It was one of the worst defeats in one of history's most dramatic conquests: Only a year after Hernan Cortes landed in Mexico, hundreds of people in a Spanish-led convey were captured, sacrificed and apparently eaten.

1 comment

Adjust slider to filter visible comments by rank

Display comments: newest first

not rated yet Nov 18, 2010
That is clearly awesome. Amirite?

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.