You might think you are the world authority on your network of friends and acquaintances. But the cellphone in your pocket may be better at tracking your relationships than you are.
That's a finding of a study which opens new possibilities for social scientists, epidemiologists and other researchers who want to know how people connect and interact socially.
Nathan Eagle of the Santa Fe Institute in New Mexico and his colleagues Sandy Pentland of the Massachusetts Institute of Technology and David Lazer of Northeastern University in Boston handed out cellphones to 94 volunteers at MIT. The phones were modified with software that logged the volunteers' calls, and used Bluetooth to detect when another of the phones was close by.
By looking for simple patterns in the logs of calls and times when phones were close together, the researchers found they could predict who the volunteers would identify as their friends with 95 per cent accuracy.
For example, being nearby on campus during work hours meant little, but if two phones were close together for several hours on a Saturday evening their owners were likely to be friends. "You can think of it as a behavioural signature," Eagle says.
The researchers were also able to link the phone data to the volunteers' satisfaction at work. Those who reported themselves less satisfied were less likely to have friends in close proximity and more likely to call friends during work hours.
The phones proved more accurate than the volunteers themselves at measuring how much time they spent physically near to others: people typically overestimated how much time they spent close to friends and underestimated how much they spent with more casual contacts.
Although some of these findings may sound obvious, the study provides an important proof of principle – the gadgets we carry day-to-day can accurately record the nuances of our relationships. Using cellphones for social science research could replace interviews, which are laborious and sometimes unreliable, to find out about people's lives.
The cellphone approach may also have immediately practical applications such as helping epidemiologists predict how swine flu will spread from person to person.
An incident that took place during the study hints at further possibilities. On 27 October 2004, when the Boston Red Sox won baseball's World Series, many volunteers in the study massed in the centre of Boston to celebrate. City transport planners could have used cellphone towers to track people's movements during this unusual migration, to help better understand how their network performs.
Some companies are already using the data that cellphones can provide on your friendships. Teradata, based in Dayton, Ohio, has helped Canadian network operator Rogers identify richly connected people it calls "social leaders" and target them with "refer-a-friend" offers. Results so far suggest this approach works better than conventional marketing campaigns.
The potential for cellphone records to be used without the owner's permission to track or even predict our behaviour raises the spectre of Big Brother-style surveillance, however. "There are very serious privacy issues," says Gueorgi Kossinets, who researched online social networks at Cornell University in Ithaca, New York, before joining Google in Mountain View, California, as a quantitative analyst.
All the volunteers in the MIT study signed detailed consent forms detailing the information that would be taken from the phones and how it would be used. That isn't practical for larger studies encompassing thousands or millions of people, says Eagle, but they can be conducted if data is anonymised to obscure private information.