Email updates six degrees theory

By Kimberly Patch, Technology Research News

The world has known about the small-world phenomenon since sociologist Stanley Milgram's 1967 study found that it took, on average, six exchanges among acquaintances to get a letter from a random correspondent in Omaha, Nebraska to a Boston recipient identified only by a brief description.

But the experiment, which started with 96 messages, 18 of which eventually reached the recipient, has been criticized for not being thorough.

Columbia University researchers have filled in the blanks by carrying out a larger, more detailed experiment over the Internet. The results match many of the broad conclusions of Milgram's work, but show that Milgram's conclusion about the importance of hubs -- people who have many connections -- may be off, at least in regards to social networks.

The results could contribute to better knowledge bases and peer-to-peer network design.

The researchers' global social search experiment, posted on the Internet, prompted 24,163 email volunteers to attempt to reach one of 18 target persons in 13 countries by forwarding messages to acquaintances. "People were given the description of the target individual and asked to send an email to a contact of theirs who they thought was in some way closer to the target," said Peter Sheridan Dodds, an associate research scientist at Columbia University.

Recipients of these messages were instructed to do the same until the message reached the target individual. The researchers also asked participants for demographic data and their reasons for choosing the contacts they did, said Dodds.

The experiment generated a total of 61,168 emails that ran through 166 countries; 384 of the original 24,163 reached their target. The researchers concluded that individual apathy or disinclination to participate was the major reason for broken chains. When they questioned senders who did not forward their messages after one week, only 0.3 percent said they could not think of an appropriate recipient, according to Dodds.

The experiment confirmed that, in social searches, a message initiated by a random person reaches its destination in a median of five to seven steps, depending on the separation of source and target, said Dodds. "People can find other people regardless of how distant they are," he said. "Search in large-scale networks is a very difficult problem, and yet people working collectively are able to do quite well."

With a greater number of successful searches to analyze, however, they found something surprising: the primary avenues from source to target are not the highly connected social hubs that Milgram's experiments pointed to. "Successful chains were... far less likely to use hubs than unsuccessful chains," said Dodds. "Hubs -- people with many friends -- don't seem to be so important for this kind of social search."

Participants in successful chains were less likely to send messages to hubs -- 1.6 percent versus 8.2 percent -- than those in incomplete chains.

The participants' answers also reflected this. They rarely chose a sender based on the number of friends that person had, according to Dodds. The main reasons for choosing the next person were geography- and work-related," he said. These two categories accounted for at least half of all choices, with geography dominating in the early stages of the chain.

When the researchers compared successful chains to those that did not reach their target, they found that successful chains involved more professional ties -- 33.9 percent versus 13.2 percent -- and fewer familial relationships -- 59.8 percent versus 83.4 percent.

The experiments also showed that the success of a search is highly dependent on individual incentives, said Dodds. This is because a small change in the attrition rate -- the probability that people don't send a message on -- leads to a substantial change in the number of chains getting through, said Dodds.

One of the researchers' targets, a U.S. professor, received many more completed chains than any of the other 10 targets reached. This was probably because the professor appeared reachable, which makes sense because the participants were largely college-educated and 50 percent lived in the U.S., said Dodds. "We interpret this as meaning that people's perceptions greatly affect their chances of success," he said. "In a nutshell, if you think the world is small, it is."

The study is a confirmation of the six degrees of separation in social networks, but also debunks some ideas associated with the six degrees that have entered the popular culture, said Stephen Strogatz, a professor of applied mathematics at Cornell University. "Milgram didn't really have enough participants to figure out what kind of methods people were using" to reach the target, he said.

The new study shows social networks don't depend on super-connected people, or hubs, said Strogatz. "There was no evidence for hubs in this study, and yet people were still able to get the messages to the targets in the successful chains in five to seven steps," he said.

Although there are many similarities between social networks and virtual networks like the Internet, it makes sense that hubs may not be as prominent in the real world, said Strogatz. When hubs are virtual, like on the Internet, "there's no physical or economic cost to having many people point to your Web page," he said. In the real world, however, network hubs contain costs -- maintaining a rolodex of 100,000 people, for instance, takes time, and maintaining a powerplant with many transmission lines costs money.

In the Milgram study, all successful chains went through one person -- a well-connected tailor. The Columbia study did not show this funnel effect, however, said Strogatz. It showed, rather, "that there are a lot of roads to Rome," he said.

The new research also shows that the key to good social searches is weaker friends, or more distant acquaintances, said Strogatz. This makes sense -- closer friends are less useful in this case because people who know each other well tend to have the same friends, he said.

"This is good work," said Jim Moody, an assistant professor of sociology at Ohio State University. "By understanding the structure of email communication networks we might be able to better design tools for spreading information or stopping virus flow," he said.

The weak-tie findings are consistent with research about how people use their acquaintances to find information when they're looking for work, said Moody.

More work on the structure of close ties is needed, Moody said. "The weak-tie findings... will eventually be quite important for information flow," he said. "For virus flow through email, the key rests on the structure of close, trusted email contact -- we won't open attachments from people we don't know well."

The next step in the research is an experiment currently on the researchers' Web site that is designed to gain more information about how the small-world phenomenon works, said Dodds. "People can now send more than one email regarding any given target," he said. Also, "we've altered the questions we ask about why people choose the people they do, and extended the descriptions of the targets."

The current results could be used now to improve databases and networks, according to Dodds. "This... could be useful in the design of knowledge bases or in the construction of peer-to-peer networks," he said.

The researchers are also working to model a range of social and economic problems including the spread of contagion agents like diseases or fads, the evolution of cooperation, and the structure of modern organizations, said Dodds.

Dodds's research colleagues were Roby Muhamad and Duncan J. Watts. The work appeared in the August 8, 2003 issue of Science. The research was funded by the National Science Foundation (NSF), the James F. McDowell Foundation and the Office of Naval Research (ONR).

Timeline:   Now
Funding:   Government, Institution
TRN Categories:  Internet; Applied Technology; Computers and Society
Story Type:   News
Related Elements:  Technical paper, "An Experimental Study of Search in Global Social Networks," Science, August 8, 2003; Researcher's book, "Six Degrees, The Science of a Connected Age," by Duncan Watts; Current small-world phenomenon experiment Web site:


August 27/September 3, 2003

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