The theory that almost everyone on Earth is connected to anyone else via a small number of acquaintances seems to hold true for e-mail, too.
An experiment has found that messages only have to be forwarded between five and seven times to reach almost any other e-mail user.
The idea was tested by asking participants to forward an e-mail to friends, relations or colleagues they thought were closer to a randomly chosen target e-mail user.
The experiment updates a pioneering test of the small world idea carried out in the late 1960s.
In that investigation, social psychologist Stanley Milgram asked randomly selected people in the US Mid-West to help get letters to a stockbroker friend in Boston on the East Coast.
The letters could not be posted. Instead, those taking part were asked to hand them to people they knew well who might have social ties that might take the message closer to the target.
The results of the experiment established the idea that almost everyone is only six friends or acquaintances distant from anyone else.
Some websites such as Friendster use such chains of acquaintances to help people meet and make new friends.
Since Professor Milgram's work established the small world idea, it has been tested a few times, and there are signs of similar intimate, interconnectedness in many physical systems.
The small world idea has now gained support from the work of a research team headed by Peter Dodds and colleagues from the Institute for Social and Economic Research and Policy at the Columbia University in New York, US.
In their experiment, the scientists recruited 61,168 individuals and asked them to try to relay messages to one of 18 target people in 13 countries.
As in Professor Milgram's experiment, the message could not be sent direct. Instead, participants were asked to forward it to a friend they thought was closer.
The researchers tracked 24,163 distinct message chains, only 384 of which managed to get the message to the target.
The experiment revealed that messages had to be forwarded between five and seven times to get from a starting point to a target, which confirms Professor Milgram's result that people are separated by only a small number of steps.
The researchers said that the results did not seem to rely on people who had so many acquaintances that they act as "hubs" for messages.
"We conclude that social search appears to be largely egalitarian," the researchers say, "not one whose success depends on a small minority of exceptional individuals."
The researchers also point out that the enthusiasm of participants and their perceptions play a vital part in explaining the results.
"Network structure alone is not everything," they conclude.
The results of the experiment are published in the journal Science.
