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entists for basic biological investigations.The method should also work on organ-

isms even more complex than yeast, says thestudy’s lead author, Jason W. Chin, who isnow at the Medical Research Council Lab-

oratory of Molecular Biology in Cambridge,England. That’s because yeast contain pro-tein-building machinery that’s similar tothat in multicellular organisms, from peasto people. Yeast cells and those of the morecomplex organisms are classified as eukary-otes because, unlike bacteria, they containdistinct nuclei and organelles.

In fact, says Cropp, the team has alreadybegun experiments in which it’s trying to gethuman cells to use unnatural amino acidswhen making proteins. —J. GORMAN

Small WorldAfter AllShort e-mail chains reachtargets worldwide

Six degrees of separation—the notion thatevery person on the planet can reach everyother through a chain of about six socialties—has been borne out by the first large-scale study of social networks.

The more than 24,000 e-mail users whoparticipated in the study were randomlyassigned one of 18 targets in 13 countries,including a police officer in Australia, a vet-erinarian in the Norwegian army, and a pro-fessor at an Ivy League university. The par-ticipants were asked to help relay a messageto their target by forwarding it to just oneacquaintance whom they regarded as“closer” than themselves to the target.

A total of 384 chains reached their target;the others fizzled out when, for example, arecipient mistook the message for spam orwas too busy to forward it to a new person.The successful chains averaged 4.05 e-mails.Taking into account the lengths of the unsuc-cessful chains, researchers estimate that twostrangers are typically connected to eachother via five to seven e-mails, says PeterSheridan Dodds, a member of the ColumbiaUniversity team that performed the study.

“Through not very many links, peoplecan actually find someone who is incredi-

bly different from themselves on the otherside of the world with a completely differ-ent job,” Dodds says. “No one along thechain knows how the rest of the chain isfunctioning—they’re all just doing some-thing local, moving one step.”

The study shows that the six-degreesidea, originally mooted in 1967 by socialpsychologist Stanley Milgram, is “not justan urban legend,” says Steven Strogatz, amathematician at Cornell University.

The Columbia team asked participantsseveral questions, including how they chosethe next recipient of the e-mail. The chainsthat succeeded in reaching their targetstended to contain many connections tocasual acquaintances. The chains that failedrelied more heavily on close friends.

This phenomenon, called the strength ofweak ties, is not surprising, says Mark Gra-novetter, a sociologist at Stanford Univer-sity. His research has shown that peopletypically find jobs through acquaintancesrather than close friends.

“Your close friends tend to know eachother, but your acquaintances tend to knowpeople you don’t know,” he says. “They’remuch more your windows on the world.”

Study participants usually chose to for-ward the message to someone closer to thetarget in location or profession rather thanto simply send it to a friend who knew manypeople. This belies the widely held beliefthat hubs, individuals with many social ties,

are crucial to the success of chains, theColumbia researchers note. Searching thesocial network is “largely an egalitarianexercise, not one whose success depends ona small minority of exceptional individu-als,” they say in the Aug. 8 Science.

It’s likely, however, that the successful e-mail chains involved more hubs than thesenders realized, Granovetter cautions. “Thestudy doesn’t prove that people with a lot ofties aren’t important in the network,” he says.

Although six degrees of separationseems like a small number of steps, insocial terms it represents an enormousgulf, Strogatz says. “With the people whoare two steps away from you, the friendsof your friends, the connection is alreadygetting a little hazy,” he says. “Once thenumber is three, you have very little psy-chological connection to these people—they’re three whole universes away.

“Six or seven steps is unfathomable,” headds. “It’s meaningless socially.”

The new research could be relevant foranalyzing not just social ties but also peer-to-peer file sharing and computer networks,Dodds says.

The Columbia researchers havelaunched a new study in which partici-pants may forward their message to manyacquaintances instead of just one. Thescientists are recruiting volunteers at theWeb site www.smallworld.columbia.edu. —E. KLARREICH

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