Yahoo! and Facebook are partnering to test one of the most famous U.S. social psychology experiments of the 1960s, the "Small World" hypothesis. The 1967 experiment was carried out by Stanley Milgram, a professor at Yale University. Milgram sent packages to 160 random residents of Omaha, Neb., and tried to determine how many steps through friends or acquaintances would be needed to get them to someone in Boston. Milgram decided the average was six steps, leading to the phrase "six degrees of separation." But later researchers have cast doubt on his work, and now Facebook and Yahoo! are working up an online test, the San Jose (Calif.) Mercury News reported. Researchers now are looking at how many steps it would take on Facebook to get a message from a person in one country to someone in another. "You really couldn't have done this until very recently," said Duncan Watts, a scientist with Santa Clara, Calif.-based Yahoo! who is leading the experiment. "It's a milestone, in terms of it's the kind of research question you can answer now that you could have imagined 50 years ago but that you couldn't have answered 50 years ago -- or even 15 years ago."