A month before the fall of the Berlin Wall, 70,000 people gathered in the streets of Leipzig, East Germany, on Oct. 9, 1989, to demonstrate against the communist regime and demand democratic reforms. Clearly, no central authority planned this event; so how did all of these people decide to come together on that particular day?

A new study by researchers at Northwestern University sheds light on how individuals might obtain information about the decisions and preferences of other individuals with whom they do not have a relationship or even contact. The findings are published online this week (Aug. 2) by the Proceedings of the National Academy of Sciences (PNAS).

The Leipzig demonstration is an example of a complex system, the result of an evolving process. The common characteristic of complex systems, whether they be social or biological in nature, is that they display organization without any external organizing principle being applied.

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