Unbeknownst to their writers, the first two articles in your Science and Technology Section go very well together (“The Kindness of Crowds” and “Primates on Facebook,” February 28, 2009). The first argues that crowds police themselves while the second points out that the number of people we know well is determined by the cognitive power of our brains rather than technology. Now, for many tens of thousands of years people lived in small nomadic communities of up to hundred-and-fifty people. This is the number of people that still corresponds to the average number of friends on Facebook or other social networks on the World Wide Web. These small communities occasionally came into contact with other nomadic communities of the same size, whence the wider circle of acquaintances also observable on Facebook. Once a year, all the small nomadic communities inhabiting the same geographic area would gather together for a few days to exchange goods and services, experience and gossip, as well as sexual favors so as to prevent inbreeding. This is what crowds are for, which is why they tend to be self-policing.