One month after the 9/11 attacks, David Plotz wondered why he didn't know anyone who had died in the tragedy when the event seemed to touch the lives of so many of his friends. He looked at the data available on Americans' individual social networks—before the days of Facebook, mind you—and estimated the number of people who may have had a personal connection to the attacks. In honor of the 10th anniversary of 9/11, the piece is reprinted below. (Note: This story was written at a time when the death toll was believed to be about 6,000.)
Lead author University of Florida anthropology professor H. Russell Bernard and his four co-authors have spent the last 15 years trying to figure out how many people the average person knows. (What does it mean to know someone?
If the average American knows 290 people, then the World Trade Center victims would seem to know about 1.8 million Americans (and vice versa: about 1.8 million Americans would seem to know a victim). But there is overlap in the networks of the victims. Someone who knew one victim at, say, Cantor Fitzgerald is very likely to have known two or more. Some poor souls lost dozens of friends. The authors account for this with an estimated "lead-in factor"—essentially a measure of non-randomness. The lead-in factor helps adjust for the fact that people who worked at the World Trade Center are likely to travel in the same social circles because they work in the same place, live in the same city, and have similar kinds of jobs.
The lead-in factor deflates the total number of Americans who know a victim from 1.8 million to about 1.1 million. (The lead-in factor is admittedly fuzzy. The authors simply do not know how likely it is that someone who knows one victim knows more than one. The lead-in factor they chose is based on past experience with other surveys [Click
One-point-one million is a gigantic number of people to be struck by tragedy: Imagine the entire state of New Hampshire in mourning. But it is also a small number: Fewer than one American in 200 is likely to know one of the 6,000.
What does this mean for Slate or any other small group? Co-author Peter Killworth of Southhampton Oceanography Centre in England did a back-of-the-envelope calculation for me to show the odds that Slate would be left untouched by the disaster. Even if you assume Slate staffers are, for demographic reasons, about twice as likely as the average American to know a World Trade Center victim, there is only a 25 percent chance that any of our editorial staffers would know someone. Our staff would have to number around 50 people before there would be even a 50-percent likelihood that one of us knew a victim. So, it is not unusual that Slate doesn't know anyone. It would be unusual if we did.
The math offers a kind of solace, too. Yes, it shows that we are less directly connected than we think to 9/11. But it also shows that we're more indirectly connected. The most amazing statistic of Bernard et al. is how many people knew someone who knew a victim. According to their estimates, essentially all Americans—more than 80 percent of them—know someone who knows someone. We are all mourners at the second degree.