Life’s Odds and Sept. 11

No one I know personally was on the list. Why?

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.)

For days after 9/11, I was waiting for the awful news. After all, I live in Washington, D.C., just a few miles from the Pentagon. Most of my extended family, all my in-laws, and many friends live in New York City. Dozens of friends from high school and college work in Manhattan finance. Dear ones in Washington, New York, and Boston regularly take cross-country airplane flights.

I heard about friends who had close calls and friends of friends who had died, but there was no victim whose face I knew, whose hand I had shaken. My anticipation was not fueled by egocentrism, by the sense that the tragedy only mattered if it struck me. It was something like the opposite. I mourned for the 6,000. But without a name to give to my grief, my sorrow felt vague and insufficient. It lacked the sharpness the victims deserved. A hole was ripped in my country. A hole was ripped in hundreds of thousands of lives. Shouldn’t a hole be ripped in my life?

Expecting but not knowing, I soon realized, is a surprisingly common 9/11 experience. As Slate staffers began to share stories, we realized that none of our 20-odd editorial employees knew any of the World Trade Center victims. The Seattleites thousands of miles away didn’t know anyone. But neither did the staffer who lives a few blocks from the twin towers, nor the New Yorker who’s engaged to a financial journalist, nor the business columnist who’s a veteran of New York financial magazines. Neither did many friends and professional associates.

Of course, Slate is no random sample of America: We’re more East Coast, more connected to New York, more college-educated, more Internet-using. But these are all factors that should make us more likely to know World Trade Center victims. In the only national poll about who knows victims, Pew Research Center found that 20 percent of Americans know or have a friend or relative who knows someone injured or killed on 9/11. But that’s true of 29 percent of college graduates, 28 percent of Internet users, 32 percent of East Coasters, and 37 percent of people whose family income tops $100,000.

We thought ourselves so close and found ourselves so distant. We wondered what explained our lack of connection. Why would Slate—or any group of 20 people who expected to be connected—not be connected? (We wondered about Slate because it was the best sample we had. It is a stand-in for any group.)

Colleagues quickly proposed theories. Several insisted that not knowing wasn’t surprising because even though we think of New York as one city, it’s actually a thousand different, largely self-contained communities. It would be foolish to expect the journalistic universe of Slate to intersect the universes of finance or firefighting. Others discounted the significance of our friendships with people in Manhattan business. Hundreds of thousands of people work for New York financial firms, but only a teeny fraction of them worked in the twin towers, and an even teenier fraction worked on their upper floors. The likelihood that our acquaintances in that world happened to work on the top floors of the skyscrapers is extremely small.

A few colleagues speculated about a class divide. Slate friends in Manhattan business, like many Slate staffers, tend to be graduates of elite colleges who work at swankier investment houses and law firms. The World Trade Center housed few such businesses. It was home to smaller firms, boutique investors, branch offices, foreign concerns, and government agencies. (Though the towers were what’s called “Class A”—that is, top-of-the-line—real estate, they were not as desirable as buildings like the World Financial Center next door, which holds several high-profile corporate headquarters.) And many city dwellers are realizing these days, with embarrassment, how unconnected they are to the police and firefighters who protect our lives.

Others thought the high percentage of foreign victims—perhaps one-quarter of the dead were foreign nationals—lowered the chances of Americans knowing someone.

But basic math may be the most powerful tool for understanding why we don’t know anyone. The idea that we ought to know someone is built on a mathematical fallacy.

The World Trade Center question falls into a category that mathematicians call “small-world” or “degrees of separation” problems. The key work in the field for our purpose is being done by a team of five social scientists and mathematicians who have already written a paper on 9/11, “Estimating the Ripple Effect of a Disaster.” It is being published in the journal Connections. You can read it here.

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? Here is their definition, which is only one of many possible ones.) According to their surveys, the average American has a “personal social network” of about 290 people. Bernard says that figure is probably a low estimate and that the average network could be more than 400. How do they arrive at the figure of 290? Read about it here.

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 here to read how they picked it.] The World Trade Center deaths are unlike the other examples they compare them to, so it’s impossible to know how accurate their lead-in factor is.)

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.