'Sorry I Haven't Written': A Scientific Explanation

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If you're like most people, you do a lot of routine things in any given day — running errands, say, or e-mailing or doing laundry. And like most people, you've probably never stopped to wonder whether the pattern of your routine behavior fits into some sort of universal mathematical law.

But Dean Malmgren has. A faculty member in Northwestern University's department of chemical and biological engineering, Malmgren thinks a lot about universal laws and how they might explain human behavior, even seemingly spontaneous actions like writing an e-mail to a friend.

In the current issue of Science, Malmgren and several colleagues explain that as far as human correspondence goes, when and how long you choose to sit down and e-mail your friends or family has less to do with your desire to get in touch than with a larger — and less random — system of outside factors.

To reach that conclusion, Malmgren's team analyzed the letter-writing oeuvre of 16 people important enough that their correspondence has been thoroughly archived — people like Einstein, Darwin and Hemingway. Initially, Malmgren says, researchers believed that old-fashioned letter-writing would follow different rules of behavior from e-mailing, but the new analysis suggests that they're actually very similar. "It's analogous to some areas of physics," says Malmgren, "where you might have two fluids with very different densities and viscosities but they ultimately follow the same laws of fluid dynamics."

It turns out that just three mechanisms combine to explain both activities. The first is our propensity to continue repeating a task once we've started: "Once you send one e-mail or write one letter, you tend to do another," says Malmgren. The second is our circadian sleep-wake cycle, which limits the available time we have to devote to letter-writing. The third is that we typically work on the same days each week, further restricting when and how long we spend getting in touch with friends.

These three fundamentals are complicated by individual situations, of course. Someone with a full-time job, for example, can't necessarily engage in personal correspondence except at night or on weekends. Some people have lots of friends and family to keep in touch with, while others are naturally more solitary. And circumstances can change over time. "In the early part of his life," says Malmgren, "Einstein didn't write many letters. Later, as he became famous — and had a secretary to help him — he wrote a lot more. Freud was steadier. Each had a personal writing rate."

Nevertheless, both celebrities' letters fit into the same underlying model, as did those of Karl Marx, Robert E. Lee, Marcel Proust — and presumably a horde of unfamous letter-writers as well. "I really wish we could have gotten some ordinary people," says Dean. "But unfortunately, their letters are rarely preserved in a comprehensive way. Nobody cares about the letters of Joe Schmo who lived in 1873."

This study is just the beginning for Malmgren. "Our model only describes how we do one activity," he says, "but we actually juggle lots of things. So it's interesting to consider how we transition between them." One way to get a handle on how people multitask is to look at online activity, the focus of his group's next analysis: it involves a lot of different behaviors — such as chatting, game-playing and reading — but under a single umbrella. "There's potentially a lot wrapped up into one," says Malmgren.

On a practical level, Malmgrem's research could help explain a range of other apparently unique human behaviors, like running errands, making phone calls, checking books out of the library and doing homework. In the meantime, the study offers at least a few possible excuses for why it's taken you so long to respond to that e-mail from your mother — like "The universal mathematical model made me do it," or maybe "You wouldn't complain if I were Einstein."