No matter what their profession, all the letter writers behaved the same way. They adhered to a circadian cycle; they tended to write a number of letters at one sitting, which is more efficient; and when they wrote had more to do with chance and circumstances than a rational approach of writing the most important letter first.

"We are interested in identifying and understanding patterns of human behavior, in learning how we make choices," said Luís Amaral, professor of chemical and biological engineering in the McCormick School of Engineering and Applied Science. Amaral led the research. "There are patterns to how we spend our days, and these models of probability, of how people allocate their time to do certain tasks, can be applied to many different areas."