On commuter trains, the middle seat in a bank of three is always the last to be occupied. Some passengers will even stand for an hour by doorways rather than sit between patrons. Why? This quirky aversion may be a case of psychological security trumping physical comfort.

The tension often begins when greedy window- and aisle-seat occupants discourage access to the middle seat by blocking it with a briefcase or studiously avoiding eye contact with approaching seat searchers. Many passengers would sooner walk by than initiate a strained interaction.

And if a commuter does squeeze in, the trials continue. According to Richard E. Wener, an environmental psychologist at Polytechnic University in Brooklyn, unplanned encounters such as brushing an arm against a neighbor raise anxiety by marring one's sense of “predictability and control.” (For more on travel stress, see “Commuting Takes Its Toll,” on page 14.)

The phobia has become so prevalent that as transit authorities from Washington, D.C., to Seattle update their fleets, they are commissioning cars containing only pairs of seats, even if that means more cars per train or that more commuters must stand. No one has really examined how to reduce the problem, but Wener offers one suggestion: armrests, like those on airplanes. “These help each traveler demarcate territory,” which fosters a perception of increased control, lessening stress.