Yesterday's New York Times Magazine explored the Harvard psychologist Jerome Kagan's work on temperament. To simplify: Anxious infants grow up to be anxious teenagers and then anxious adults (though sometimes "anxiety" can be a good thing, as when it translates as alertness, or lack of complacency). For another look at the debate surrounding Kagan's theories--and a quicker read--check out this 2004 Ideas piece, "The Temperamentalist."