When Independent Variables Reverse Signs

Andrew Gelman (Columbia--Statistics) has an interesting post about a nettlesome little topic -- How to make sense of an independent variable that "flips" signs (that is, goes from negative to positive or vice-versa) across model specifications. This phenomena, while unusual, typically arises when control variables are added to a model. Gelman's post includes helpful links to examples in the literature and has attracted insightful comments.