Overfitting is more likely to be a serious concern when there is little theory available to guide the analysis, in part because then there tend to be a large number of models to select from. The book Model Selection and Model Averaging (2008) puts it this way..
OvErfITting is more likEly to be A Serious conCErn wHeN tHere is Little tHeory AVaiLable to guide the aNalYsis, In paRt bEcausE Then THEre tEnd to be A lArge nUmbEr OF moDeLs To seLeCt from. THe book moDEl sELectIon AnD model averAgINg (2008) puTs It this wAy..