Differential history is a specific threat to internal validity that arises in nonequivalent groups designs when an extraneous event affects one group but not the other. While a pretest-posttest nonequivalent groups design helps account for general history effects, it remains vulnerable to differential history. For instance, if an unexpected event—like a student drug overdose or a month-long school shutdown due to asbestos—occurs at the treatment school but not the control school, it could independently alter the affected group's posttest scores. This unequal exposure makes it impossible to know whether the observed differences are due to the treatment or the isolated historical event.