A retrospective study is a scientific study that tries to determine the causes of subjects' present characteristics by looking for significant connections between the present characteristics of subjects and what happened to those subjects in the past, before the study began
Because retrospective studies of human subjects must use the subjects' reports about their own pasts, however, such studies cannot reliably determine the causes of human subjects' present characteristics
Every scientific study in which researchers look for significant connections between the present characteristics of subjects and what happened to those subjects in the past must use the subjects' reports about their own pasts.