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
Whether or not a study of human subjects can reliably determine the causes of those subjects' present characteristics may depend at least in part on the extent to which that study uses inaccurate reports about the subjects' pasts.