IT IS WIDELY, if not universally, accepted that psychological events are multidetermined; that there are no simple, unique explanations for most such events. In Psychological research, however, the investigation of several variables at once necessitates large numbers of subjects;* and for practical reasons it is common to explore one variable at a time.
Historical cross-sectional studies are common in psychological and epidemiological research. Generally they seek to determine the frequency of occurrence of some crucial event or class of events in the histories of individuals in a particular pathological population in comparison with their frequency in a "normal" population.† Typical examples are: (1) the incidence of psychopathology in the mothers of schizophrenics, (2) the incidence of early parental death in the histories of depressive patients, and (3) the incidence of early illness or trauma in the histories of childhood schizophrenics. The inference
WENDER PH. On Necessary and Sufficient Conditions in Psychiatric Explanation. Arch Gen Psychiatry. 1967;16(1):41-47. doi:10.1001/archpsyc.1967.01730190043005