“Proneness” can be interpreted either as vulnerability or as being at heightened risk. The two are not synonymous, and using them interchangeably can lead to confusion among conceptual and empirical issues.

“Vulnerability” refers to perceivable, palpable, or measurable variation in structure or function that represents a predisposition to a specific disease process. Examples of vulnerability traits include the absence of the enzyme glucose-6-phosphate dehydrogenase for hemolytic anemia, the presence of hemoglobin S, which leads to the formation of sickle-shaped red blood cells for sickle-cell anemia, a high pepsinogen level for peptic ulcer, and the absence of phenylalanine hydroxylase for phenylketonuria. All of these traits are identifiable prior to the onset of the disease and place the person possessing such traits in a position of being susceptible to the specific disorders, given certain known or as yet unknown environmental conditions. These traits, moreover, are not intrinsically the disease. They represent a necessary but not sufficient condition for the disease to develop.

In contrast, “risk factors” refers to statistical probabilities that some groups of people will become affected by a particular disorder. Thus, being a child of a schizophrenic parent puts that child in a group with a larger statistical risk than the general population runs, although that child may not be at all vulnerable because he or she may not have the specific – but as yet unknown – structural underpinning that predisposes to schizophrenia.