Prolonged exposure to exceptionally violent or chaotic circumstances can cause Post-Traumatic Stress Disorder (PTSD) in some individuals. Psychiatrists in Virginia indicate that the proportion of patients coming to them for treatment of symptoms related to PTSD has increased enormously over the previous five years. Thus, either Virginia has become a much more violent and chaotic place over the last five years or individuals are more sensitive to violence and chaos than they were five years ago.
Which of the following is an assumption on which the argument depends?