It seems obvious that 2 key attributes of health hazards, their perceived p
robability and perceived severity, do not act independently on the motivati
on to engage in protective behavior. If a health problem is perceived to ha
ve no chance of occurring, there should be no interest in acting against it
, regardless of how serious it might be. Nevertheless, researchers seldom o
bserve the expected interaction between probability and severity. A case st
udy approach was used to examine how probability and severity combine to in
fluence interest in protection. Ratings of motivation to act, probability,
and severity for 201 hazards were collected from 12 participants, and data
were analyzed for each person separately. Analyses revealed the expected Pr
obability x Severity interaction. Additional calculations showed why it is
difficult to detect this interaction using between-subjects designs. The da
ta also revealed that people are surprisingly insensitive to variations in
hazard probability when probabilities are in the moderate to high range.