Doctor: Medical researchers recently examined a large group of individuals who said that they had never experienced serious back pain  Half of the members of the group turned out to have bulging or slipped disks in their spines, conditions often blamed for serious back pain
Since these individuals with bulging or slipped disks evidently felt no pain from them, these conditions could not lead to serious back pain in people who do experience such pain
An effect that occurs in the absence of a particular phenomenon might not occur when that phenomenon is present.