When listening to rock music played backward, people often perceive an evil message only if specifically forewarned what to listen for. this best illustrates the dangers of:

The answer is perceptual set.

Explanation:

This concept refers to perceiving some features of available data, while ignoring others. This occurs because our perception is influenced by expectations and emotions.

In the example, many people state that they hear evil subliminal messages in the songs because they have assumed it to be true beforehand. Their expectations make them hear the words, even if there aren't any.