There are different factors that may influence selective perception. Basically, all of a human's previous experience have an effect on his selective perception mechanism. According to advertising researcher Seymour Smith, people screen out advertisements depending on their beliefs, attitudes, conditioning, habits, usage preferences, and others. Biological factors such as gender, age, and race may also affect selective perception. In addition to these, an individual's needs, expectations and emotional states have heavy influences on what he or she "chooses" to perceive.
There are two types of selective perception: perceptual vigilance and perceptual defense. The low level of selective perception, perceptual vigilance refers to the process in which the individual notices and recognizes the stimuli that may be significant to him at some degree. On the other hand, perceptual defense occurs in an attempt of a person to create a barrier between him and the stimuli so that he could protect himself from having awareness of it. More often than not, these stimuli are perceived to be threatening or unpleasant, such as obscene words and violent actions. This is the high level of selective perception wherein violent actions are not accurately seen or foul words are not precisely heard. Researchers say that people with high level perceptual defense have a strong "perceptual wall" which serves as the filtering mechanism, making them unable to perceive unwanted stimuli.
Selective perception, when done consciously, may lead to "seeing" things that the person wants to and disregarding the opposite of such. A classic research on selective perception included subjects from Princeton University and Darthmouth University. The respondents were asked to watch a film of a football game between Princeton and Dartmouth. The results showed that the Dartmouth subjects reported noticing almost twice as much infraction in the rules by the Princeton team as that which was perceived by the Princeton viewers regarding the Dartmouth team. In this case, the subjects experience selective perception relative to the opposing team.
Selective Perception Tests
Since the early 21st century, many researchers performed experiments and studies to develop more knowledge regarding the concept of selective perception. In 1999, psychologists Daniel called "The Invisible Gorilla Test", which revealed that people can be concentrated on one stimulus or situation and become "blind" to an incoming or unexpected situation. This effect was termed as "inattentional blindness". Watch the test here.