Just recently in my introductory psychology course, a student did not know the difference between an "illusion" and a "delusion." Since this is a common source of confusion, it is worth explaining.
An illusion is a misleading perception, usually visual. You see something, but you consistently misjudge its length, shape, motion or direction. To avoid further confusion, illusions are distinctly different from hallucinations which involve sensing something that is not actually present. In contrast, illusions deal with stimuli that are actually present, but they are misinterpreted or hard to interpret.
For example, look at the Mόller-Lyer illusion shown to the right. Look at the horizontal lines. Typically the top line will be seen as being longer. However, they are both the same length. If you don't believe me, measure them yourself.
Another illusion involves the Necker cube shown to the left. Are you looking down on a cube from above? Or are you looking up at a cube from below? If you concentrate on the solid dot in the upper left, you will seem to be looking down on the cube. If you concentrate on the open dot in the lower right, it will seem that you are looking up at a cube from underneath it. For most people, as their concentration shifts from dot to dot, the cube seems to "flip-flop" the front and back sides seem to reverse themselves. For people who are used to seeing things only one way in this case, usually looking down on a cube they might find it difficult to get the cube to shift.
On the other hand, a delusion is a deeply held false belief that is maintained even when other information contradicts the belief. The contradictory information is either ignored completely or discounted in some way. Many prejudices rely on stereotypes that apply to a small minority in a group, but these stereotypes become delusional when they are used to judge everyone in that group. Beside race, religion, sex, ethnic group and nationality, occupational and age groups can develop stereotypes. Very few computer experts are "nerds," and only a small minority of athletes are "dumb jocks." Likewise, few teenagers are "delinquents," and only a small percentage of the elderly are "old fogies." However, people who hold strong prejudices against these groups will ignore the contradictory information and characterize all or most members of the group with these stereotyped labels.
In their extreme form, delusions and hallucinations are symptoms of people who are psychotic they cannot clearly distinguish what is real from what is not. With psychotic disorders, hallucinations are most frequently auditory like hearing voices.
There are some psychotic disorders that suffer only from delusions. Although delusions are relatively rare among mental disorders as a whole, they are typical of a delusional disorder previously called paranoid psychosis. The paranoid component includes delusions of persecution, grandeur and reference, with persecution at the central core.
Delusions of persecutionare mistaken beliefs that people are out to "get you" in some way. They are trying to get you fired, cheat you, poison you or even take over the world. The evidence offered for this is unlikely to convince those who do not share the delusion. Of course, if you disagree with them or suggest that they need psychological help you are likely to be seen as "part of the plot."
Delusions of grandeurare mistaken beliefs that you are better than others. If others are trying to get you fired, it is because they are jealous of your "special talents." Delusions of reference include mistaken beliefs that innocent events relate especially to you. When other workers are talking, they are talking about you. Even television newscasters could be talking specifically about you or to you.
Since those with paranoid delusions are not sure who is plotting against them, they are not likely to share their suspicions with others. Suspicious and hostile they are likely to separate themselves from other people. Their delusions can expand to cover more people. For example, one person couldn't trust anyone except his wife and he wasn't too sure about her.
If their delusions involve someone trying to kill them, these people can be dangerous to others. A person with a hand in the pocket might be viewed as holding a concealed gun and become a target for "self-defense." This might account for individuals who seem to senselessly kill a group of people like the slaughter that occurred in 1984 at a McDonald's restaurant in the San Diego area.
This should clear up the confusion between an illusion and a delusion, but I hope it doesn't make you paranoid about people who have a delusional disorder.
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