This is the tendency to search primarily for information that confirms our preconceptions. We discussed this phenomenon in class. Suppose you give subjects a sequence of three numbers {2, 4, 6} and ask them to come up with the rule for the sequence (the actual rule is any three ascending numbers). To discover the rule, they are allowed to generate sequences which are then either confirmed our disconfirmed. This is done until the subject thinks the rule has been found. Most often, people initially hypothesize that the rule is consecutive even numbers. As a result, they generate sample sequences such as {8, 10, 12} which confirm the hypothesis. They do not generate sequences which might disprove their hypothesis. Thus, most of the time the subjects guess the wrong rule.
This is a thinking strategy people use to judge the liklihood of something by how well it matches a particular prototype. It leads people to ignore useful information. Here's an example from Myers (p. 53): Consider Linda, who is 31, single, outspoken, and very bright. She majored in philosophy in college. As a student she was deeply concerned with discrimination and other social issues, and she participated in antinuclear demonstrations. Based on this description, which would you say is more likely: (a) Linda is a bank teller or (b) Linda is a bank teller and active in the feminist movement. Most people answer (b) because it matches their impression of Linda. But this is an error because it is not possible for the conjunction of two events to be more likely than one of the events alone. So (b) can never be more likely than (a).
Here's another example from Myers (p. 53) . Consider the following two questions: Roberta is a university student who spends about three hours studying outside of classes in an average week. What would you guess her grade point average to be? Judith is a university student who spends about three hours studying outside of classed in an average week. Judith has four plants in the place she's living now. On an average weekday, she goes to sleep around midnight. She has a brother and two sisters. Two months is the longest period of time she has dated one person. She describes herself as being often a cheerful person. What would you guess her grade point average to be? People almost always estimate a higher grade point average for the second question. However, both passages give the same useful information: that the student studies three hours per week. People seem to pay attention to irrelevant information in the second passage and forget that the student only studies for three hours a week. When useless information is added, it dilutes the useful information.
Suppose you have flipped a coin and it has come up heads several straight times. The gambler's fallacy is the notion that tails is more likely to occur on the next toss. The chances of heads or tails is 50/50 on every throw. Each toss is completely independent of the other tosses. Gamblers often base their theories on the incorrect assumption that something is "bound to happen".
Barnum was the guy who said, "There's a sucker born every minute". This is the tendency for people to accept favorable predictions and descriptions that are generally true of everyone, such as in horoscopes. This is a phenomenon that is quite apparent today with the popularity of the Psychic Friends Network, etc.
This is the tendency for people to be more confident than correct in their judgements. Studies have shown that when people answer multiple choice questions like, "Which is longer, the Mississippi River or the Nile River?", if the answers are 60% correct, the subjects are 75% certain of their answers.
Reference: Myers, David G. (1993). Social Psychology (4th ed.). New York: McGraw-Hill, Inc.