A heuristic is a mental short-cut used to make judgments and decisions.
Many decisions and judgments that humans make every day would require massive amounts of attention and brain power if every bit of relevant information was to be considered. Human cognition, while amazingly powerful, is still a limited resource. Heuristics are mental short-cuts that are used to simplify judgments and decisions, allowing them to be made more quickly and easily, freeing up cognitive capacity for other tasks. In some cases individuals are aware that they are using a heuristic, but in many cases the heuristic is used so naturally that the individual is not aware that a short-cut has been used.
Heuristics were first studied by psychologists and researchers in judgment and decision making in the 1970s. After that, researchers discovered a wide variety of heuristics, and a debate developed about whether heuristics have an overall effect that is positive or negative. Generally, most researchers agree that heuristics are highly beneficial in that they produce goodenough answers to problems most of the time, at a great cognitive savings. However, in some instances where the outcome is important, or for which the heuristic is not well calibrated, costly incorrect answers can be produced, and efforts should be taken to overcome the instinct to use a heuristic in certain situations.
One of the most studied heuristics is the availability heuristic. The availability heuristic is the tendency to use how easy an event is to recall as a proxy for its prevalence. For example, when asked to guess whether death from plane crashes or food poisoning is more common, many people guess plane crashes. Fatalities caused by food poisoning are actually much more common but are less likely to be covered by the media. When trying to decide which is more common, it is easier to recall instances of plane crashes (as they tend to be shown with graphic images in the media), and people incorrectly infer from this that deaths from plane crashes are more common. In many situations, using how easy it is to retrieve examples of something from memory is a good proxy for how common it is (for example, how common the name John is among American college students). However, factors that are particularly salient can create incorrect conclusions.
In the representativeness heuristic, the similarity between two subjects is used to make a judgment, ignoring the relative likelihood of those factors (i.e. ignoring the base rate). In many cases this is a good enough way to make judgments, but in some cases, like in the Tom example, it can lead to erroneous reasoning and decision making. This is not to say that base rate information should be used without reference to additional available information but rather that the base rate should be taken into account when determining the likelihood of an event.
Kahneman, Daniel. Thinking, Fast and Slow. New York: Farrar, Straus and Giroux, 2013.
Chater, Nick, ed. Judgment and Decision Making. Los Angeles: SAGE, 2009.
Wilhelms, Evan, and Valerie Reyna, eds. Neuroeconomics, Judgment, and Decision Making. New York: Taylor and Francis, 2015.