Attribute substitution is a psychological process thought to underlie a number of cognitive biases and perceptual illusions. It occurs when an individual has to make a judgment (of a target attribute) that is computationally complex, and instead substitutes a more easily calculated heuristic attribute. This substitution is thought of as taking place in the automatic intuitive judgment system, rather than the more self-aware reflective system. Hence, when someone tries to answer a difficult question, they may actually answer a related but different question, without realizing that a substitution has taken place. This explains why individuals can be unaware of their own biases, and why biases persist even when the subject is made aware of them. It also explains why human judgments often fail to show regression toward the mean.

The theory of attribute substitution unifies a number of separate explanations of reasoning errors in terms of cognitive heuristics. In turn, the theory is subsumed by an effort-reduction framework proposed by Anuj K. Shah and Daniel M. Oppenheimer, which states that people use a variety of techniques to reduce the effort of making decisions.

In a 1974 paper, psychologists Amos Tversky and Daniel Kahneman argued that a broad family of biases (systematic errors in judgment and decision) were explainable in terms of a few heuristics (information-processing shortcuts), including availability and representativeness.

In 1975, psychologist Stanley Smith Stevens proposed that the strength of a stimulus (e.g., the brightness of a light, the severity of a crime) is encoded neurally in a way that is independent of modality.^{[citation needed]} Kahneman and Frederick built on this idea, arguing that the target attribute and heuristic attribute could be unrelated.

In a 2002 revision of the theory, Kahneman and Shane Frederick proposed attribute substitution as a process underlying these and other effects.

[P]eople are not accustomed to thinking hard, and are often content to trust a plausible judgment that comes to mind.

Kahneman and Frederick propose three conditions for attribute substitution:

Attribute substitution explains the persistence of some illusions. For example, when subjects judge the size of two figures in a perspective picture, their apparent sizes can be distorted by the 3D context, making a convincing optical illusion. The theory states that the three-dimensional size of the figure (which is accessible because it is automatically computed by the visual system) is substituted for its two-dimensional size on the page. Experienced painters and photographers are less susceptible to this illusion, because the two-dimensional size is more accessible to their perception.