ARE you intelligent — or rational? The question may sound redundant, but in recent years researchers have demonstrated just how distinct those two cognitive attributes actually are.

It all started in the early 1970s, when the psychologists Daniel Kahneman and Amos Tversky conducted an influential series of experiments showing that all of us, even highly intelligent people, are prone to irrationality. Across a wide range of scenarios, the experiments revealed, people tend to make decisions based on intuition rather than reason.

In one study, Professors Kahneman and Tversky had people read the following personality sketch for a woman named Linda: “Linda is 31 years old, single, outspoken and very bright. She majored in philosophy. As a student, she was deeply concerned with issues of discrimination and social justice, and also participated in antinuclear demonstrations.” Then they asked the subjects which was more probable: (A) Linda is a bank teller or (B) Linda is a bank teller and is active in the feminist movement. Eighty-five percent of the subjects chose B, even though logically speaking, A is more probable. (All feminist bank tellers are bank tellers, though some bank tellers may not be feminists.)

In the Linda problem, we fall prey to the conjunction fallacy — the belief that the co-occurrence of two events is more likely than the occurrence of one of the events. In other cases, we ignore information about the prevalence of events when judging their likelihood. We fail to consider alternative explanations. We evaluate evidence in a manner consistent with our prior beliefs. And so on. Humans, it seems, are fundamentally irrational.