Meet Linda. She’s thirty-one years old, single, outspoken, and very bright. In college, Linda majored in philosophy. As a student, she was deeply concerned with issues of discrimination and social justice, and participated in antinuclear demonstrations.
Before I tell you more about Linda, let me ask you a question about her. Which is more likely?
a. Linda is a bank teller.
b. Linda is a bank teller and is active in the feminist movement.”
“Faced with this question, most people answer (b). It makes intuitive sense, right? A justice-seeking, antinuke philosophy major? That sure sounds like someone who would be an active feminist. But (a) is—and must be—the correct response. The answer isn’t a matter of fact. Linda isn’t real. Nor is it a matter of opinion. It’s entirely a matter of logic. Bank tellers who are also feminists—just like bank tellers who yodel or despise cilantro—are a subset of all bank tellers, and subsets can never be larger than the full set they’re a part of. In 1983 Daniel Kahneman, he of Nobel Prize and DRM fame, and his late collaborator, Amos Tversky, introduced the Linda problem to illustrate what’s called the “conjunction fallacy,” one of the many ways our reasoning goes awry.
When researchers have posed the Linda problem at different times of day—for instance, at 9 a.m. and 8 p.m. in one well-known experiment—timing often predicted whether participants arrived at the correct answer or slipped on a cognitive banana peel. People were much more likely to get it right earlier in the day than later. There was one intriguing and important exception to the findings (which I’ll discuss soon). But as with executives on earnings calls, performance was generally strong in the beginning of the day, then worsened as the hours ticked by.
The same pattern held for stereotypes. Researchers asked other participants to assess the guilt of a fictitious criminal defendant. All the “jurors” read the same set of facts. But for half of them, the defendant’s name was Robert Garner, and for the other half, it was Roberto Garcia. When people made their decisions in the morning, there was no difference in guilty verdicts between the two defendants. However, when they rendered their verdicts later in the day, they were much more likely to believe that Garcia was guilty and Garner was innocent. For this group of participants, mental keenness, as shown by rationally evaluating evidence, was greater early in the day. And mental squishiness, as evidenced by resorting to stereotypes, increased as the day wore on.
“Scientists began measuring the effect of time of day on brainpower more than a century ago, when pioneering German psychologist Hermann Ebbinghaus conducted experiments showing that people learned and remembered strings of nonsense syllables more effectively in the morning than at night. Since then, researchers have continued that investigation for a range of mental pursuits—and they’ve drawn three key conclusions.
First, our cognitive abilities do not remain static over the course of a day. During the sixteen or so hours we’re awake, they change—often in a regular, foreseeable manner. We are smarter, faster, dimmer, slower, more creative, and less creative in some parts of the day than others.
Second, these daily fluctuations are more extreme than we realize. “[T]he performance change between the daily high point and the daily low point can be equivalent to the effect on performance of drinking the legal limit of alcohol,” according to Russell Foster, a neuroscientist and chronobiologist at the University of Oxford. Other research has shown that time-of-day effects can explain 20 percent of the variance in human performance on cognitive undertakings.”
Third, how we do depends on what we’re doing. “Perhaps the main conclusion to be drawn from studies on the effects of time of day on performance,” says British psychologist Simon Folkard, “is that the best time to perform a particular task depends on the nature of that task.”