Behaviorism dominated psychology from the 1930s to the 1950s. As the psychologist Ulric Neisser recalled, “Behaviorism was the basic framework for almost all of psychology at the time…That was the age when it was supposed that no psychological phenomenon was real unless you could demonstrate it in a rat” (quoted in Baars, 1986, p. 275). But behaviorism wouldn’t dominate the field forever, and Neisser himself would play an important role in developing the perspective that replaced it.

Even at the height of behaviorism, a few psychologists continued to study mental processes. For example, the German psychologist Max Wertheimer (1880–1943) studied psychological illusions, which are errors of perception, memory, or judgment in which subjective experience differs from objective reality. In one of Wertheimer’s experiments, two lights flashed quickly on a screen, one after the other. When the time between the two flashes was relatively long (one fifth of a second or more), observers correctly reported seeing two lights going on and off in sequence. But when Wertheimer reduced the time between flashes to around one twentieth of a second, observers incorrectly reported seeing a single light moving back and forth (Fancher, 1979; Sarris, 1989). Wertheimer argued that during perception, the mind brings many disparate elements together and combines them into a unified whole, which in German is called a gestalt. Wertheimer’s ideas led to the development of Gestalt psychology, a psychological approach that emphasizes the active role that the mind plays in generating perceptual experience. According to Gestalt psychology, the mind imposes organization on what it perceives. The German psychologist Kurt Lewin (1890-1947) was strongly influenced by Gestalt psychology and argued that the best way to predict a person’s behavior was not to understand the stimuli to which they were responding, but to understand their subjective interpretation or construal of those stimuli. A pinch on the cheek can be pleasant or unpleasant depending on who we think has administered it and under what circumstances (as well as to which set of cheeks). For Lewin, the person’s inner experience was paramount.

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In the 1930s and 1940s, most psychologists ignored Wertheimer and Lewin—and ignored mental processes as well. In the 1950s, that was changed—not by yet another German psychologist, but by the invention of the computer. Although people and computers differ in important ways, both seem to register, store, and retrieve information, which led some psychologists to wonder whether the computer might be useful as a model for the human mind. Computers are information-processing systems, and the flow of information through their circuits is clearly no fairy tale. If mental events—such as remembering, thinking, believing, evaluating, feeling, and assessing—were simply words we use to describe different kinds of information processing, then the events that took place inside a human mind could be studied as objectively as the events that took place inside a computer. This way of thinking gave rise to a new approach called cognitive psychology, which is the scientific study of mental processes, including perception, thought, memory, and reasoning.

For example, during World War II, the military turned to psychologists to help understand how soldiers could best learn to use new technologies, such as radar. Radar operators had to pay close attention to their screens for long periods while trying to decide whether blips were friendly aircraft, enemy aircraft, or flocks of wild geese in need of a good chasing (Ashcraft, 1998; Lachman, Lachman, & Butterfield, 1979). How could radar operators be trained to make quicker and more accurate decisions? The British psychologist Donald Broadbent (1926–93) observed that pilots can’t attend to many different instruments at once and must actively move the focus of their attention from one to another (Best, 1992). Broadbent (1958) showed that the limited capacity to handle incoming information is a fundamental feature of human cognition and that this limit could explain many of the errors that pilots (and other people) made. At about the same time, the American psychologist George Miller (1956) pointed out a striking consistency in our capacity limitations across a variety of situations—we can pay attention to and briefly hold in memory about seven (give or take two) pieces of information. Cognitive psychologists began conducting experiments and devising theories to better understand the mind’s limited capacity. The emergence of cognitive psychology was made possible by the advent of the computer, but it was also energized by the appearance of a book by B. F. Skinner called Verbal Behavior, which offered a behaviorist analysis of language (Skinner, 1957). But linguist Noam Chomsky (b. 1928) thought that Skinner’s unwillingness to talk about the mind had led him to seriously misunderstand the nature of human language. Chomsky argued that just as a computer program contains a set of step-by-step rules for generating output, language relies on mental rules that allow people to understand and produce novel words and sentences. The ability of even the youngest child to generate new sentences that he or she had never heard before flew in the face of Skinner’s claim that children learn to use language the same way that rats learn to press levers—namely, by reinforcement. Chomsky provided a clever, detailed, and thoroughly cognitive account of language that explained many of the phenomena that Skinner’s account could not (Chomsky, 1959). These and other developments set the stage for an explosion of research in cognitive psychology, which dominated the landscape of academic psychology for the next 25 years.

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If cognitive psychologists studied the software of the mind, they had little to say about the hardware of the brain. And yet, as any computer scientist knows, the relationship between software and hardware is crucial: Each element needs the other to get the job done. Our mental activities often seem so natural and effortless—noticing the shape of an object, using words in speech or writing, recognizing a face as familiar—that we fail to appreciate the fact that they depend on intricate operations carried out by the brain. This dependence is revealed by dramatic cases in which damage to a particular part of the brain causes a person to lose a specific cognitive ability. Recall Broca’s patient who, after damage to a limited area in the left side of the brain, could not produce words, even though he could understand them perfectly well. Such striking—sometimes startling—cases remind us that even the simplest cognitive processes depend on the brain.

Karl Lashley (1890–1958), a psychologist who studied with John Watson, conducted a famous series of studies in which he trained rats to run mazes, surgically removed parts of their brains, and then measured how well they could run the maze again. Lashley hoped to find the precise spot in the brain where learning occurred. Alas, no one spot seemed to uniquely and reliably eliminate learning (Lashley, 1960). Rather, Lashley simply found that the more of the rat’s brain he removed, the more poorly the rat ran the maze. Lashley was frustrated by his inability to identify a specific site of learning, but his efforts inspired other scientists to take up the challenge. They developed a research area called physiological psychology. Today, this area has grown into behavioral neuroscience, an approach to psychology that links psychological processes to activities in the nervous system and other bodily processes. To learn about the relationship between brain and behavior, behavioral neuroscientists observe animals’ responses as the animals perform specially constructed tasks, such as running through a maze to obtain food rewards. The neuroscientists can record electrical or chemical responses in the brain as the task is being performed, or they can later remove specific parts of the brain to see how performance is affected.

Of course, experimental brain surgery cannot ethically be performed on human beings; thus, psychologists who want to study the human brain often have to rely on nature’s cruel and inexact experiments. Birth defects, accidents, and illnesses often cause damage to particular brain regions, and if that damage disrupts a particular ability, then psychologists deduce that the region is involved in producing the ability. For example, in the Memory chapter, you’ll learn about a patient whose memory was virtually eliminated by damage to a specific part of the brain, and you’ll see how this tragedy provided scientists with remarkable clues about how memories are stored (Scoville & Milner, 1957). But in the late 1980s, technological breakthroughs led to the development of non-invasive brain scanning techniques that made it possible for psychologists to watch what happens inside a human brain as a person performs a task such as reading, imagining, listening, and remembering (see FIGURE 1.1). Brain scanning is an invaluable tool because it allows us to observe the brain in action and to see which parts are involved in which operations (see the Neuroscience and Behavior chapter). In fact, there’s a name for this area of research. Cognitive neuroscience is the field of study that attempts to understand the links between cognitive processes and brain activity (Gazzaniga, 2000).

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Psychology’s renewed interest in mental processes and its growing interest in the brain were two developments that led psychologists away from behaviorism. A third development also pointed them in a different direction. Recall that one of behaviorism’s key claims was that organisms are blank slates on which experience writes its lessons, and hence any one lesson should be as easily written as another. But in experiments conducted during the 1960s and 1970s, psychologist John Garcia and his colleagues showed that rats can learn to associate nausea with the smell of food much more quickly than they can learn to associate nausea with a flashing light (Garcia, 1981). Why should this be? In the real world of forests, sewers, and garbage cans, nausea is usually caused by spoiled food and not by lightning, and although these particular rats had been born in a laboratory and had never left their cages, millions of years of evolution had prepared their brains to learn the natural associations causing nausea more quickly than the artificial one. In other words, it was not only the rat’s learning history but the rat’s ancestors’ learning histories that determined the rat’s ability to learn.

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Although that fact was at odds with the behaviorist doctrine, it was the credo for a new kind of psychology. Evolutionary psychology explains mind and behavior in terms of the adaptive value of abilities that are preserved over time by natural selection. Evolutionary psychology has its roots in Charles Darwin’s theory of natural selection, which, as we saw earlier, holds that the features of an organism that help it survive and reproduce are more likely than other features to be passed on to subsequent generations. Evolutionary psychologists think of the mind as a collection of specialized “modules” that are designed to solve the human problems our ancestors faced as they attempted to eat, mate, and reproduce over millions of years. According to evolutionary psychology, the brain is not an all-purpose computer that can do or learn one thing just as easily as it can do or learn another; rather, it is a computer that was built to do a few things well and everything else not at all. It is a computer that comes with a small suite of built-in applications that are designed to do the things that previous versions of that computer needed to have done.

Consider, for example, how evolutionary psychology treats the emotion of jealousy. All of us who have been in romantic relationships have experienced jealousy, if only because we noticed our partner noticing someone else. Jealousy can be a powerful, overwhelming emotion that we might wish to avoid, but according to evolutionary psychology, it exists today because it once served an adaptive function. If some of our hominid ancestors experienced jealousy and others did not, then the ones who experienced it might have been more likely to guard their mates and aggress against their rivals and thus may have been more likely to reproduce their “jealous genes” (Buss, 2000, 2007; Buss & Haselton, 2005).

Critics of the evolutionary approach point out that many current traits of people and other animals probably evolved to serve different functions than those they currently serve. For example, biologists believe that the feathers of birds probably evolved initially to perform such functions as regulating body temperature or capturing prey and only later served the entirely different function of flight. Likewise, people are reasonably adept at learning to drive a car, but nobody would argue that such an ability is the result of natural selection; the learning abilities that allow us to become skilled car drivers must have evolved for purposes other than driving cars.

Complications such as these have led the critics to wonder how evolutionary hypotheses can ever be tested (Coyne, 2000; Sterelny & Griffiths, 1999). We don’t have a record of our ancestors’ thoughts, feelings, and actions, and fossils won’t provide much information about the evolution of mind and behavior. Testing ideas about the evolutionary origins of psychological phenomena is indeed a challenging task, but it is not an impossible one (Buss et al., 1998; Pinker, 1997a, 1997b).

Start with the assumption that evolutionary adaptations should also increase reproductive success. So if a specific trait or feature has been favored by natural selection, it should be possible to find some evidence of natural selection at work in the numbers of offspring that are produced by the trait’s bearers. Consider, for instance, the hypothesis that men tend to have deep voices because women prefer to mate with baritones rather than sopranos. To investigate this hypothesis, researchers studied a group of modern hunter–gatherers, the Hadza people of Tanzania. Consistent with the evolutionary hypothesis, the researchers found that the pitch of a man’s voice did indeed predict how many children he would have, but the pitch of a woman’s voice did not (Apicella, Feinberg, & Marlowe, 2007). This kind of study provides evidence that allows evolutionary psychologists to test their ideas. Not every evolutionary hypothesis can be tested, of course, but evolutionary psychologists are becoming increasingly inventive in their attempts.

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