24-1 Why do we forget?

Amid all the applause for memory—all the efforts to understand it, all the books on how to improve it—have any voices been heard in praise of forgetting? William James (1890, p. 680) was such a voice: “If we remembered everything, we should on most occasions be as ill off as if we remembered nothing.” To discard the clutter of useless or out-of-date information—where we parked the car yesterday, our old phone number, restaurant orders already cooked and served—is surely a blessing. The Russian memory whiz Solomon Shereshevskii (or S, as he was known) was haunted by his junk heap of memories. They dominated his consciousness. He had difficulty thinking abstractly—generalizing, organizing, evaluating. After reading a story, he could recite it but would struggle to summarize its gist.

A more recent case study of a life overtaken by memory is Jill Price, whose experience has been studied and verified by a University of California at Irvine research team, along with several dozen other “highly superior autobiographical memory” cases (McGaugh & LePort, 2014; Parker et al., 2006). Price compares her memory to “a running movie that never stops. It’s like a split screen. I’ll be talking to someone and seeing something else. . . . Whenever I see a date flash on the television (or anywhere for that matter) I automatically go back to that day and remember where I was, what I was doing, what day it fell on, and on and on and on and on. It is nonstop, uncontrollable, and totally exhausting.” A good memory is helpful, but so is the ability to forget. If a memory-enhancing pill becomes available, it had better not be too effective.

More often, however, our unpredictable memory dismays and frustrates us. Memories are quirky. My [DM’s] own memory can easily call up such episodes as that wonderful first kiss with the woman I love, or trivial facts like the air mileage from London to Detroit. Then it abandons me when I discover I have failed to encode, store, or retrieve a student’s name, or where I left my sunglasses. See how you do with remembering this sentence when we ask you about it later: The fish attacked the swimmer.

For some, memory loss is severe and permanent. Consider Henry Molaison (1926–2008). Surgeons removed much of his hippocampus in order to stop severe seizures. This resulted “in severe disconnection of the remaining hippocampus” from the rest of the brain (Annese et al., 2014). For 55 years after the procedure, Molaison was unable to form new conscious memories. He was, as before his surgery, intelligent and did daily crossword puzzles. Yet, reported neuroscientist Suzanne Corkin (2005, 2013), “I’ve known H. M. since 1962, and he still doesn’t know who I am.” For about 20 seconds during a conversation he could keep something in mind. When distracted, he would lose what was just said or what had just occurred. Without the neural tissue for turning new information into long-term memories, he never could name the current president of the United States (Ogden, 2012).

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Molaison suffered from anterograde amnesia—he could recall his past, but he could not form new memories. (Those who cannot recall their past—the old information stored in long-term memory—suffer from retrograde amnesia.)

Neurologist Oliver Sacks (1985, pp. 26–27) described another patient, Jimmie, who had anterograde amnesia resulting from brain damage. Jimmie had no memories—thus, no sense of elapsed time—beyond his injury in 1945.

When Jimmie gave his age as 19, Sacks set a mirror before him: “Look in the mirror and tell me what you see. Is that a 19-year-old looking out from the mirror?”

Jimmie turned ashen, gripped the chair, cursed, then became frantic: “What’s going on? What’s happened to me? Is this a nightmare? Am I crazy? Is this a joke?” When his attention was diverted to some children playing baseball, his panic ended, the dreadful mirror forgotten.

Sacks showed Jimmie a photo from National Geographic. “What is this?” he asked.

“It’s the Moon,” Jimmie replied.

“No, it’s not,” Sacks answered. “It’s a picture of the Earth taken from the Moon.”

“Doc, you’re kidding! Someone would’ve had to get a camera up there!”

“Naturally.”

“Hell! You’re joking—how the hell would you do that?” Jimmie’s wonder was that of a bright young man from 40 years earlier reacting with amazement to his travel back to the future.

Careful testing of these unique people reveals something even stranger: Although incapable of recalling new facts or anything they have done recently, Molaison, Jimmie, and others with similar conditions can learn nonverbal tasks. Shown hard-to-find figures in pictures (in the Where’s Waldo? series), they can quickly spot them again later. They can find their way to the bathroom, though without being able to tell you where it is. They can learn to read mirror-image writing or do a jigsaw puzzle, and they have even learned complicated job skills (Schacter, 1992, 1996; Xu & Corkin, 2001). They can be classically conditioned. However, they do all these things with no awareness of having learned them. “Well, this is strange,” Molaison said, after demonstrating his nondeclarative memory of skillful mirror tracing. “I thought that would be difficult. But it seems as though I’ve done it quite well” (Shapin, 2013).

Molaison and Jimmie lost their ability to form new explicit memories, but their automatic processing ability remained intact. Like Alzheimer’s patients, whose explicit memories for new people and events are lost, they could form new implicit memories (Lustig & Buckner, 2004). These patients can learn how to do something, but they will have no conscious recall of learning their new skill. Such sad cases confirm that we have two distinct memory systems, controlled by different parts of the brain.

For most of us, forgetting is a less drastic process. Let’s consider some of the reasons we forget.

Much of what we sense we never notice, and what we fail to encode, we will never remember (FIGURE 24.1). English novelist and critic C. S. Lewis (1967, p. 107) described the enormity of what we never encode:

[We are] bombarded every second by sensations, emotions, thoughts . . . nine-tenths of which [we] must simply ignore. The past [is] a roaring cataract of billions upon billions of such moments: Any one of them too complex to grasp in its entirety, and the aggregate beyond all imagination. . . . At every tick of the clock, in every inhabited part of the world, an unimaginable richness and variety of ‘history’ falls off the world into total oblivion.

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Age can affect encoding efficiency. The brain areas that jump into action when young adults encode new information are less responsive in older adults. This slower encoding helps explain age-related memory decline (Grady et al., 1995).

But no matter how young we are, we selectively attend to few of the myriad sights and sounds continually bombarding us. Consider: You have surely seen the Apple computer logo thousands of times. Can you draw it? In one study, only 1 of 85 UCLA students (including 52 Apple users) could do so accurately (Blake et al., 2015). Without encoding effort, many potential memories never form.

Even after encoding something well, we sometimes later forget it. To study the durability of stored memories, Ebbinghaus (1885) learned more lists of nonsense syllables and measured how much he retained when relearning each list, from 20 minutes to 30 days later. The result, confirmed by later experiments, was his famous forgetting curve: The course of forgetting is initially rapid, then levels off with time (FIGURE 24.2; Wixted & Ebbesen, 1991). Harry Bahrick (1984) found a similar forgetting curve for Spanish vocabulary learned in school. Compared with those just completing a high school or college Spanish course, people 3 years out of school had forgotten much of what they had learned (FIGURE 24.3 below). However, what people remembered then, they still remembered 25 and more years later. Their forgetting had leveled off.

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One explanation for these forgetting curves is a gradual fading of the physical memory trace. Cognitive neuroscientists are getting closer to solving the mystery of the physical storage of memory and are increasing our understanding of how memory storage could decay. Like books you can’t find in your campus library, memories may be inaccessible for many reasons. Some were never acquired (not encoded). Others were discarded (stored memories decay). And others are out of reach because we can’t retrieve them.

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Often, forgetting is not memories faded, but memories unretrieved. We store in long-term memory what’s important to us or what we’ve rehearsed. But sometimes important events defy our attempts to access them (FIGURE 24.4). How frustrating when a name lies poised on the tip of our tongue, just beyond reach. Given retrieval cues (“It begins with an M”), we may easily retrieve the elusive memory. Retrieval problems contribute to the occasional memory failures of older adults, who more frequently are frustrated by tip-of-the-tongue forgetting (Abrams, 2008; Salthouse & Mandell, 2013).

Do you recall the gist of the second sentence we asked you to remember? If not, does the word shark serve as a retrieval cue? Experiments show that shark (likely what you visualized) more readily retrieves the image you stored than does the sentence’s actual word, fish (Anderson et al., 1976). (The sentence was “The fish attacked the swimmer.”)

But retrieval problems occasionally stem from interference and, perhaps, from motivated forgetting.

INTERFERENCE As you collect more and more information, your mental attic never fills, but it surely gets cluttered. Sometimes the clutter wins, and new and old learning collide. Proactive (forward-acting) interference occurs when prior learning disrupts your recall of new information. If you buy a new combination lock, your well-rehearsed old combination may interfere with your retrieval of the new one.

Retroactive (backward-acting) interference occurs when new learning disrupts recall of old information. If someone sings new lyrics to the tune of an old song, you may have trouble remembering the original words. It is rather like a second stone tossed in a pond, disrupting the waves rippling out from the first.

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Information presented in the hour before sleep suffers less retroactive interference because the opportunity for interfering events is minimized (Diekelmann & Born, 2010; Nesca & Koulack, 1994). Researchers John Jenkins and Karl Dallenbach (1924) first discovered this in a now-classic experiment. Day after day, two people each learned some nonsense syllables, then tried to recall them after up to eight hours of being awake or asleep at night. As FIGURE 24.5 shows, forgetting occurred more rapidly after being awake and involved with other activities. The investigators surmised that “forgetting is not so much a matter of the decay of old impressions and associations as it is a matter of interference, inhibition, or obliteration of the old by the new” (1924, p. 612).

The hour before sleep is a good time to commit information to memory (Scullin & McDaniel, 2010), though information presented in the seconds just before sleep is seldom remembered (Wyatt & Bootzin, 1994). If you’re considering learning while sleeping, forget it. We have little memory for information played aloud in the room during sleep, although the ears do register it (Wood et al., 1992).

Old and new learning do not always compete with each other, of course. Previously learned information (Latin) often facilitates our learning of new information (French). This phenomenon is called positive transfer.

MOTIVATED FORGETTING To remember our past is often to revise it. Years ago, the huge cookie jar in my [DM’s] kitchen was jammed with freshly baked chocolate chip cookies. Still more were cooling across racks on the counter. Twenty-four hours later, not a crumb was left. Who had taken them? During that time, my wife, three children, and I were the only people in the house. So while memories were still fresh, I conducted a little memory test. Andy admitted wolfing down as many as 20. Peter thought he had eaten 15. Laura guessed she had stuffed her then-6-year-old body with 15 cookies. My wife, Carol, recalled eating 6, and I remembered consuming 15 and taking 18 more to the office. We sheepishly accepted responsibility for 89 cookies. Still, we had not come close; there had been 160.

Why do our memories fail us? This happens in part because memory is an “unreliable, self-serving historian” (Tavris & Aronson, 2007, p. 6). Consider one study, in which researchers told some participants about the benefits of frequent toothbrushing. Those individuals then recalled (more than others did) having frequently brushed their teeth in the preceding two weeks (Ross et al., 1981).

FIGURE 24.6 below reminds us that as we process information, we filter, alter, or lose much of it. So why were my family and I so far off in our estimates of the cookies we had eaten? Was it an encoding problem? (Did we just not notice what we had eaten?) Was it a storage problem? (Might our memories of cookies, like Ebbinghaus’ memory of nonsense syllables, have melted away almost as fast as the cookies themselves?) Or was the information still intact but not retrievable because it would be embarrassing to remember?²

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Sigmund Freud might have argued that our memory systems self-censored this information. He proposed that we repress painful or unacceptable memories to protect our self-concept and to minimize anxiety. But the repressed memory lingers, he believed, and can be retrieved by some later cue or during therapy. Repression was central to Freud’s psychoanalytic theory and was a popular idea in mid-twentieth-century psychology and beyond. While an American study revealed that 81 percent of university students, and 60 to 90 percent of therapists (depending on their perspective), believe “traumatic memories are often repressed” (Patihis et al., 2014a,b), increasing numbers of memory researchers think repression rarely, if ever, occurs. People succeed in forgetting unwanted neutral information (yesterday’s parking place), but it’s harder to forget emotional events (Payne & Corrigan, 2007). Thus, we may have intrusive memories of the very same traumatic experiences we would most like to forget.