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Memory Errors


Memory is the outcome of a constructive process, and information loss (forgetting) may occur at each stage of processing.

Forgetting is information loss. Some information is lost early in memory processing due to encoding failures. People may fail to pay attention or misinterpret information. Information may also be lost due to the limited capacity of short-term memory. Proactive interference may disrupt processing of information that came before a to-be-remembered event, and retroactive interference may disrupt processing of information that comes after. Even when information is successfully processed through the short-term memory bottleneck, forgetting may occur due to the gradual decay of stored information over time. Retrieval failures may also occur if ineffective cues are used to search memory.

Most forgetting occurs quickly, within about a day of forming memories. However, the ability to retrieve memories continues to decline over time. Relearning information typically takes less time than the initial learning of that information. The rate of forgetting is also slower for relearned information. For example, a student learning a foreign language is likely to forget the meaning of most new words learned in class by the next day. However, as the student reviews the list, they will find it takes less time to memorize word meanings than it did the first time they saw those words. If the student stops studying and using the foreign language, they will eventually forget the meaning of many words they once knew. They will also forget grammatical rules. However, if the student goes back to studying the language years later, they are likely to relearn vocabulary and grammar more quickly, and retain more of that information, than during their initial learning experience.

Forgetting over Time

Most forgetting occurs within a day of forming memories. Memories continue to decay at a slower rate over time. Relearning improves recall of information, leading to slower rates of forgetting and higher recall over time.
Forgetting is a normal process that occurs for all people over time. Drugs, injuries, diseases, and intense stress can cause more severe forms of forgetting. For example, traumatic head injuries (such as occur in car accidents) can disrupt memory for events that immediately preceded or immediately followed the accident. The accident victim may arrive at the hospital feeling bewildered as to what happened or how they got there. Retrograde amnesia refers to the inability to retrieve information that was stored earlier. Typically, retrograde amnesia is caused by an injury or disease. It is more likely to affect recall of explicit memories for facts and experiences than procedural memories of how to carry out tasks. Medical reports describe severe cases of retrograde amnesia. For example, in 2011, 55-year-old Kay Delaney hit her head at work. This accident erased more than 20 years of memories. After the accident, she believed she was 34 years old. Her last memory was of putting her oldest son and daughter to bed decades ago. In 2008, Scott Bolzan, a former NFL player, slipped and fell in the bathroom. His head injury wiped out 46 years of memory. In some cases of retrograde amnesia, people are able to recover lost memories. However, memories lost for the time surrounding a brain injury may never have been stored and thus will never be retrievable.

Anterograde amnesia refers to memory loss due to an inability to transfer information from short-term to long-term memory. Anterograde amnesia can be caused by brain injuries or diseases. Anterograde amnesia caused by chronic alcohol abuse is called Korsakoff syndrome. It occurs because alcohol inhibits the absorption of thiamine, a B vitamin that is vital to brain function. In severe cases of anterograde amnesia, people become unable to form new memories for facts or experiences. They can hold information in short-term memory but are unable to effectively transfer that information to long-term memory. For example, a viral infection damaged memory formation areas of the brain in British conductor Clive Wearing. He developed anterograde amnesia, making him unable to remember anything for more than a few seconds. Following the infection, he could not recognize entries he'd written in his diary minutes before. He could still recognize his wife but could not remember his wedding. Wearing also developed retrograde amnesia, causing him to lose access to earlier memories. He knew he was a music conductor but had no memory of ever having conducted a concert. He could not recognize musical scores but could play them effortlessly on the piano. Wearing did not forget how to play piano, because implicit memory (particularly procedural memory) is typically spared in anterograde amnesia. Amnesia patients often can learn new skills (such as riding a bicycle or programming a computer). They also often show priming effects, in which their behavior is shaped by experiences of which they have no memory. In anterograde amnesia, memories cannot be recovered because they have not been stored. In some cases, however, people may heal from injuries or diseases and regain the ability to store memories.

False Memories

Memory is not as reliable as a digital video recording; memory errors may occur due to suggestibility and false memories.
A false memory is a recollection that is believed to be true but is actually contrary to fact. False memories are possible because memory formation is a constructive process. Memories can be modified or distorted each time they are retrieved. Even when people accurately remember information, they may forget when, where, or how that information was acquired, a phenomenon called source amnesia. For example, someone might know that Denver is the capital of Colorado but not remember where or when they learned that fact. When source memory (when, where, how) is lost, the retrieved memory may be assigned to the wrong source. For instance, memory researcher Donald Thomson was accused of rape based on the victim's detailed recollection of his face. But at the time of the rape, Thomson was giving a live television interview. The victim was assaulted shortly after she viewed the interview. The trauma of the assault apparently disrupted encoding of the source memory. This led her to wrongly attribute the face of the man she saw on television to the man who assaulted her. Thomson was cleared on the basis of his airtight alibi.


Human memory is also vulnerable to suggestibility, a phenomenon in which misleading information from external sources is incorporated into personal recollections. In the 1980s misuse of hypnosis and suggestive therapeutic techniques led many people to believe they had been victims of ritual sexual abuse. One case involved a woman named Diana Halbrooks. In therapy she began "recalling" incidents of satanic ritual physical and sexual abuse at the hands of her parents during her childhood. Her parents denied that these incidents had ever occurred. After searching for and failing to find objective evidence to corroborate her recollections, Diana eventually came to realize the "recovered memories" were false.

Psychoanalytic theories emphasize the power of unconscious information and desires. According to these theories, repression is a mental process in which people remove unwanted thoughts and memories from conscious awareness. The role of the psychotherapist is to bring these memories to consciousness so that they can be resolved. Unfortunately, the techniques used to recover memories, such as hypnosis and coached interviewing, can sometimes foster the creation of false memories. Research has compared abuse memories in people who never forgot the abuse, people who spontaneously remembered it outside of therapy, and people who remembered abuse only after therapeutic interventions. The results showed that memories recovered in therapy were rarely corroborated by objective evidence. Spontaneously recovered memories of abuse were corroborated by objective evidence at the same rate as memories of abuse reported by people who never forgot their abuse.

Researchers have demonstrated the ability to implant false memories of events that never occurred. These implanted memories differ from true memories in several regards. Namely, implanted memories are recalled in less detail than true memories and are recalled with less confidence. In 1995, American researchers Elizabeth Loftus and Jacqueline Pickrell reported implanting false memories of having been lost in a shopping mall as a child in 24 adults. During the first interview after the false memory manipulation, only 29 percent of participants "remembered" the false event. Over the course of two interviews, 75 percent resisted the suggestion that they had been lost in a mall as children. The small minority who "remembered" the false events used fewer words to describe them than they did when describing true events that actually happened to them. Their ratings for the clarity of memories and their confidence in them was lower for false memories than for true memories. Researchers have found similar results across studies in which they implanted false memories of participants taking hot-air balloon rides as children or having an encounter with the police.

Eyewitness Testimony

Although human memory is fairly robust, fabrications and errors can and do occur. For example, when people view a series of common objects (e.g., tables, vases, umbrellas) and are later shown a new object similar to the other objects, they often falsely identify the new object as one they've seen before. The sense of familiarity causes them to remember seeing the object before. But when given a choice between the new, similar object and the one they originally saw, they typically correctly choose the object they originally saw.

Some individuals may be more prone to suggestibility than others. This has significant implications for eyewitness testimony. In fact, faulty eyewitness memory turned out to be a contributing factor in more than 75% of the first 250 convictions overturned using DNA evidence. For this reason, certain vital procedures must be followed to reduce eyewitness error. Eyewitnesses should not be shown a photograph of a sole individual and asked whether that individual is the perpetrator. Instead, they should be asked whether the perpetrator appears in an array of photographs of similar-looking individuals.

Poor police interviewing techniques also can contaminate memories and make eyewitness testimony unreliable. Police interviews often include "bait questions"—questions that mislead people into believing that certain evidence exists when it does not. When people are asked these kinds of misleading questions, their memory is significantly less accurate. The effect occurs even when people are warned in advance that some questions may be misleading.

An interview technique called the cognitive interview has been shown to improve the accuracy of eyewitness testimony. It involves four steps:

  • Reinstate the context: Eyewitnesses are asked to think about what the surrounding environment looked like and how they felt during the incident.
  • Report everything: Eyewitnesses are asked to report everything that comes to mind, even things they think may not be important.
  • Recall the events in different orders: Eyewitnesses are asked to recall the event from beginning to end, in reverse order, and then starting with the part of the incident that left the strongest impression.
  • Change perspectives: Eyewitnesses are asked to try to describe the incident from the perspectives of others present.