Memories reconstruct the past, rather than replicate it. Because our memories need to rebuild the past, they can make mistakes. Beyond simple forgetting errors, many errors in memory are caused by mix-ups in the associative network. In this section, we will examine the failures of memory.

Transience refers to the gradual loss of information in memory over time. As we saw in the section of Decay and Interference, there are two main causes of transience: decay and interference. Decay refers to the loss of memories over time simply due to the passage of time. Interference, on the other hand, refers to the loss of memories due to the presence of other memories. In proactive interference, old memories cause new memories to be lost, and in retroactive interference, new memories cause old memories to be lost. An example of transience would be forgetting the name of your third-grade teacher. You certainly used to know it, but over time, the memory has decayed, and the names of other teachers have caused interference, blocking retrieval of that specific memory.

Absent-mindedness is closely related to transience. However, while transience focuses on the process of losing memories already in storage, absent-mindedness refers to the problem of creating reliable memories in the first place. Absent- mindedness occurs when we don't devote enough attention to an item to encode it sufficiently into memory. For example, if you walk into your room and toss your keys on the table without thinking about it (perhaps you are distracted by a message on the answering machine), you may not be able to remember where you put them later. The memory has not faded; rather it was never encoded into long-term storage at all.

Blocking occurs when the memory is intact, but we can't access it. We may not have quite the right cues to lead us to the proper information, or we may be retrieving other items that interfere with the retrieval of the desired memory. People are aware of the block, feeling as though they have the memory and yet can't quite reach it: "I know the answer, I just can't think of it right now." Blocking can result in a phenomenon called tip-of-the-tongue state, in which people are able to retrieve parts of the memory, such as the first letter of a word, but not its entirety. Most items are subsequently retrieved soon after, within half a minute or so, but some blocks can last for longer periods of time, preventing retrieval of the item for several days. Many blocks seem to be caused by retrieval of incorrect items. Perhaps you are trying to remember the state capital of Vermont, but Concord, the capital of New Hampshire, keeps springing to mind. "I know Concord's not right," you say to yourself, "but what is Vermont's capital?" Blocking can be one of the more frustrating memory errors.

The prior errors all dealt with missing memories, items that were not encoded or couldn't be retrieved. However, errors in memory can also cause distortions to existing memories. In misattribution, information is correctly remembered, but is associated with an incorrect time, place, or person. For example, your sister tells you a funny story about her weekend, and later you mention some incident from the story to your co-worker, thinking that he had told it to you. You misattributed the source of your memory of the story to your co-worker rather than to your sister. In another type of misattribution, people may not have any specific memory, but rather may experience a general feeling of familiarity for the learned item. This can lead to the "false-fame effect." If subjects study a series of names, then take a test the next day asking them to identify a series of names as either famous or not famous, they tend to call the names that they studied famous. The studied names sound familiar, but the subjects can't remember why, so they assume the name is famous. They err by misattributing their memory. Misattribution can cause misidentification by eyewitnesses in criminal cases, especially when the accused has been encountered somewhere else.

Misleading suggestions can cause people to create entirely false memories. They may believe that they heard or saw something that wasn't there due to questions or statements that strongly suggest its presence. Changing a memory through suggestion is easy. In one study, subjects viewed a video of an auto accident, a small fender-bender. After the video, subjects were asked to estimate how fast the cars were going when they "hit" or "smashed" into each other. Then, they were asked if there was any broken glass on the ground (there was not). Subjects who heard the word "smashed," suggesting a more severe accident, were more likely to claim that there was broken glass present. Beyond simply changing a memory, studies have shown that it is possible to create an entirely new memory for an event that never happened. Over a series of interviews about childhood experiences, researchers insisted to each subject that he or she had been lost in the supermarket as a child (researchers confirmed with the family that this had, in fact, not happened). Many of the subjects came to believe in this memory, and even added details of their own, such as how they felt at the time or embellishments about what happened. Overall, research suggests that a person's confidence in his or her memory has no relation to whether that memory is true or false. Research into suggestibility is often applied to cases of recovered memories of childhood sexual abuse, especially when such memories are "recovered" after strong suggestions from a therapist.

Memories can be biased, distorted as a result of present knowledge and beliefs. People might show selective memory, remembering only the good or only the bad about a particular event or situation, or their memories might be changed to agree with their beliefs. Depressed individuals often show a negative bias in their memories; they find it much easier to recall negative aspects of past events than to come up with positive memories. The biases that emerge in memory can reveal important information about the stereotypes some people hold. For example, subjects who viewed a picture of a white man attacking a black man with a knife tended to later say, incorrectly, that the black man was the attacker and the white man was the victim.

The final type of memory error to be discussed here is persistence. Persistence is not the loss of memory, nor is it the distortion of memory. A person suffering from persistence is doomed to remember events that he or she would prefer to forget. Persistence is often seen in post-traumatic stress disorder. After a traumatic event, such a violent event in the armed services or a rape, people often reexperience their memories of the event while awake and while asleep. Trauma victims seem to lose cognitive control over the retrieval of their trauma-related memories, so that the memories are constantly being pulled into awareness by the slightest cue. Persistence can occur in non-traumatic situations as well. Depressed individuals are often bothered by negative memories that intrude when they are not wanted. In addition, normal subjects can experience a rebound effect; if I tell you not to think of a pink elephant, the pink elephant is bound to spring to mind, and will be difficult to get rid of as long as you try not to think about it.