Recall is remembering without any external cues. For example, essay questions test recall of knowledge because nothing on a blank sheet of paper will jog the memory.

Recognition is identifying learned information using external cues. For example, true or false questions and multiple-choice questions test recognition because the previously learned information is there on the page, along with other options. In general, recognition is easier than recall.

When using the relearning method to measure retention, a researcher might ask a subject to memorize a long grocery list. She might measure how long he has to practice before he remembers every item. Suppose it takes him ten minutes. On another day, she gives him the same list again and measures how much time he takes to relearn the list. Suppose he now learns it in five minutes. He has saved five minutes of learning time, or 50 percent of the original time it took him to learn it. His savings score of 50 percent indicates that he retained 50 percent of the information he learned the first time.

The way information is encoded affects the ability to remember it. Processing information at a deeper level makes it harder to forget. If a student thinks about the meaning of the concepts in her textbook rather than just reading them, she’ll remember them better when the final exam comes around. If the information is not encoded properly—such as if the student simply skims over the textbook while paying more attention to the TV—it is more likely to be forgotten.

According to decay theory, memory fades with time. Decay explains the loss of memories from sensory and short-term memory. However, loss of long-term memories does not seem to depend on how much time has gone by since the information was learned. People might easily remember their first day in junior high school but completely forget what they learned in class last Tuesday.

Interference theory has a better account of why people lose long-term memories. According to this theory, people forget information because of interference from other learned information. There are two types of interference: retroactive and proactive.

Forgetting may also result from failure to retrieve information in memory, such as if the wrong sort of retrieval cue is used. For example, Dan may not be able to remember the name of his fifth-grade teacher. However, the teacher’s name might suddenly pop into Dan’s head if he visits his old grade school and sees his fifth-grade classroom. The classroom would then be acting as a context cue for retrieving the memory of his teacher’s name.

Psychologist Sigmund Freud proposed that people forget because they push unpleasant or intolerable thoughts and feelings deep into their unconscious. He called this phenomenon repression. The idea that people forget things they don’t want to remember is also called motivated forgetting or psychogenic amnesia.

Anterograde amnesia is the inability to remember events that occur after an injury or traumatic event. Retrograde amnesia is the inability to remember events that occurred before an injury or traumatic event.