Short-term memory is defined by the ability to briefly hold small bits of knowledge for instant recall. The length of time afforded to such memory, without premeditation, is generally limited to a few seconds. Alternately known as "active memory," short-term memory stands in contrast to long-term memory, which is characterized by a lengthy—in some cases lifelong—ability to store unlimited amounts of information in the back of one's mind.

Short-term memory is also different from working memory: a longer-lasting, though temporary, retention of data for the purpose of manipulation and imminent use. This sample research paper was written to explain the differences in memory.

The dual-memory model

The concept of a short-term/long-term dual memory model has been analyzed within scientific circles for the last two centuries. Since the 1960s, experts have advanced a theory that memories pass into a long-term storage after a short-term gestation process. The foremost proponent of this theory—known as the "modal model"—is American cognitive science professor Richard Shiffrin (Atkinson). Nonetheless, the theory raises several key questions: 

• What causes memories to move from one mental store to another?
• Is there really a tangible distinction between short-term and long-term memory storage?
• Are all memories stored permanently, or only a fraction?

The separate store theory: support and criticism

Evidence supporting the separate store theory is provided by the condition known as anterograde amnesia, the sufferers of which are almost entirely unable to hold memories for more than a brief number of seconds. The condition indicates that short-term memories are in fact held in a distinct store that is spared of amnesia.

The separate store theory has also been challenged by numerous findings. In a 1974 study published in the journal Cognitive Psychology, subjects were given pairs of words to memorize, but between each pair the subjects had to spend 12 seconds on simple math problems (Bjork and Whitten). Regardless of where a word pair appeared in the sequence, the subjects were just as likely to remember it, which ultimately supported two theories that contradict Shiffrin's model: the recency effect, defined as the likelihood of remembering the final items in sequence; and the primacy effect, marked by a subject's ability to remember the earliest items from a list.

The essence of long-term memory

The last stage of Shiffrin's dual memory model is long-term memory, or the ability to store vast amounts of info over days, months, and years; as opposed to the mere 20–30 second retention of short-term memory. Alternately, long-term memory has also been dubbed "reference memory," because it consists of information that people reference when doing chores that are based on skills and knowledge, such as operating machinery or taking exams. When broken down, long-term memory serves as an umbrella category for a variety of more specific memory concepts, such as episodic memory and implicit memory.

Sleep and long-term memory

According to some theories, sleep serves as an organizer and solidifier of long-term memory (Tarnow). As such, the knowledge contained in a memory can help a person complete a task without being consciously aware of the memory's impact. In a 2012 study conducted by Jessica Payne, Ph.D. of the University of Notre Dame, a random mix of word pairs were given to a range of people divided into two groups. One group received the info at 9am and the other at 9pm, and each group was given three quizzes on the word pairs at intervals of 30 minutes, 12 hours, and 24 hours after receiving the info. Among both groups, the test results were better after periods of sleep had transpired (Payne).

The length of short-term memory

With no effort on the part of a subject to retain a piece of information, a short-term memory will typically expire in under 20 seconds. Evidence of this has been used to support the decay assumption, which asserts that memories quickly evaporate unless a subject proactively retains the info through means of verbal and metal repetition. Essentially, a person must recite and internalize all important info received in order to remember it accurately when needed. Through this process of repetition, the info then passes beyond the short-term store and enters a more permanent place in the mind.

Scientists have yet to determine why vast amounts of info fail to last inside the human mind beyond the short term, but most agree that it limits a person's ability to gain much knowledge over brief spans of time. The term "finite capacity" is often used in reference to the limits of short-term memory.

Quantity versus time: The magic number seven

Measuring memory in terms of quantity rather than seconds, 20th century cognitive psychologist George Miller posited that forward memory accounts for roughly seven items (Miller). The "magical number seven," as it's known in academic circles, has shown to be true when taking the average college student's short-term memory of digits into account; but when it comes to memorizing other pieces of data, the results tend to be more divergent, especially when tested among broader cross sections of people.

The ability to memorize words, for example, can be impacted by numerous factors; test subjects generally find it easier to remember groups of words that are familiar or related to the same topic, but more difficult if the words are lengthier or similar sounding. 

Methods that aid memory retention

In a sense, memory is like strength; both need exercise in order to function at their maximum potential. The following exercises are two of today's most popular methods for keeping information fresh in the mind.

• Rehearsal – This process is used to keep info recurrent in the short-term memory store. With each rehearsal, a piece of info is kept fresh and active within a person's mind for up to 20 seconds. Rehearsal can help a person memorize mundane details—such as the names of other people at a party—as well as boring information that might be needed in order to pass a test.
• Chunking – The process of memorizing things in chunks makes it easier to retain large pieces of info for instant recall. Typically, the short-term memory store accommodates only four units of info at any given time. With chunking, larger pieces of information are broken down into groups, such as the area code (666), prefix (867), and suffix (5309) of a phone number.

While the above methods can help with memory retention, certain factors like age and neurodegenerative diseases like Alzheimer's can seriously impair a person's memory banks. The link between age and memory loss has been a frequent topic of exploration; in a 2012 French study, subpar results were delivered on memory tests in several categories—spatial, verbal, visual—by subjects over age 55 (Fournet et al.)  

Four explanations for why we forget things

In the 21st century, questions still linger as to why the human mind has trouble remembering vital pieces of information. According to American cognitive psychologist and author Elizabeth F. Loftus, forgetfulness is symptomatic of the following four factors (Cherry):

1. Retrieval failure – For most people, the inability to remember key pieces of info at inconvenient times is an all-too familiar situation. The concept known as decay theory—which suggests that each memory is a mental trace, but that all traces fade over time if left unused or un-recited—is used by psychologists to explain why people often fail to retrieve memories. Basically, if a person never talks or even thinks about a piece of info that's entered his or her mind, the info will waste and decay.
   
   
2. Interference – According to the concept of interference theory, the ability to remember one set of information could come into conflict with earlier knowledge if the two pieces of info are similar. The theory accounts for two interference types: proactive and retroactive. The former occurs when an older piece of news (my brother's ex-girlfriend was a Scientologist) makes it difficult to remember a clarification or correction (my brother's ex-girlfriend was an Objectivist; not a Scientologist.) Retroactive interference refers to when newer information (punk was the biggest cultural phenomenon of the '70s) causes people to forget what they had known previously (disco was the biggest cultural phenomenon of the '70s.)
   
   
3. Encoding failure – When exposure to info has been received, but not retained, the problem might not be explained by forgetfulness; it could be a matter of info not being registered in the first place. When info fails to register, it often consists of details that hold little interest to a person, such as specifics about the wall decorations in a frequently visited bank or post office. 
   
   
4. Motivated forgetfulness – With some people, memories of an unpleasant nature are withheld from the mind. For instance, a war veteran might consciously suppress his memory's of mutilated bodies on a battlefield. By refusing to mentally visualize what he saw, or even revisit any of the details in his mind, the whole experience becomes a subject of motivated forgetfulness. A more controversial variant of motivated forgetting involves repression of memory, where the process occurs subconsciously. Psychologists have been split on whether to accept this possibility, because it's impossible to scientifically verify if the denial of memory could ever be an unconscious process. 

Works Cited

Atkinson, Richard C., and Richard M. Shiffrin. "Human memory: A proposed system and its control processes." The psychology of learning and motivation, vol. 8. Eds. K.W. Spence and J.T. Spence. London: Academic Press, 1968. 89–195. Print.

Bjork, Robert A., and William B. Whitten. "Recency-sensitive retrieval processes in long-term free recall." Cognitive Psychology, vol. 6. Ed. G.D. Logan. Amsterdam: Elsevier B.V., 1974. 173–189. Print.

Tarnow, Dr. Eugen. "How Dreams And Memory May Be Related." Neuro-Psychoanalysis. Eds. Edward Nersessian and Mark Solms. Madison, Conn.: International Universities Press, 2003. 177–182. Print.

Payne, Jessica D., Matthew Tucker, Jeffrey Ellenbogen, Erin Wamsley, Matthew P. Walker, Daniel L. Schacter, and Robert A. Stickgold. "Memory for semantically related and unrelated declarative information: the benefit of sleep, the cost of wake". PLOS ONE. Ed. Damian Pattinson. San Francisco: Public Library of Science, 2012. 1–8. Print.

Miller, George. "The Magical Number Seven, Plus or Minus Two: Some Limits on Our Capacity for Processing Information." The Psychological Review, vol. 63. Ed. Theodore M. Newcomb. Washington D.C.: American Psychological Association, 1956. 81–97. Print.

Fournet, Nathalie; Jean-Luc Roulina, Fanny Valleta, Marine Beaudoina, Stefan Agrigoroaeib, Adeline Paignona, Cécile Dantzera, and Olivier Desrichard. "Evaluating short-term and working memory in older adults: French normative data." Aging & Mental Health, vol. 6. London: Taylor & Francis (Routledge), 2012. 922–930. Print.

Cherry, Kendra. "Explanations for Forgetting: Reasons Why We Forget." about education. IAC. n.d. Web. 25 Aug. 2015.