The serial position effect is a psychological phenomenon that suggests that people are more likely to remember the first and last items on a list, rather than the middle ones. The term was invented in 1913 by Herman Ebbinghaus, a German psychologist who was interested in testing his ability to remember items on a list (free-recall). To do so he performed free-recall experiments on himself using a list of 2000 made-up syllables. He observed that how well he would remember an item depended on the item's position on the list and is reflected by the serial position curve. The serial position curve reflects two cognitive biases. The first 2-3 items and up to the last 8 items in a list are always better remembered than those in the middle (in lists with 20 or more items). These findings are not influenced by the size and presentation rate (how fast items were presented) of the list. They show a primacy and recency effect.

Free Recall 
Free recall is an experimental method used in psychology to test memory. It is the most common method of studying the serial position effect. It involves presenting participants with a list of items (normally words) and asking them to remember them in any order. However, one major disadvantage is that it is very artificial because experiments are often conducted in labs. So results of free-call experiments may not fully reflect the real world.

Primacy effect 
The tendency to remember the first piece(s) of information given to us is known as the primacy effect. Many psychologists propose that the primacy effect may be a result of rehearsal. Participants may be able to rehearse previous items in the list before being introduced to new ones (repeating the first item in their head by itself, then the first followed by the second when it is presented, then the third after the first two items, and so on). Therefore, items at the start of the list are more likely to be rehearsed than middle and end items. Slower presentation rates allow participants to rehearse a larger number of items, causing a larger primacy effect.

In the 1970s psychologists became interested in testing the role rehearsal played on the primacy effect. In 1972 psychologist Phillip Marshal tested whether eliminating rehearsal would affect the primacy effect. He did this by engaging his participants with a word-sorting task while presenting a list of 18 common nouns. Participants were not asked to learn the nouns in this experimental condition but were still tested on recall accuracy. Being unaware that researchers were going to test the participants’ memory meant that there was no need for rehearsal. Marshal found that eliminating rehearsal also eliminated any significant primacy effect. Which proved that rehearsal accounted for the existence of a primacy effect. 

In 1971, Dewey Rundus developed a technique called overt rehearsal (participants were asked to recite their thoughts out loud), to test how and when participants rehearsed each item in a list. In 1977 Brodie and Murdock, used the overt rehearsal technique to offer an alternate explanation for the primacy effect. They discovered that even as participants got towards the end of the list, they were still rehearsing initial terms. Because of this, the initial items were still present in the participant's short-term memory (STM) towards the end of the list, creating a recency effect. Though Brodie and Murdock acknowledged that rehearsal is essential to the primacy effect, they asserted that the primacy effect may indirectly be a type of recency effect.

Recency effect 
The tendency to remember items towards the end of a list is known as the recency effect. Unlike primacy, the recency effect is not influenced by changes in presentation rates and list length. Instead, psychologists have found that asking participants to list the items in the ‘order they were presented’ decreases the recency effect. Additionally, Glanzer and Kunitz's famous 1966 study found that delaying the retention period (the time between the presentation of the last item and recall test) by just 30 seconds, got rid of the recency effect. Interestingly in 1990, researchers tried to engage participants with a ‘distractor task’ during the retention period. This is a task that suppresses short-term memory (rehearsal), for example counting backward from 3. And results showed that the presence of a distractor task also eliminated the recency effect.

Therefore the recency effect is commonly explained by short-term memory (STM)(also known as primary or working memory). Our STM can store a small amount of information (2-4 items) for a limited time. The final few items of a list being present in our STM during recall may be one reason for the primacy effect. This theory explains why the recency effect is generally larger than the primacy effect. More effort is needed to retrieve initial items from long-term memory (LTM). However, distractor tasks can easily suppress or distract STM, explaining why they eliminate the recency effect.

Newer research, however, shows that the recency effect can exist without STM. For instance, Bjork and Whitten showed that even when STM was engaged there may be a delayed, long-term recency effect. During their study, they added a continuous distractor task to free recall to suppress STM. This meant participants did a distractor task after each item was presented. Because this did not eliminate the recency effect as expected, some psychologists suspected that the recency effect is based on a theory of temporal context (literally meaning the context of time). Although theories in this field are vague and still developing. The general idea remains that a contextual element is attached to each item on the list. During recall, the test’s context will act as a retrieval cue (a prompt that will help us remember). The later the item in the list, the similar the presentation context would be to test context. However, recall is probably dependent on the effectiveness of the retrieval cue.

Link to models of memory 
Earlier research directly related the serial position effect to single store mechanisms such as LTM (primacy) and STM (recency). And research on brain-damaged patients with amnesia (with damaged LTM) who showed a recency effect but not a primacy effect during recall tasks, provides good evidence for this. 

On the other hand, the serial position effect is also supported by dual-store or multi-store models of memory. For instance, research on the serial position effect provides some of the strongest evidence for the multi-store model of memory by Atkinson and Shieffrin. Plus, research around the long-term recency effect may provide evidence for theories of the working memory model.

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