The Serial Position Effect and the Distinction Between Short-Term and Long-Term Memory

One of the foundational features of the modal model of memory is the postulate that there is a clear distinction between short-term memory (STM) and long-term memory (LTM).
For this model to be considered correct, researchers must be able to demonstrate that these two memory stores are separate and independent from one another.
Some of the most robust evidence for this separation comes from studies on the serial position effect.

Discovery: The serial position effect was identified by researchers Rundus (1971) and Murdock (1962).
Experimental Design:
- Participants were asked to learn lists of words that varied in length from $10$ to $40$ words.
- Each word was presented for a duration of $1 \text{ to } 2 \text{ seconds}$ .
- Examples of list items: Words such as dog, cat, cup, pen, notebook, wall, tree, etc.
- Task: The participants' objective was to remember the words and ultimately recall as many as possible at the conclusion of each trial.
Analytical Results:
- The data is typically represented on a graph where the horizontal axis ( $x$ -axis) shows the serial position of the word in the study list (i.e., its order from first to last).
- The vertical axis ( $y$ -axis) shows the probability of correctly recalling the words.
- The results demonstrate a U-shaped function.
Definition: The serial position effect describes the tendency for the probability of recalling a specific word to depend heavily on its position in the list. Specifically, words at the beginning and the end of the list are recalled more often than words in the middle, which are more likely to be forgotten.
Components: The serial position effect is composed of two distinct phenomena: the primacy effect and the recency effect.

Definition: The primacy effect refers to the superior recall of words presented at the beginning of a list.
Mechanism of Action: The primacy effect is believed to stem from the opportunity for rehearsal.
- Rehearsal is the primary vehicle that allows information to be transferred from short-term memory to long-term memory.
- The number of opportunities to rehearse a word declines from the start of the list to the end.
Rehearsal Example: If a list begins with "cat, dog, tree," a participant might mentally repeat the words as follows:
- "Cat, cat, cat, cat…"
- "Cat, dog, cat, dog, cat, dog…"
- "Cat, dog, tree, cat, dog, tree…"
At a certain point, the participant can no longer keep up with the incoming words, but the early items have already received sufficient rehearsal to be encoded into long-term storage.
Theoretical Implication: The primacy effect is thought to represent the retrieval of information held in long-term memory.

Definition: The recency effect refers to the superior recall of words presented at the very end of a list.
Mechanism of Action: Unlike the primacy effect, items at the end of the list have not had the chance to be rehearsed or transferred to long-term memory.
The Store: These items are thought to still reside in sensory memory or short-term memory at the moment recall begins.
Capacity and Function:
- Short-term memory is a very brief store with a limited capacity.
- The capacity of short-term memory is approximately $7 \text{ items}$ .
- Consistent with this theory, the recency effect typically affects approximately the last $7 \text{ items}$ of a list.
Theoretical Implication: The recency effect relies primarily on the short-term memory store.

Research by Glanzer and Kenyans (1966) provided further evidence that primacy and recency rely on different stores by showing that the two effects can be manipulated independently.
Experimental Variables: Participants were asked to delay their recall after the list was presented by specific intervals:
- $0 \text{ seconds}$ (no delay)
- $10 \text{ second delay}$
- $30 \text{ second delay}$
Impact on Primacy Effect: The primacy effect remained relatively constant regardless of the delay. Because this information is stored in long-term memory, which can potentially last forever, a $30 \text{ second}$ wait does not significantly hinder recall.
Impact on Recency Effect: The delay had a profound effect on the recency effect:
- With no delay, the recency effect was strong, showing much better recall for the last words compared to intermediate words.
- As the delay increased, the magnitude of the recency effect decreased.
- By the time a $30 \text{ second delay}$ was introduced, the recency effect disappeared entirely. Words at the end of the list were remembered no better than words in the middle.
Conclusion: This occurs because short-term memory has a short duration. A $30 \text{ second}$ window is long enough for information in sensory or short-term memory to decay. This dissociation proves the recency effect relies on a temporary store while the primacy effect does not.

Conversely, there are factors that influence the primacy effect while leaving the recency effect unaffected.
Variable: The speed of presentation (the rate at which words are read to the participant).
Effect of Rapid Presentation: The faster the words are presented, the smaller the magnitude of the primacy effect.
Explanation: When words are presented rapidly, there is less time for the participant to rehearse each word before the next one is introduced.
Conclusion: Fewer words are rehearsed sufficiently to be transferred into long-term memory. Since this reduces the primacy effect but does not affect the items currently sitting in the short-term store at the end of the list, it further distinguishes the two memory systems.