Peterson and Peterson (1959) tested how long short-term memory lasts without rehearsal. Twenty-four participants recalled trigrams after counting backwards for delays of 3 to 18 seconds, which blocked rehearsal. Recall fell from 80% to under 10% as the delay grew, so memory fades within about 18 seconds unrehearsed.
Aim
To investigate the duration of short-term memory.
Lloyd and Margaret Peterson aimed to test the hypothesis that information that is not rehearsed is lost quickly from short-term memory.
Procedure
A lab experiment was conducted in which 24 participants (psychology students) had to recall trigrams (meaningless three-consonant syllables), such as TGH, CLS.
The trigrams were presented one at a time. Each had to be recalled after a retention interval of 3, 6, 9, 12, 15 or 18 seconds.
No two successive trigrams contained any of the same letters.
After hearing a trigram, participants counted backward in threes or fours from a random number. They kept counting until a red light appeared, then recalled the trigram.
This is known as the Brown Peterson technique, which aimed to prevent rehearsal.
The independent variable was the retention interval: the time between hearing the trigram and recalling it after the red light appeared. This ranged from 3 to 18 seconds across trials.
The dependent variable was the number of trigrams correctly recalled by the participants after every trial. There were six trials in total.
Findings
Recall accuracy fell as the retention interval grew longer.
- After 3 seconds 80% of the trigrams were recalled correctly.
- After 6 seconds this fell to 50%.
- After 18 seconds less than 10% of the trigrams were recalled correctly.
Conclusion
Short-term memory has a limited duration of about 18 seconds when rehearsal is prevented. Peterson and Peterson attributed this loss to trace decay.
Short-term memory is therefore shorter-lived than long-term memory, supporting the multi-store model of memory. Information that cannot be rehearsed fails to transfer into long-term memory.
Critical Evaluation
The Peterson and Peterson study has clear strengths and important limitations, summarized below.
Strengths:
- Experimental Control: Fixed retention intervals and standardized procedures, with noise and other distractions eliminated, gave every participant the same experience.
- Replicability: Because the procedure is standardized, the study can be repeated to check whether the findings are a one-off result.
Limitations:
- Ecological Validity: The experimental method lacked mundane realism and external validity, since people rarely try to recall meaningless trigrams in everyday life.
- Material Specificity: The study only tested short-term memory for consonant trigrams, so it says nothing about how other material, such as pictures or melodies, is retained.
- Small Sample: Only 24 participants took part, so the size and consistency of the effect across ages and populations remain uncertain.
- Interference, Not Just Decay: Keppel and Underwood (1962) later showed that much of this forgetting reflected interference from earlier trials, not decay alone.
One important practical application of this study is that it demonstrates how interference in the form of verbal distractions can affect our ability to retain information. This implies that when revising for an exam, or memorizing a shopping list, we should take care to avoid distractions.
Brown’s Original Decay Study (1958)
Aim: John Brown tested whether a trace in immediate memory decays within seconds unless rehearsal renews it.
Method: Participants tried to remember small strings of consonants, then read a list of digits aloud for a short interval before recalling the letters.
Results: Recall fell sharply across only a few seconds whenever the reading task filled the interval, but was almost perfect when no distractor task was given.
Conclusion: Brown concluded that an unrehearsed trace decays rapidly, and that this decay, not the simple passage of time alone, explains the collapse in recall.
The Proactive-Interference Challenge: Keppel and Underwood (1962)
Aim: Keppel and Underwood asked whether Peterson and Peterson’s forgetting curve reflected decay of a single trigram, or interference building up from earlier trials in the same session.
Method: They re-ran a Brown-Peterson-style task and analyzed recall by trial position, comparing the very first trial to later trials at the same retention intervals.
Results: Recall on the first trial was almost perfect at every interval, including 18 seconds. The classic steep forgetting curve only appeared from the second trial onwards.
Conclusion: Because interference from earlier trigrams, not time alone, explained most of the forgetting, psychologists now treat proactive interference as at least as important as decay in this paradigm.
Contemporary Research
Aim: Ricker, Vergauwe and Cowan (2016) reassessed six decades of evidence descended from Brown (1958) to ask whether time-based decay contributes to forgetting in immediate memory at all.
Method: They reviewed three kinds of evidence: distractor tasks that also block attention, not just rehearsal; designs that vary the retention interval while holding the number of intervening items constant; and complex-span tasks that manipulate time and memory load separately.
Results: Across these more tightly controlled designs, memory still weakened with time even after interference was accounted for, though the effect was smaller than Peterson and Peterson’s original curve suggested.
Conclusion: Ricker and colleagues argue that both decay and interference contribute to short-term forgetting, reopening a debate that many textbooks had treated as settled in favor of interference alone.
References
Brown, J. (1958). Some tests of the decay theory of immediate memory. Quarterly Journal of Experimental Psychology, 10(1), 12-21. https://doi.org/10.1080/17470215808416249
Keppel, G., & Underwood, B. J. (1962). Proactive inhibition in short-term retention of single items. Journal of Verbal Learning and Verbal Behavior, 1(3), 153-161. https://doi.org/10.1016/S0022-5371(62)80023-1
Peterson, L. R., & Peterson, M. J. (1959). Short-term retention of individual verbal items. Journal of Experimental Psychology, 58(3), 193-198. https://doi.org/10.1037/h0049234
Ricker, T. J., Vergauwe, E., & Cowan, N. (2016). Decay theory of immediate memory: From Brown (1958) to today (2014). Quarterly Journal of Experimental Psychology, 69(10), 1969-1995. https://doi.org/10.1080/17470218.2014.914546
FAQs
What is the Brown Peterson task method?
The Brown Peterson task is a cognitive psychology experimental method used to study the duration of short-term memory. Participants are presented with a series of items to remember, and then engaged in a distracting task to prevent rehearsal. They are later tested on their ability to recall the original items.
What was the dependent variable in Peterson and Peterson’s 1959 study on short-term memory?
The dependent variable in Peterson and Peterson’s 1959 study on short-term memory was the participants’ ability to recall and correctly reproduce the trigram (three-letter nonsense syllables) after a specified delay interval. The researchers measured the accuracy of recall to assess the duration of short-term memory.
What was the independent variable in Peterson and Peterson’s 1959 study on short-term memory?
The independent variable was the length of the delay interval between the presentation of the trigram (three-letter nonsense syllables) and the recall task. Different delay intervals, such as 3, 6, 9, 12, 15, or 18 seconds, were used to examine the effect of time on the decay of information in short-term memory.
