Why does a phone number you just heard slip away so fast?
Forgetting a number seconds after hearing it isn't bad memory. From Miller's magic seven to Cowan's four chunks, here is why.
You repeat a phone number under your breath so you can call to book a table. Then someone asks you a question, and the whole number is gone. This happens to everyone, and it isn’t a sign of a bad memory. The space where we briefly hold information is simply very small.
Cognitive scientists call that space working memory.
Gone in seconds without rehearsal
In 1959 the American psychologists Lloyd and Margaret Peterson ran a simple experiment. Participants heard three consonants and were immediately asked to count backward by threes, which kept them from rehearsing. When asked to recall the consonants after a few seconds, accuracy fell quickly as the delay grew, and after about 18 seconds most of the letters were gone.
The phone number vanishing when someone speaks to you works the same way. The moment silent rehearsal stops, the information scatters.
Seven, or four?
So how many items can we hold at once? In 1956 George Miller gathered experimental results into a famous paper arguing that people can handle about seven items at a time, “the magical number seven, plus or minus two.”
In 2001 Nelson Cowan revisited that number. When rehearsal and grouping tricks are blocked, Cowan concluded, true capacity is closer to four chunks. We feel we can hold about seven because we are already grouping items in our heads.
Grouping widens the door
The key is the chunk. Remember 4155550132 digit by digit and it is ten items; break it into 415-555-0132 and it becomes three. If the area code is one you already know, only two new chunks remain.
The habit of writing phone numbers with hyphens and card numbers in blocks of four are both devices that help information through the narrow door of working memory.
The mental workbench
In 1974 Alan Baddeley and Graham Hitch described working memory not as a single box but as a workbench with parts: a phonological loop that briefly holds speech sounds, a visuospatial sketchpad for location and shape, and a central executive that divides attention between them. Rehearsing a number is the phonological loop’s job, and the voice of someone talking to you competes for the same loop, pushing the number out.
What the brain is doing
In monkey studies, neurons in the prefrontal cortex kept firing for seconds after a stimulus disappeared. Researchers have long studied this delay-period activity as a cellular trace of working memory. Holding information in mind comes down to particular neural circuits keeping their activity going.
In short
A number that slips away is not a malfunction but a design feature. Working memory is small to begin with and empties quickly when rehearsal stops. What we do instead is group information into chunks so the same space can carry more. Next time you jot down a number, count how many chunks you are putting on your mental workbench.
Sources
- Short-term retention of individual verbal items. Peterson LR, Peterson MJ. Journal of Experimental Psychology 58, 193–198 (1959)
- The magical number seven, plus or minus two: some limits on our capacity for processing information. Miller GA. Psychological Review 63, 81–97 (1956)
- The magical number 4 in short-term memory: a reconsideration of mental storage capacity. Cowan N. Behavioral and Brain Sciences 24, 87–114 (2001)
- Working memory. Baddeley AD, Hitch G. Psychology of Learning and Motivation 8, 47–89 (1974)
- Cellular basis of working memory. Goldman-Rakic PS. Neuron 14, 477–485 (1995)
