Why the middle of a list slips away

Read someone a list of words and ask what they remember. The first few and the last few come back; the middle mostly doesn't. A 1962 study mapped the shape.

Try reading a friend twenty unrelated words, once, and ask them to write down as many as they can, in any order. Look at which ones they got. Chances are they'll have several from the end of the list, a few from the very start, and not many from the middle.

Psychologists call this the serial position effect. The term goes back to Hermann Ebbinghaus, the German pioneer who studied memory by testing himself. It has two parts. The advantage of the first items is the primacy effect, and the advantage of the last ones is the recency effect.

Mapping the curve

In 1962 the psychologist Bennet Murdock, at the University of Vermont, measured its shape. He read lists of common English words aloud to 103 students, one word every one or two seconds, in lists of 10 to 40 words. After each list the students had a minute and a half to write down whatever they remembered.

The curves all had the same basic shape. Recall was best for the last words, and the advantage stretched back over roughly the final eight. The first three or four words were also remembered well, though that boost was smaller. In between, recall was low and roughly flat.

A common explanation is that the two ends are remembered for different reasons. Early words get more rehearsal: the first word can be repeated in your head on its own, while each later one has to share that time with all the words before it. The extra repetition helps them settle into long-term memory. The last words have had little rehearsal, but they are still in working memory when recall begins.

Experiments fit that split. A distracting task such as arithmetic, lasting around 15 to 30 seconds before recall, can wipe out the recency effect. Presenting the words faster weakens the primacy effect. And people with amnesia, who struggle to form new long-term memories, show no primacy effect but still remember the last items when tested straight away.

Not every researcher accepts this two-store account, and some models explain both ends with a single mechanism. The curve itself is well established.

So if you want someone to remember a point, the beginning and the end are the places to put it.