in the 1970s, psychologists began testing the effect of rehearsal on the primacy effect. in 1972, psychologist phillip marshal tested if removing rehearsal would have an affect on the primacy effect. he did this by having the people sort the words while he was giving them a list of 18 common nouns. the people in the test were not told that their memmory was being tested. since they did not know this, they did not repeat the words as they heard them. the experiments showed marshal that removing the repeating also removed most of the primacy effect. it showed that rehearsal was a main cause for the primacy effect.