McGeoch & Mcdonald (1931) studied retroactive interference by changing the level of similarity between 2 lists. Participants learned a list of 10 words off by heart. Then they learnt a new list in which the 6 groups learnt a new different type of list; (synonyms, antonyms, unrelated, consonant syllables, 3 digit numbers, & no new list). They found that when they recalled the original list, accuracy depended on the similarity of the 2nd list (synonym=worst recall). This shows interference is strongest when memories are similar.