Harlow experimented with monkeys and attachment.

He gave caged monkeys two surrogate mothers

One was wiry and contained food. The other was cloth and provided comfort.

Harlow found that the monkeys spent only 1 hour a day with the wire mother (only when it needed feeding) and the rest of its time was spent with the cloth mother.

This was used as an argument against the Learning Theory- showing that babies attach for reasons other than food. Instead they attach based on comfort and possibly an innate drive.

Harlow later found that when returned to normal life with other monkeys- the young testees never adapted back - evidence of a knock-on effect.

Psychology- Attachment