The learning theory proposes that attachment is learned through classical conditioning. It suggests that infants become attached to their caregiver because they learn that the caregiver will meet their needs like food. However, there are some weaknesses with this theory. Studies by Shaffer and Emerson found that 39% of babies attached to those who played with them rather than the person who fed them. Research by Harlow also showed that baby monkeys preferred a surrogate mother that comforted them over one that just provided food. This indicates that attachment may not solely be learned through association with being fed.