Behaviorism
A person learning an L2 starts off with the habits formed in the L1 and these habits would interfere with the new ones needed for the L2.
Behaviorism was often linked to the Contrastive Analysis Hypothesis (CAH): It predicts that where there are similarities between the L1 and the target language, the learner will acquire target-language structures with ease; where there are differences, the learner will have difficulty