is separate from the ability of a learner's brain to absorb language (the "language acquisition device" in Krashen's terminology). That is, these emotional factors act to impede the reception of information. In this way, a learner can receive a great deal of input or instruction, but relatively little of it will "stick" or be retained. Classrooms that promote low anxiety among students or that seek to remove stress from the learning environment are said to encourage "low" affective filters and thus help promote better language retention.

The affective filter hypothesis is one component of the principles of second language acquisition that Stephen Krasher developed in the 1980s. Krashen was concerned with identifying variables that facilitate or hinder acquiring a second language but that are not specific causes of language acquisition. Among these factors are motivation and confidence or the anxiety that can reduce confidence. Those who are highly motivated, for example, are more likely to succeed in second language acquisition. Along the same lines, low self-esteem and high anxiety will serve to reduce the likelihood of, or filter out, the chances of success.

In extreme situations, these filters can combine to create a mental block, which actually impedes acquisition. These factors play important roles but are not to be considered sufficient to assure success. For students experiencing difficulty, efforts to reduce the filter should be employed to help them learn.