Chapter II

Second Language Acquisition Theory

This chapter summarizes current second language acquisition theory. To do this, it first describes some very important hypotheses. The first three, the acquisition-learning distinction, the natural order hypotheses, and the Monitor hypothesis, are reviewed somewhat briefly, as they have been dealt with a great deal in several other books and professional papers. Enough detail will be provided, however, to give the uninitiated reader a good idea of the hypotheses and the sort of evidence that exists to support them. The fourth hypothesis, the input hypothesis, may be the single most important concept in second language acquisition theory today. It is important because it attempts to answer the crucial theoretical question of how we acquire language. It is also important because it may hold the answer to many of our everyday problems in second language instruction at all levels. Following the discussion of the input hypothesis, we turn to the concept of the affective filter, a hypothesis as to how affective variables relate to the process of second language acquisition.

The second portion of this chapter reviews a variety of factors that have been thought to be related to second language acquisition success, including instruction, different measures of exposure to the second language, and the age of the acquirer. These factors, it will be claimed, are not really causative factors. While they seem to relate to success or failure to acquire second languages, the true causative variables in second language acquisition derive from the input hypothesis and the affective filter--the amount of comprehensible input the acquirer receives and understands, and the strength of the affective filter, or the degree to which the acquirer is "open" to the input.

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