This paper sets out to peruse the role of various theories or more precisely, hypotheses invoked in SLA research by surveying three empirical studies pertaining to the critical period hypothesis in the SLA of phonetics and phonology. In particular, the three studies which will be reviewed in the next section are titled in chronological order as (1) A critical period for learning to pronounce foreign languages? (Flege, 1987); (2) Reexamining the critical period hypothesis: A case study of successful adult SLA in a naturalistic environment. (Ioup et al., 1994); (3) Ultimate attainment in L2 Phonology: The Critical Factors of Age, Motivation, and Instruction (Moyer, 1999).

Theory plays a central role in the development of human knowledge. In essence, theory solves puzzles, or questions about observable phenomena that need to be answered, (Kuhn, 1996). Theorizing about solutions to these puzzles requires working at the edge of uncertainty, making bold postulations, and engaging in what renowned philosopher of science Karl Popper terms critical rationalism; it is through the development of theories that are falsifiable and the subsequent empirical testing of those theories that our knowledge about the world (i.e., of natural phenomena) will progress (Popper, 1968).