The actuation problem asks why a linguistic change occurs in a particular language at a particular time and space. Responses to this problem are multifaceted. This review approaches the problem of actuation through the lens of sound change, examining it from both individual and population perspectives. Linguistic changes ultimately actuate in the form of idiolectal differences. An understanding of language change actuation at the idiolectal level requires an understanding of ( a) how individual speaker-listeners’ different past linguistic experiences and physical, perceptual, cognitive, and social makeups affect the way they process and analyze the primary learning data and ( b) how these factors lead to divergent representations and grammars across speakers-listeners. Population-level incrementation and propagation of linguistic innovation depend not only on the nature of contact between speakers with unique idiolects but also on individuals who have the wherewithal to take advantage of the linguistic innovations they encountered to achieve particular ideological projects at any given moment. Because of the vast number of contingencies that need to be aligned properly, the incrementation and propagation of linguistic innovation are predicted to be rare. Agent-based modeling promises to provide a controlled way to investigate the stochastic nature of language change propagation, but a comprehensive model of linguistic change actuation at the individual level remains elusive.
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