This paper analyses recently discussed evidence of over-imitation in Acheulean biface construction. First, it evaluates the argument for over-imitation using the available archaeological and cognitive science evidence. Next, it applies the four major theories of over-imitation, (1) Copy and Correct (C&C), (2) Automatic Causal Encoding (ACE), (3) social affiliation and (4) normative theory, as potential explanations for Acheulean over-imitation. ACE theory is the most likely explanation for early biface over-imitation (before 500,000 years bp), with social affiliation becoming increasingly likely after that. Normative over-imitation probably did not occur until around 300,000 years bp, when both the necessary hominin cognitive capacities and social conditions were present. An important conclusion emerging from this analysis is that over-imitation requires an integration of social and technical intelligence. Thus, the origins of cognitive fluidity may date back to as early as a million years ago, well before material evidence of fluidity is present.