This thesis analysed whether functional implicit learning differences existed in two areas that have produced promising, but equivocal, findings: individual differences in typical populations (e.g., Gebauer & Mackintosh, 2010) and group differences between Autism Spectrum Condition (ASC) and Typically Developing (TD) individuals (e.g., L. G. Klinger, Klinger, & Pohlig, 2007). Overall, the results from the four studies presented in this thesis emphasised a lack of functional differences in implicit learning between individuals. Study I investigated whether there were functional individual differences in implicit learning among a typical population by examining the inter-correlation between the performances of academic psychologists on three implicit learning tasks; the independence of those performances from IQ; the relationships between those performances, intuitive aspects of personality and occupational tacit knowledge; and, finally, whether the performances were related to occupational achievement. There was no evidence of inter-correlation between the implicit learning task performances, nor relationships between any of those performances, and occupational achievement, or personality. The study did replicate a finding that is important to the distinction between implicit and explicit learning: indices of explicit processing, but not performance on implicit learning tasks, were correlated with IQ (e.g., Gebauer & Mackintosh, 2007). Additionally, the study found that Academic Psychology and Business Management Tacit Knowledge Inventories measured knowledge that predicted occupational achievement in academic psychology incrementally to IQ and personality, and was general to both occupations. However, tacit knowledge appeared to be acquired primarily as a function of practice and experience, rather than individual differences in implicit learning. Overall, I asserted that a consideration of the results from Study I with the wider literature currently leads to the conclusion that there are minimal individual differences in implicit learning, which signifies that there is no general implicit learning ability that is critical to how much is learnt implicitly. In the absence of a general ability that determines how much is learnt implicitly, it was argued that there could still be general, prerequisite processes, which are always necessary for implicit learning but without those processes determining the variation in how much was learnt implicitly. Such prerequisite processes would not constitute a psychometric ability but could be SUMMARY vi conceptualised as general implicit learning processes. This conceptualisation of implicit learning would be supported by the existence of an atypical population who consistently demonstrated profound deficits on all implicit learning tasks and skills associated with an implicit acquisition. There is no convincing evidence of such a patient group, although the ASC population is a plausible candidate (e.g., L. G. Klinger, et al., 2007). Therefore, Study II compared IQ-matched ASC and TD individuals on a range of implicit learning tasks. The study, taken together with other recent reports (e.g., Barnes, et al., 2008), provided convincing evidence that implicit learning is actually intact in ASC and it was argued that deficits reported in previous studies must have resulted from differences in task procedures (e.g., L. G. Klinger, et al., 2007). In particular, the earlier studies used procedures that encouraged explicit strategies, which disadvantaged the ASC groups who had not been matched for IQ. A further analysis supported that interpretation: TD and ASC groups who were not matched for IQ exhibited differences on an explicit learning task, but not on the implicit learning tasks. In order to determine whether those previously identified implicit learning deficits in ASC resulted just from differences in IQ, or whether there was also a contribution from an ASC difficulty in explicit learning, Study III compared ASC individuals with IQ-matched TD individuals on an implicit learning task, the Serial Reaction Time (SRT) task, with a procedure that encouraged explicit strategies. The SRT procedure was combined with a contextual cueing task that provided an indirect, ongoing index of the extent to which sequence learning was explicit (Jiménez & Vázquez, in press). Study III indicated a difference in initial explicit sequence learning in ASC, which was independent of IQ. Study IV replicated the difficulty and by using a pre-task manipulation the study was also able to elaborate the nature of that difficulty: ASC individuals were able to learn sequence information explicitly, but they had a specific difficulty with learning to apply that explicit information. Thus, there was good evidence that implicit learning is intact in ASC and that instead ASC individuals have more difficulties with aspects of explicit learning. These findings refute the idea that ASC individuals successfully compensate for implicit deficits with explicit compensatory strategies. Instead, together with the ASC propensity for using explicit strategies, an ASC difficulty with explicit processing might explain some ASC deficits in a range of learnt skills, although I acknowledge that there are also plausible alternatives. More generally, these SUMMARY vii findings and ideas accord with ASC literature concerning impairments in executive functions, which require flexible and intentional processing (e.g., Russell, 1997a) and emphasise that future research is focused on how explicit, executive differences emerge and affect behaviour. In conclusion, the thesis provided no evidence for the proposal that there are functional differences between individuals in implicit learning. I propose that, taken together with the equivocal evidence discussed in my reviews of the wider literature, it is parsimonious to conclude that there is neither a general implicit learning ability, nor general, prerequisite implicit learning processes. However, in line with previous literature, the thesis did support functional distinctions between implicit and explicit learning: explicit, but not implicit, learning was related to IQ; and ASC individuals have difficulties with explicit but not implicit learning. Therefore, I assert that a descriptive distinction between explicit and implicit learning remains both useful and valid. This is true even though implicit learning seems to be defined by the absence, or minimal influence, of explicit processing rather than the general presence of an implicit learning ability or processes. Beyond the issue of functional differences, I argue that these findings and conclusions make modest, but not decisive, contributions to some of the other fierce debates in the wider implicit learning literature. Finally, I propose some recommendations, and directions, for future research.