There are several factors that influence the level of professional achievement. First and foremost, extensive experience of activities in a domain is necessary to reach very high levels of performance. Extensive experience in a domain does not, however, invariably lead to expert levels of achievement. When individuals are first introduced to a professional domain after completing their basic training and formal education, they often work as apprentices and are supervised by more-experienced professionals as they accomplish their work-related responsibilities. After months of experience, they typically attain an acceptable level of proficiency, and with longer experience, often years, they are able to work as independent professionals. At that time most professionals reach a stable, average level of performance, and then they maintain this pedestrian level for the rest of their careers. In contrast, some continue to improve and eventually reach the highest levels of professional mastery.
Traditionally, individual differences in the performance of professionals have been explained by an account given by Galton (1869/1979, see Ericsson, 2003a, for a description). According to this view, every healthy person will improve initially through experience, but these improvements are eventually limited by innate factors that cannot be changed through training; hence attainable performance is constrained by one's basic endowments, such as abilities, mental capacities, and innate talents. This general view also explains age-related declines in professional achievement, owing to the inevitable degradation of general capacities and processes with age (see also Krampe & Charness, Chapter 40).