Sources of large individual differences in professional achievement are still inadequately understood. Experience in a domain of activity appears to be necessary to perform adequately, but extensive experience does not invariably lead people to attain superior (expert) performance. When individuals are first introduced to a professional domain after completing their training, preparation, or education, they are often confused and must rely on advice from more-experienced colleagues to complete their responsibilities. After months or even years of mentored experience, their independent work reaches an acceptable level. Virtually everyone in a given domain of activity tends to improve with experience during this initial phase. After this point there are, however, considerable individual differences in development. Some professionals continue to improve steadily during many years or even decades, and are eventually recognized by their peers as having attained the highest levels as experts or masters. In contrast, most professionals in a domain reach a stable, average, undistinguished level of achievement within a relatively short time frame and maintain this mediocre level for the rest of their careers.

The nature of these individual differences in attained achievement is still controversial. The most common and traditional explanation is that individuals' achievements in any domain are limited by innate factors that cannot be changed through experience and training; hence, limits of attainable performance are determined by the person's basic endowments, such as physical and anatomical characteristics, mental capacities, and inborn talents.