Yet the range of findings and level of disagreement are themselves hints that there are likely to be factors involved in building expertise that are neither genetic nor related to the amount of practice time.
One is the age at which a person picks up a violin, or a basketball, or a language. People who grow up in bilingual households fully integrate both languages at the same time that language-specialized areas in their brains are developing. The same may be true of many other skills — there may exist a critical window of learning in childhood that primes the brain to pick up skills quickly later on.
Other factors are much easier to control. For instance, scientists have shown that performance itself — that is, testing oneself, from memory — is a particularly strong form of practice. One of the studies that the new review paper includes found that chess masters with similar abilities varied widely in the amount of hours they reported practicing, from 3,000 to more than 25,000.