It may sound appealing that you can become an expert in something with 10,000 hours of practice, but this varies greatly by domain and by person. In the 1970s, researchers William Chase and Herbert Simon discovered that experts have about 100,000 patterns within their domain in their heads, which would take about 10 years to develop. This research was subsequently picked up by psychologist K. Anders Ericsson, who conducted research on excelling in something. According to his research, individual differences in performance can be explained by differences in practice. Subsequently, author Malcolm Gladwell embraced both studies in his book Outliers (2008). He saw some merit in the theory that training is the foundation of genius. He also figured that you have about 4 hours a day on average to practice something like a musical instrument. 10 years x 4 hours a day of practice is roughly ‘10,000 hours of practice to become an expert’. Since then, this has become a widely used rule of thumb. However, this rule of thumb is incorrect. Ericsson writes that Gladwell did not properly understand his original research. The research does not mean that they were simply engaged in repeating a practice for 10,000 hours (‘mediocrity culture’), but were deliberately working to reach a higher level. Often this was even more than 10,000 hours: a top-level violinist often reaches 15,000+ hours. The word 'deliberate' is very important here: a very experienced doctor who has been performing a certain surgery for years will not necessarily be better at it than a medical student who has just graduated. Moreover: people who do not continue to engage in something deliberately can sometimes even become relatively worse at it, as shown by Ericsson's research. It is also important to note that Ericsson strongly believes in nurture (competencies are learned), while other studies also indicate that nature (competencies are innate) can indeed play a determining role. Both the number of hours practiced and the overall malleability of human expertise are thus a myth. Of course, you can certainly get better at something by (deliberately) practicing.
De Bruyckere, P., Kirschner, P. & Hulshof, C. (2019). Juffen zijn toffer dan meesters. Nog meer mythes over leren en onderwijs . Amsterdam, Nederland: Lannoo Campus | Anderz.