[miguelaeh]

There is some genetic part, but I would say this also highly depends on the environment where you grow up. From my observation, people who grow up in villages are smarter than people who grow up in the city. They just know more about the world, the nature, the have been more "free" as it was safer for them to explore the environment. As you grow, being considered smart is usually more related to business, and I would say it is actually a matter of how good you are predicting the future. We all can learn the same things, just putting in more or less time, but what is more complex to change is the internal process you follow to reach a conclusion, and that's basically based on your previous experiences. So, the more things you live, the more things you interact with, the more technologies you learnt, the more books you read, etc is what makes you "smarter", basically because your internal thinking process has more information to contrast.