[robertk]

What about Salman Khan's TED talk, where he shows that it is a frequent occurrence that kids stumble on a math concept, but then rapidly catch up or exceed their peers. If learning as a general phenomenon occurs in plateaus, but turns out linearly the same in the long run for most everybody (like a graph of the prime counting function seems choppy up close but a line out far), perhaps it is the case we intuitively think some people have a talent, because they didn't experience any early plateaus, but experience some later instead, when they aren't under as careful scrutiny. In other words, it might be a statistical phenomenon of the learning progression (assuming the plateaus are uniformly randomly distributed). Having thought a lot about this issue, I think it is closer to the truth.

In any case, I second the request for more studies.

[sthlm]

What you're talking about is known generically as the General Learning, which is an aptitude in itself. Some people learn better than others. It also depends on what you learn.

Nurture: At various ages, the ability to learn changes (http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.88....). It might well be true that there is a lot of optimization to be done in the way we teach -- something that Salman Khan illustrates well with his new teaching methodologies. That doesn't necessarily mean however that at all stages, ages, circumstances, people will be equally able to learn something.

Nature: Some people have an innate aptitude; I won't look for papers, as I find this to be truly obvious.