alt Hacker NewsThat depends immensely on the type of effect you're looking for.
Within-subject effects (this happens when one does A, but not when doing B) can be fine with small sample sizes, especially if you can repeat variations on A and B many times. This is pretty common in task-based fMRI. Indeed, I'm not sure why you need >2 participants expect to show that the principle is relatively generalizable.
Between-subject comparisons (type A people have this feature, type B people don't) are the problem because people differ in lots of ways and each contributes one measurement, so you have no real way to control for all that extra variation.
Precisely, and agreed 100%. We need far more within-subject designs.
You would still in general need many subjects to show the same basic within-subject patterns if you want to claim the pattern is "generalizable", in the sense of "may generalize to most people", but, precisely depending on what you are looking at here, and the strength of the effect, of course you may not need nearly as much participants as in strictly between-subject designs.
With the low test-retest reliability of task fMRI, in general, even in adults, this also means that strictly one-off within-subject designs are also not enough, for certain claims. One sort of has to demonstrate that even the within-subject effect is stable too. This may or may not be plausible for certain things, but it really needs to be considered more regularly and explicitly.