There are many many examples of this. For example, if you give girls math tests and tell them your purpose in giving them the test is to check for gender differences in math performance, the girls do significantly worse than if you tell them nothing.
And it's not just intelligence - this is why sports psychologists can make a living coaching athletes.
Of course when you carry out research like this at school level, you can assume that the material is not impossible to master even for a child of average intelligence. I don't know whether these results hold at the world-class researcher level. Still, the school years provide an important foundation for later life, so it is proper that people look into what affects performance.
And it's not just intelligence - this is why sports psychologists can make a living coaching athletes.
Of course when you carry out research like this at school level, you can assume that the material is not impossible to master even for a child of average intelligence. I don't know whether these results hold at the world-class researcher level. Still, the school years provide an important foundation for later life, so it is proper that people look into what affects performance.