The marshmallow test is not really testing hunger or self control. It tests how willing people are to align with authority/the bigger picture.

The ideal participant isn't someone doing the calculus that 2 > 1. It's someone who recognizes that they are being tested, and cares about that more than any number of marshmallows.

The question isn't "how hungry am I?", but "what does adult attention mean to me?".

And that's why all of this stuff will stop replicating eventually, why new psychotherapies revert to the mean - it doesn't have the same amount of meaning for the test-givers after decades of trials.