“Adding the marshmallow test results to the index does virtually nothing to the prognosis, the study finds.” This does not mean that the test is not predictive

“Adding the marshmallow test results to the index does virtually nothing to the prognosis, the study finds.” This does not mean that the test is not predictive. It means that the index (a bunch of measurements) contains statistical dependencies. From a practical view, the marshmallow test result depends on many cognitive factors unrelated to self-control. The child must understand the instructions, remember them for the duration of the test, trust the provider, value the second marshmallow, and then make a decision. To be of any value, it should have been tested against a standard cognitive battery, which it almost certainly would have failed to improve upon. Cognitive tests have worked extremely well to predict life outcomes for decades now if not centuries.