The study you link to confirms the correlation between the ability to delay gratification in the marshallow test and later success in life. However, it also says that when taking other factors in to account, in particular the wealth of the parents, the marshallow test does not add a significant amount of information any more.

So all we know now is that we have a number of variables that correlate:

- succeeding at the marshmallow test

- having wealthy parents

- be successful in life

Which one of these causes what is open for interpretation. For example, growing up in a stable and reliable environment (which more affluent parents can easier provide) might help to develop long-term thinking, which helps both at the marshmallow test and generally in life.

In any case, the study confirmed that if you don't know anything else about a person, the marshmallow test provides significant information about the probable success later in the life of that person.

>Ultimately, the new study finds limited support for the idea that being able to delay gratification leads to better outcomes. Instead, it suggests that the capacity to hold out for a second marshmallow is shaped in large part by a child’s social and economic background—and, in turn, that that background, not the ability to delay gratification, is what’s behind kids’ long-term success.