The reason these tests were inconclusive is likely because young children do not have a concrete conception of time. A young child can understand little difference in meaning between 'ten minutes' and 'ten hours', and so the child will take the marshmallow or whatever else is being offered, rather than wait for a period of time that they cannot fathom.

This changes somewhere between the ages of 5 and 6, depending on the child. TLC ran a special series about childhood last week that talked about this, among many other intriguing facets of development.

Of course, the original concept of this node still holds. It's often quite easy to differentiate between 1- and 2-marshmallow individuals. As a result of experience (and hell, maybe a little genetics thrown in there too), some of us come to see that ten minute wait for the second marshmallow as a positive thing, well others see it as a profoundly negative experience. Sadly, it seems that in the West, this later category, concerned only with instant gratification, has become the dominant one.