Oh, I suspect there are lots of variations. Is there a "development" of delayed gratification (do we learn to put it off)? Are there thresholds of difference -- one marshmallow (or M&M) now versus how many and how long? How much now and how much later? Does it make a difference if we have just eaten or not (satiation)? Room for tons of follow-up studies.

I wonder if anyone has tried this recently in a really controlled experiment? Be interesting to compare with the older studies and see whether kids are really getting less able to postpone or not. I know what everyone says, but I prefer measurements :-)