I don't get this cartoon; please explain.

The cartoonist should have used a circus peanut instead.

They won’t even tell me if I was in that study, but I distinctly remember marshmallows somewhere in my past.

I remember believing marshmallows grew on a plant. I didn’t know how to account for the varying sizes.

I always feel like key personality info comes from these questions:

Do you want to hear the good news first or the bad news first?

Do you eat your favorite thing on the plate first, or your least favorite?

I remember reading something about the marshmallow test being a better indicator of whether the kids thought adults were trustworthy. Or, in other words, if kids believed that the adult would show up with 2 marshmallows, they were willing to wait; if they were used to grownups breaking promises all the time and thought that there was no second marshmallow on the way, well, there was no good reason to wait.
It does seems likely that children who have reliable adults in their corners could likely do better in school than children with flaky ones.