It didn't occur to me either, but ironically it was the first thing my wife suggested when I told her about the marshmallow experiment yesterday (it came up in the context of that professor's comments about fat people, self control, and PhD programs recently). This post's timing was thus quite serendipitous.

It probably occurred to her because she is a doctor who works with primarily poor patients, many of whom are black and hispanic, and so is used to the associated mistrust when crossing cultural and socio-economic lines.