This study is from over a year ago, but I came across it only yesterday. Here's an excerpt from The Marshmallow Study revisited: Delaying gratification depends as much on nurture as on nature.
For the past four decades, the "marshmallow test" has served as a classic experimental measure of children's self-control: will a preschooler eat one of the fluffy white confections now or hold out for two later?
Now a new study demonstrates that being able to delay gratification is influenced as much by the environment as by innate ability. Children who experienced reliable interactions immediately before the marshmallow task waited on average four times longer—12 versus three minutes—than youngsters in similar but unreliable situations. [...]
Children who experienced unreliable interactions with an experimenter waited for a mean time of three minutes and two seconds on the subsequent marshmallow task, while youngsters who experienced reliable interactions held out for 12 minutes and two seconds. Only one of the 14 children in the unreliable group waited the full 15 minutes, compared to nine children in the reliable condition.
Michael Bourne's meditation on this study alerts us to the possibility that the way the original study was presented to the public was essentially an appeal to our own tendency towards instant gratification: we are all suckers for simple stories that gel with our own worldview.