A follow-up study found that the children who were able to defer gratification by waiting for the second marshmallow were also later described by their parents as "competent adolescents", and achieved higher SAT scores than those who ate the marshmallow immediately.
Looking at my own family, I'm not sure whether Older Daughter would have waited for the second marshmallow, but I can confidently predict that Younger Daughter would have eaten the first marshmallow, and possibly taken a bite out of the furniture and the returning researcher as well.
There is nothing new under the sun (Ecclesiastes). It turns out that Bronson Alcott, transcendentalist, abolitionist, and the barking-mad father of Louisa May, performed a similar experiment on his own daughters, Anna and Louisa, then aged 5 and 4.
Bronson, knowing that both girls loved apples, left an unguarded apple near Louisa and her older sister, Anna, with the restriction that it belonged to him and the girls were not to eat it. Bronson knew that the girls would be tempted by this literally forbidden fruit, and he felt that the struggle would reveal important information about his daughters. Since Louisa ate the apple, and then unrepentantly stated that she had done so "cause I wanted it," Bronson's intended lesson in self-sacrifice was obviously only half learned. Anna did not eat the apple, and apologized for even thinking about it. Louisa, on the other hand, may have struggled with her will, but in the end she gave in to it, despite her fear of Bronson's displeasure.Which daughter grew up to be a famous and still widely-read novelist?
Adults favor obedient, disciplined children, and it is no surprise that such children are spoken of favorably by their parents and do well on the SATs. But the child who insists on her own point of view may have more to offer the world in the long run.
*Overachiever's footnote: I'm still looking for an authoritative source for the apple story. If you know of one, please let me know. I dimly remember a book or article called something like "Rebellion in the Nursery", about Bronson and Louisa Alcott, but I haven't been able to find it.