I can speak to this test a bit from experience: as a very young child, I was in a pilot study used to design a large longitudinal study, and my younger sibling

I can speak to this test a bit from experience: as a very young child, I was in a pilot study used to design a large longitudinal study, and my younger sibling was in that large longitudinal study. At about age 4, I ended up literally maxing out the delayed gratification test and being sent home with a ridiculously large bag of M&M's, much to this dismay of my mom. With that as context, I wonder whether some of the changes/lack of reproducibility are actually measures of decreasing economic mobility and economic agency within the US. Early studies on ability to delay gratification were done during the favorable economic conditions baby boomers grew up in. More recent studies were done in eras with far less economic mobility. It's quite likely you'd see a smaller effect today, not because the impact isn't there, but because it's so much harder today to make a significant upward change in your economic status.