One of the big psychology books mentions this study. Maybe Thinking Fast and Slow?

All it tells you really is whether the person has to grab what joy they can now because their life experience has taught them that promises about tomorrow sometimes do not come. You see that marshmallow, you enjoy it while you can.

And that’s also ignoring the joy of small things. Three marshmallows is as enjoyable as fifty. So now you need to decide if one is enough joy or if you should wait for ten or whatever the reward is.