'do' depends on context
I am capable of driving fast, and weaving in and out of traffic on the freeway. But I choose not to, due to the risk/reward trade-off.
If I were in a different situation (e.g. sick child needing to get to hospital urgently), I might act differently.
Measuring my behaviour in one context may tell you little about what I can and might do in another.
>Measuring my behaviour in one context may tell you little about what I can and might do in another.
It may tell you little, or it may tell you a lot. That is why you run a study and see how predictive it is.
If someone drives fast and reckless today, they might not tomorrow. You can still run a study to see how predictive today's driving is for tomorrow. It won't be 100%, but I bet you would see some correlation. Someone who smoked a cigarette today is more likely to smoke tomorrow than someone that who didn't smoke today. That people can quit and can start doesn't change this
The question of why some children can delay gratification is different that what how predictive delayed gratification is of life outcomes.
It can also be true that broken promises are a better prediction of future outcomes.