In the famous marshmallow test, a young child can eat one treat now or wait and get two. A study long ago found that kids who waited did better when they grew up. People turned this into a simple story: if you have willpower as a kid, you will succeed in life.
In 2018, scientists tried the test again with about 900 children, many more than before. This time the link between waiting and doing well later was much weaker. When they took each child's family and home into account, most of the link faded away.
It turns out that waiting may depend a lot on a child's home, not just willpower. A child who trusts that the second treat will really come may find it easier to wait. This shows why two things happening together does not mean one caused the other.