Those, who believe [that people would not achieve anything without rewards and punishments] (...), have made a number of mistaken assumptions.

For starters, they've assumed that people who were raised to believe they are basically competent, have no reason to accomplish anything.

I once heard someone defend that belief by declaring that "human nature is to do as little as necessary".

This prejudice is refuted not just by a few studies, but by the entire branch of psychology dealing with motivation.

Normally, it’s hard to stop happy, satisfied people from trying to learn more about themselves and the world, or from trying to do a job of which they can feel proud.

– Alfie Kohn, "Unconditional Parenting: Moving from Rewards and Punishments to Love and Reason", https://www.goodreads.com/quotes/10061757-for-starters-they-ve-assumed-that-people-who-are-raised-to