Predict how people respond to incentives, including the unintended consequences of well-meant rules.
People respond to incentives — rewards pull behavior, penalties push it. This is the closest thing economics has to a law of physics. Prices are incentives (high prices summon producers and repel buyers), wages are incentives, grades, fines, likes, discounts — all levers. When you want to predict behavior, find the incentives and you're most of the way there.
The catch: people respond to the incentive you BUILT, not the one you MEANT. Pay per rat tail delivered and people farm rats (this actually happened in Hanoi). Reward hours worked and you get slow work. Every rule is a lever, and the economist's habit is asking "what does this ACTUALLY reward — and what will clever people do about it?"
Mastery looks like: They can take any rule and articulate the behavior it actually rewards, including the exploit.
Common stumbles: Assuming people follow intentions rather than incentives; calling exploiters "cheaters" instead of fixing the lever.