What do dead rats, dopamine, performance metrics and peacock’s tails have in common? A new ‘Target Article’ in ‘Behavioral and Brain Sciences’ suggests they all, at one time or another, reveals the principle of proxy failure. It might be most pithily summarized as ‘When a measure becomes a target, it can cease to be a good measure’. This principle of proxy failure has received numerous names, including Goodhart’s Law, Campbell’s Law, the Cobra Effect, the unethical optimization principle, and many more. The paper explores how these phenomena relate, and what we can learn from this. Find out more in this interview (https://www.uni-bonn.de/en/news/what-do-peacock2019s-feathers-have-to-do-with-test-scores-in-education) or read the article (https://doi.org/10.1017/S0140525X23002753).