Really interesting article. Got me pondering the extent to which the peacock’s tail is an example of overfitting and Goodhart's
The female peacock is using the make peacock’s tail as a proxy for fitness - with beautiful consequences, but the males with the largest, showiest tails are clearly less fit, and more prone to predation.
That's the point. That they are alive shows that their innate fitness minus the tail is higher than that of another alive peacock who doesn't have a tail.
Or put another way: someone who wins the Olympic 100m sprint while hopping on one leg is a better runner that everyone else in the race by a wide margin.
The female peacock is using the make peacock’s tail as a proxy for fitness - with beautiful consequences, but the males with the largest, showiest tails are clearly less fit, and more prone to predation.