Campbell's Law

Biological Parallel

When a measure becomes a target, it ceases to be a good measure. Goodhart's Law applies to biology through surrogate fitness markers. Peacock tail length correlates with genetic quality because growing extravagant tails requires robust health. But this creates selection pressure to fake the signal—males invest disproportionately in tail growth even when malnourished, sacrificing survival for display. The measure (tail length) became the target, breaking its correlation with underlying fitness. Bower bird construction demonstrates measurement corruption at extreme scale. Female bower birds originally selected mates based on bower quality as a proxy for cognitive ability and resource access. But males now invest so heavily in bower decoration—collecting hundreds of colorful objects, arranging them by size and hue—that bower quality no longer reliably indicates other fitness traits. Males neglect foraging and predator vigilance to optimize the measured variable. The proxy detached from what it originally measured. Antibiotic resistance shows Campbell's Law in microbial evolution. Susceptibility testing measures bacterial vulnerability, but when that measure becomes the treatment target, bacteria evolve resistance specifically to the measurement. MRSA evolved resistance to methicillin not because methicillin was the most effective antibiotic, but because susceptibility to it became the treatment criterion. The measurement created selection pressure to game the metric. Campbell's Law emerges from evolutionary dynamics: optimizing for measured proxies generates selection pressure to maximize the proxy independent of underlying quality. When organizations reward degrees over competence, credential mills emerge. When birds reward tail length over health, males grow unsustainable tails. The moment measurement becomes selection criterion, the correlation between measurement and underlying trait begins eroding.