Concept · Numerical Business Rules
Rule of 55
Origin: US tax code, but used in FIRE planning
Biological Parallel
Many mammals transition from active foraging to stored-resource dependence around 55-60% of maximum lifespan—elephants at age 40 (of 70-year lifespan) shift from continuous foraging to strategic water hole control and knowledge-based resource access. This transition point balances declining physical capacity against accumulated spatial memory and social capital. Too early and you haven't built sufficient reserves (fat, knowledge, alliances); too late and physical decline limits resource access. The 55% threshold emerges where experience-based efficiency compensates for age-related performance decline.