When the Curtain Opens: The Threshold Crossover from Economic Stratification to Shared Fate

Economic stratification organizes daily life but collapses under sufficiently severe shared threats. We propose a stratification–threat function S(τ) in which behavioral stratification first amplifies under moderate threat and then collapses above a critical threshold τ*, modulated by three variables: threat immediacy, escape excludability, and shared-fate visibility. We test this framework in an exploratory computational experiment: 18 autonomous LLM agents divided into three equal-sized economic classes face an escalating shared threat over 100 decision cycles across ten experimental conditions totaling 18,000 agent decisions. Lower-class cooperation is significantly lower than upper- and middle-class cooperation (64.3% vs. 95.0% and 97.3%; Cohen's d = 1.33, p < 0.0001), not from selfishness but because contributing threatens their survival. A random-action baseline confirms that 20.5 percentage points of the class gap are structural, while LLM reasoning widens it by an additional 10.2 points. Inequality deepens under shared threat (Gini: 0.470 → 0.563, d = 4.12, p < 0.0001), and when threat recedes, lower-class cooperation collapses to 30.8% while upper-class cooperation remains at 72.5%. These results suggest that the capacity for collective action is itself stratified by wealth, with implications for disaster policy, inequality research, and multi-agent AI safety.