Fig. 6: Evolution of behavioral motivation.

a The theoretical prediction of the evolutionary direction of behavioral motivations for (b/c) < (b/c)^*. The evolution results in individuals transitioning away from undemanding philanthropists (region II) and demanding aspirationalists (region IV), and towards demanding philanthropists (region I) and undemanding aspirationalists (region III), as indicated by the arrows. This transition leads to a decrease in the abundance of cooperation x_C. The blue (respectively red) regions represent the behavioral motivation that contributes to a larger abundance of cooperation than defection (respectively defection than cooperation). b The Monte Carlo simulations show the evolutionary trajectories of behavioral motivations for (b/c) < (b/c)^* using 300 simulations in random regular networks. Each simulation starts from a monomorphic population with behavioral motivations (α, λ) in one of (0.75,0.05), (0.25,0.05), (0.25,− 0.05), and (0.75,− 0.05), represented by open black dots, and undergoes 5 × 10⁷ motivation updating steps, where interactions and action updates repeat for T = 100 rounds before each motivation updating. During each behavioral motivation update, with a probability of 0.01, the imitated behavioral motivation is subject to a random fluctuation in need α (randomly sampled from the range [− 0.1,0.1]) and motivation intensity λ (randomly sampled from the range [− 0.01,0.01]). Each thin red line represents a resulting trajectory (i.e., the average behavioral motivation of the population), and each open red dot represents an ending behavioral motivation. The thick red line represents the linear regression of all final behavioral motivations, which is highly consistent with the evolutionary direction predicted analytically in (a). c The cooperation abundance throughout the evolutionary process (b/c) < (b/c)^* by simulations. The highlighted line represents the average cooperation abundance over the 300 simulations. d The theoretical prediction of the evolutionary direction of behavioral motivations for (b/c) > (b/c)^* shows that individuals evolve towards undemanding philanthropists (region II) and demanding aspirationalists (region IV). e The evolutionary trajectories of behavioral motivations for (b/c) > (b/c)^* by simulations. f The cooperation abundance throughout the evolutionary process for (b/c) > (b/c)^* by simulations. Parameter values: N = 100, d = 6, which gives (b/c)^* ≈ 6.7, b = 2, c = 1 (bc) and c = 0.2 (ef).