The Easterlin Paradox (Easterlin, 1974) is the observation that within a country, average reported happiness barely rises over decades even while real income per person grows steadily. The leading explanation is the hedonic treadmill: people don't judge income in absolute terms, they judge it against a shifting aspiration level — an internal reference point that itself climbs with their own past income (adaptation) and with what people around them earn (social comparison).
income_i(t+dt) = income_i(t) · (1 + g·dt)
aspiration_i(t+dt) = aspiration_i(t)
+ dt·[ λ·(income_i − aspiration_i)
+ s·(meanIncome − aspiration_i) ]
happiness_i(t) = 50 + 25·tanh( 3·(income_i − aspiration_i) / aspiration_i )
- g — national income growth rate. Every agent's raw income grows at this rate, exactly like real GDP per capita.
- λ (adaptation speed) — how fast an agent's own reference point chases their own income. Higher λ = faster hedonic adaptation = happiness gains fade quicker.
- s (social comparison weight) — how much an agent's aspiration is pulled toward the community's average income ("keeping up with the neighbours"). Higher s means income gains that aren't shared by everyone barely move happiness at all.
- Initial inequality — widens or narrows the starting spread of agent incomes (a lognormal-ish draw), so you can see whether the treadmill effect depends on how unequal the population starts out.
- Bar height tracks each agent's happiness; bar colour tracks how much richer they've become since the simulation started. Drag to pan the grid, scroll/pinch or use the zoom buttons to inspect individual agents, and hover a bar for its exact numbers.
Note on the formula: the reference 3D engine's own written theory omits a ×3 sensitivity constant that its code actually uses inside the tanh — verified numerically (both converge to the same plateau-below-50 signature, the constant only changes how quickly happiness saturates). This 2D engine reproduces the real, running formula (with the ×3) rather than the abbreviated prose version.
Real-world relevance: this is the same mechanism behind Brickman & Campbell's 1971 "hedonic treadmill", and why national well-being surveys (World Happiness Report, GDP-vs-life-satisfaction studies) keep finding that income growth alone explains only a small share of change in reported happiness.