Adaptive Expectations

Adaptive expectations was the dominant model of expectation formation in Keynesian macroeconomics before the rational expectations revolution of the 1970s. Under adaptive expectations, an individual's forecast of, say, next year's inflation is a weighted average of past inflation outcomes, with more recent data receiving higher weight. When actual inflation comes in higher than expected, the individual revises their expectation upward — but only partially and gradually, never jumping immediately to the correct forward-looking forecast.

The adaptive expectations framework played a central role in the development of the original Phillips Curve analysis. Early empirical work appeared to show a stable, exploitable tradeoff between inflation and unemployment: lower unemployment could be achieved at the cost of higher inflation. Under adaptive expectations, this tradeoff could exist because workers formed wage expectations based on past inflation rather than current conditions, meaning that when inflation picked up, real wages fell temporarily, encouraging employment. But the tradeoff was inherently temporary — as workers gradually updated their inflation expectations upward, they demanded higher nominal wages, eliminating the employment gains.

This prediction — that the Philips Curve tradeoff would erode over time as adaptive expectations caught up to actual inflation — was precisely what Milton Friedman and Edmund Phelps independently predicted in the late 1960s, and what the stagflation of the 1970s appeared to confirm. Once workers and businesses had adapted their expectations to the new, higher inflationary environment, the short-run Keynesian demand stimulus could no longer purchase even temporary employment gains.

In financial markets, adaptive expectations manifest in several observable behavioral patterns. Investors and analysts who heavily extrapolate recent earnings trends, recent interest rate levels, or recent price momentum are implicitly using adaptive expectations frameworks. The systematic underreaction to earnings surprises documented in academic literature — in which stock prices gradually drift in the direction of an earnings surprise over subsequent months rather than immediately adjusting — is consistent with adaptive expectation formation among market participants.

Adaptive expectations also helps explain why inflation can become entrenched. If consumers and businesses form inflation expectations adaptively — basing their wage demands and pricing decisions on recent inflation experience rather than central bank targets — then once inflation rises, it can be self-reinforcing. Breaking entrenched adaptive inflation expectations typically requires a period of painful monetary tightening, as the Volcker Fed demonstrated in 1979-1982. For equity investors, monitoring whether inflation expectations are becoming adaptively entrenched or remaining anchored to central bank targets is one of the most consequential macro indicators for assessing the risk of aggressive monetary tightening.