What makes the stock market move over the long term? While stocks have historically delivered positive returns year-over-year on average, it is not clear why stock prices rise more rapidly in one period than in any other.

With my colleagues, Martin Lettau of the U.C. Berkeley Haas School of Business and Sydney Ludvigson of New York University, I set out to investigate what makes stocks move over time. What we found was surprising.

Despite the widespread belief that firm productivity is a key driver of stock market returns, our results indicate that fluctuations in productivity play only a small role. Far more influential over long periods is the economic redistribution between workers and shareholders — meaning how a company’s profits are divided between employees and investors.

Our first step in this research was to consider which factors might be responsible for movement in the stock market in aggregate. Each firm that is represented in the stock market index produces a stream of revenues. After paying a portion to workers, the rest is left over as profits that can be distributed to shareholders as dividends. The stock price will rise whenever the rewards to the shareholders increase, which can be caused by one of three separate forces:

Productivity: The firm becomes more productive, increasing its stream of revenues. This increases the size of both slices, including the shareholders’ slice. Redistribution: The size of the pie remains fixed, but the firm pays a smaller share to the workers, increasing the shareholders’ slice. Market confidence: Neither the size nor the division of the pie changes, but more risk-tolerant investors demand more stock despite there being no change in their current dividends.

Combining theoretical analysis with statistical estimation on macroeconomic and stock market data, we were able to determine the relative strengths of these three forces.

Here’s what we found: At short-term horizons of one-quarter to several years, market confidence shocks are dominant, explaining nearly all fluctuations. Essentially, short-run swings in stock prices are simply out of proportion to the movements in the underlying cash flows, ruling out the other two explanations.

But at longer horizons of a decade or more, we found that redistributions between workers and shareholders play an increasingly important role, explaining half of the variation in overall stock prices at a horizon of 25 years.

These redistributive shifts, taken individually, are quite small, and are not major factors in explaining quarterly returns. But unlike movements in market confidence, which tend to return to normal over a few quarters or years, these redistributive shifts are virtually permanent, allowing a sequence of small shifts to build up over time — with huge overall impact.

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For example, we estimate that if there had been no redistributive shifts since 1980, which mostly went in favor of shareholders, the level of the stock market in 2012 would have been only 56% of its observed value. However, these changes do not always favor shareholders. Case in point: the period from 1960 to 1980 saw substantial gains for workers at the expense of lower stock returns.

Finally, fluctuations in productivity play little role over any time frame. Certainly, improvements in technology are important, and changes in productivity may be central to the performance of individual stocks. But for explaining why the market as a whole rose more in one period than another — be it a quarter, year, or decade — differences in aggregate productivity growth are rarely the story.

Since financial researchers often assume that productivity is the key driver of stock prices, our results suggest that a major rethinking may be in order.

In the wake of Donald Trump’s election, many are looking to the stock market to learn what is in store for the economy. While much of the short-run action likely reflects movements in market confidence or uncertainty, upcoming decisions on corporate taxes or labor policy may influence what shareholders ultimately receive. Our research implies that expectations regarding redistribution, rather than forecasts of overall economic performance, may play a crucial role in driving stock prices as the political agenda is revealed.

Daniel Greewald is an assistant professor of finance at the MIT Sloan School of Management.