After Donald J. Trump won the presidential election, Americans’ optimism about the economic future soared. But midway through the year, that optimism has not translated into concrete economic gains.

This seeming contradiction exposes a reality about the role of psychology in economics — or more specifically, how psychology is connected only loosely to actual growth. It will take more than feelings to fix the sluggishness that has been evident in the United States and other major economies for years. Confidence isn’t some magic elixir for the economy: Businesses will hire and invest only when they see concrete evidence of demand for their products, and consumers intensify their spending only when their incomes justify it.

The sharp rise in economic optimism after the election came through no matter how the question was asked or who answered, whether the survey was intended to capture consumer confidence or consumer comfort or consumer sentiment. It was true in surveys of small-business owners and of C.E.O.s of some of the biggest companies in the world. And the rise during the winter months in these surveys has mostly been sustained in the months since.

But the economy is plodding along at the same modest rate it has for the last eight years nonetheless — at least when you look at “hard” data around economic activity instead of “soft” data like surveys, as analysts put it.