Because the corporate tax is often considered one of the most distortionary taxes in the federal arsenal, cutting it would promote economic growth. The true cost of the tax cut, as estimated by dynamic scoring, would most likely be much less than its static score.

An increased child tax credit would have a very different effect on G.D.P. Because the child credit is targeted at the middle class, a family gradually loses eligibility for the credit when its income increases. As a result, a family keeps less of each additional dollar it earns. In economics-speak, this tax cut actually increases the family’s effective marginal tax rate. The disincentive from higher marginal rates could reduce G.D.P., making the true cost of the proposal greater than its static score.

For all these reasons, the case for dynamic over static scoring is strong in theory. Yet three problems make the task difficult in practice.

First, any attempt to estimate the impact of a policy change on G.D.P. requires an economic model. Because reasonable people can disagree about what model, and what parameters of that model, are best, the results from dynamic scoring will always be controversial. Just as many Republicans are skeptical about the models of climatologists when debating global warming, many Democrats are skeptical about the models of economists when debating tax policy.

Second, accurate dynamic scoring requires more information than congressional proposals typically provide. For example, if a member of Congress proposes a tax cut, a key issue in estimating its effect is how future Congresses will respond to the reduced revenue.

This raises important questions for which we have no easy answers. In the coming years, will these Congresses respond quickly to the revenue shortfall, or will they let budget deficits fester? When they act to close the budget gap, will they increase taxes, or will they cut spending? If they cut spending, will it be on consumption items, such as health care for the elderly, or on growth-promoting investments, such as education for the young? The impact of the initial tax cut depends crucially on the answers to these questions, but budget analysts usually have little to go on but speculation.

Third, dynamic scoring matters most over long time horizons. Some policy changes, such as those aimed at encouraging capital investments, take many decades to have their full impact on economic growth. Yet congressional budgeting usually looks only five or 10 years ahead. If we want to take dynamic scoring seriously, we have to think about how policy affects the next generation, not just the next election.