In the June 13 edition of the Wall Street Journal, Stephen Moore wrote an editorial entitled Real Tax Cuts Have Curves (available for a fee):

…The Laffer Curve helped launch the Reaganomics Revolution here at home and a frenzy of tax rate cutting around the globe that continues to this day.
The theory is really one of the simplest concepts in economics. Yet its logic continues to elude the class-warfare lobby whose disbelief is unburdened by the multiple real-life examples which validate its conclusions. The idea is that lowering the tax rate on production, work, investment, and risk-taking will spur more of these activities and thereby will often lead to more tax revenue collections for the government rather than less.
In the 1980s, President Ronald Reagan chopped the highest personal income tax rate from the confiscatory 70% rate that he inherited when he entered office to 28% when he left office and the resulting economic burst caused federal tax receipts to almost precisely double: from $517 billion to $1,032 billion. [Remember these numbers the next time someone tries to tell you the deficits under Reaganomics were a revenue problem and not a spending a problem!] Now we have overpowering confirming evidence from the Bush tax cuts of May 2003. The jewel of the Bush economic plan was the reduction in tax rates on dividends from 39.6% to 15% and on capital gains from 20% to 15%. These sharp cuts in the double tax on capital investment were intended to reverse the 2000-01 stock market crash, which had liquidated some $6 trillion in American household wealth, and to inspire a revival in business capital investment, which had also collapsed during the recession. The tax cuts were narrowly enacted despite the usual indignant primal screams from the greed and envy lobby about “tax cuts for the super rich.”
Last week the Congressional Budget Office released its latest report on tax revenue collections. The numbers are an eye-popping vindication of the Laffer Curve and the Bush tax cut’s real economic value. Federal tax revenues have surged in the first eight months of this fiscal year by $187 billion. This represents a 15.4% rise in federal tax receipts over 2004. Individual and corporate income tax receipts have exploded like a cap let off a geyser, up 30% in the two years since the tax cut. Once again, tax rate cuts have created a virtuous chain reaction of higher economic growth, more jobs, higher corporate profits, and finally more tax receipts.
This Laffer Curve effect has also created a revenue windfall for states and cities. As the economic expansion has plowed forward, and in some regions of the country accelerated, state tax receipts have climbed 7.5% this year already…Many of President Bush’s critics foolishly predicted that states and localities would be victims of the Bush tax cut gamble.
Alas, all of the fiscal news is not celebratory. The CBO also reports that federal expenditures are up $110 billion, or 7.2%, so far this year as the congressional Republican spending spree rolls on. Nonetheless, it now appears that the budget deficit will be at least $60 billion lower than last year and states and cities, led by California, which a few years ago were awash in debt themselves, will enjoy net surpluses of at least $50 billion. This means that total government borrowing will come in at below 2.5% of national output, which is hardly a crisis level of debt…
On the private-sector side of the ledger, what we are now witnessing is a broad-based investment boom. The lower capital gains and dividends taxes have been capitalized into higher stock values, and that in part explains why the Dow is up 24% since May of 2003 while the Nasdaq has risen 39%. Dan Clifton of the American Shareholder Association estimates that this rise in stock values has translated into roughly $3 trillion in added wealth holdings of American households. The severe slump in business capital spending in 2001 and 2002 has now taken the shape of a U-turn, with spending on capital purchases up an enormous 22% since 2003. Because higher wages and new job creation are highly dependent on business capital investment, the mislabeled “Bush tax cut for the rich” has in reality enormously benefited middle-income workers.
…Thanks to inane budget rules in Congress the capital gains and dividend tax cuts are currently set to expire in 2008. (When was the last time a spending program in Washington expired?) One thing would seem certain: Raising the tax rates on capital gains and dividends would be a formula for choking off the expansion and reversing the stock market climb. Until now, the Democrats in Congress have in unison sanctimoniously charged that the government can’t afford the price tag of making the tax cut permanent. But, of course, all this new fiscal evidence points to precisely the opposite conclusion: that we can’t afford not to make the tax cuts permanent.
Whether Mr. Bush’s critics’ ideological blinders make them capable of being persuaded by facts and evidence is an altogether different issue.

If you want even more empirical data, read this excellent article by Arthur Laffer, in which he presents historical data on the effects of marginal tax cuts from the Harding-Coolidge (1920’s), Kennedy (1960’s) and Reagan (1980’s) eras – which also turn out to be the three times of greatest economic growth in the last 100 years. In the article, Laffer explains the drivers which provide the underlying logic for the Curve:

The Laffer Curve illustrates the basic idea that changes in tax rates have two effects on tax revenues: the arithmetic effect and the economic effect. The arithmetic effect is simply that if tax rates are lowered, tax revenues (per dollar of tax base) will be lowered by the amount of the decrease in the rate. The reverse is true for an increase in tax rates. The economic effect, however, recognizes the positive impact that lower tax rates have on work, output, and employment–and thereby the tax base–by providing incentives to increase these activities. Raising tax rates has the opposite economic effect by penalizing participation in the taxed activities. The arithmetic effect always works in the opposite direction from the economic effect. Therefore, when the economic and the arithmetic effects of tax-rate changes are combined, the consequences of the change in tax rates on total tax revenues are no longer quite so obvious.

It is important to note that, in evaluating the effects of tax cuts, many opponents of such cuts (including the “pay as you go” budget deficit hawks as well as the methodology used by the Congressional Budget Office) only present a calculation of the arithmetic effect – called a “static analysis” – thereby assigning a zero value to the economic effect. Yet the empirical data from the three eras of tax cuts clearly show the error of that approach. That is why it is crucial that a “dynamic scoring” methodology be used, incorporating both arithmetic and economic effects. It is no less important to note that cash refunds from government, which do not change marginal tax rates, will have no lasting economic effect because they create no incentive to change human behavior and create new economic value.
Never underestimate the incentive power of marginal tax cuts. It’s Economics 101, after all.