Suppose that you buy a new shirt every month for $20. But now there is a high tax on shirts to help pay back the debt so that the price is $35. You might not buy that new shirt. Many other people might not, either. Then some stores go out of business and some t-shirt makers lay off workers. This will slow down economic growth in the future.
Also, if taxes are especially high, businesses will have less incentive to invest (build new factories, stores, restaurants, buy new capital, etc.). Less capital means less economic growth. The problem with taxes is that each incremental tax increase causes more harm to economic efficiency than the previous increase (and probably harms economic growth more). I will explain more of this below.
But also remember that just a small drop in the growth rate hurts us in the long run. For example, in the last 30 years or so, the annual growth rate in the real GDP in the U.S.has been about 2.8%. If per capita GDP goes up 1 percentage point less than that to take population growth into account, we would have a per capita growth rate per year of 1.8%.
If 30 years ago per capita GDP was $27,500 then today it would be about $46,900 (actually close to what it really was last year). But what if we had only grown 1.3% per year? The per capita GDP would be only $40,500. That would be $6,000 less, which is big and that kind of difference just keeps getting bigger over time and that is only a .5% lower growth. This big difference is due to compound interest.
Below is a letter to the editor of the WSJ I wrote a few years ago. It helps explain the exponentially growing damage that taxes cause:
"Stephen Moore did a great job explaining how complicated our tax code is and how high taxes have gotten relative to what was originally promised in 1913. One other way to see the insidiousness of taxes is to realize that they are just as much the "noise" in the economy as prices are the "signals." The income you get paid is the price for your services and therefore signals the value of those services. But taxes reduce the clarity of that signal (hence, they are noise) by reducing how much of your pay you actually get to keep. As taxes increase, the noise-to-signal ratio in the economy increases even more, meaning distortions, and the misallocation of resources they cause increases disproportionately. For example, if the income tax rate is 10%, you keep 90% of your income. The noise-to-signal ratio is .111 (or .1/.9). But if the tax rate goes up by .10, or to 20%, the noise-to-signal ratio goes up even more, by .15 to .25 since you keep 80% of your income. The .25 comes from .20/.80 equaling .25. Another .10 increase in the tax rate increases the noise-to-signal ratio by .179 from .25 to .429. Then going from a 30% tax rate to a 40% tax rate makes it go up by .238, from .429 to .667. Every tax increase causes increasing damage to the economy's ability to efficiently allocate resources."
This is consistent with the fact that deadweight loss also grows exponentially with tax increases. There will be some links to deadweight loss at the end of this post (my students can simply look at the appendix to chapter 3 in their textbooks). But the idea is that a tax on a good causes the problem mentioned above when the price of a shirt increases.
In supply and demand, if an excise tax has to be collected by the seller, the supply line shifts up by the amount of the tax. In the graphs below, the green triangle shows the deadweight loss or the total economic harm from the tax.
In the first graph, the tax on the good is $2 per unit, so the supply curve shifts up by $2 (the red line represents the new supply line). The area of the green deadweight loss triangle is 1 (one-half times the base times the height (I turn it sideways to make a base of 2)).
But in the second graph, the tax is doubled. It is $4 per unit, so the supply curve shifts up by $4. Now the area of the deadweight loss triangle is 4. So we doubled the tax but the damage caused has quadrupled. This shows that tax increases cause exponential damage to economic efficiency, which harms economic growth in the future.
If the supply line gets shifted up by $20 (if taxes were that high), then there would be no market left at all.
Links on deadweight loss: