John Cochrane posted a reply to my previous post on how changes in the corporate tax rate might affect investment. The key issues seem to relate to the specifics of what the tax code will allow as an expense. In my analysis, I treated investment spending by the firm as fully-expensed, which is not correct. However, the tax code does permit businesses to deduct interest on their debt, and depreciation, which I didn't include. What I'll do here follows - I think - a comment by Francois Gourio (Chicago Fed) on Twitter.
So, let's write down the firm's problem again, assuming a constant real interest rate r, which the firm's shareholders face (an important assumption - I'm neglecting taxes affecting the sharelholders). We'll assume that dividends are paid period-by-period to the firm's shareholders, with the firm maximizing the present value of dividends:
If the firm were to fund investment out of retained earnings (provided this does not violate a nonnegativity constraint) then a reduction in the corporate tax rate will indeed raise the after-tax marginal net payoff to investing. Alternatively, suppose that the firm always funds new investment by issuing debt, then pays the interest on the debt, retires debt as capital depreciates, and otherwise rolls the debt over. This implies that the firm's outstanding debt is always backed one-for-one by the firm's capital, or
1. We need to worry about how the household is taxed, which in this formulation determines what the objective function is for the firm.
2. To do a proper job here, we need to determine the optimal financial structure for the firm.
This is potentially quite complicated (not blog material), though I'm sure someone has addressed related problems in the taxation literature. To do the problem justice, we need a complete general equilibrium model. That said,
1. There's no presumption that the corporate tax rate reduction is going to matter much for intensive-margin decisions of the firm - decisions about labor input and investment.
2. Where the change in the corporate tax rate should matter is for entry decisions - here we need to start worrying about nonconvexities - e.g. fixed costs of entry. But some entry, relating to the treatment of pass-throughs, would just be a renaming of the productive unit - call yourself a business and you can be taxed at a lower rate. As well, firms may choose to relocate from other countries to the U.S., though as I mentioned in my previous post, those other countries won't give up without at fight.
3. There's a clear redistributive effect, as I mentioned in my previous post. Owners of stocks will benefit, and they tend to be richer people. Long-term, government transfers and expenditures on goods and services have to fall, and the burden of those reductions will be borne by the relatively poor.
4. If the Republican Congress actually wanted to increase investment spending, there are straightforward ways to do this through the tax code - an investment tax credit, for example.