I wish we had a better answer to this question, it would make tax policy recommendations much easier:

Who Ultimately Pays the Corporate Income Tax?, by Uwe E. Reinhardt: I ended last week’s post by asking whether anyone knows which human beings ultimately pay the corporate income tax. An intuitively appealing answer is that the tax is levied on profits... Therefore, those who made that investment — the shareholders — absorb the tax fully in the form of a lower after-tax return on their investments.

That impression would be reinforced by the short-run, partial-equilibrium model ... that we sometimes trot out in freshman economics courses. In the partial-equilibrium model, the company’s capital stock is assumed to be fixed in the short run. Imposing or raising a tax on the profits will not change the company’s decisions...

But ... all bets are off in the longer run, when the company’s capital stock relative to the input of labor can change and when the owners of investable funds can decide whether to invest their money ... abroad or in enterprises not subject to corporate taxation. General-equilibrium models accommodating this wider view of the economy and the longer run ... show that who actually pays the corporate income tax — the owners of capital or labor — is driven by a number of factors in complicated ways that elude simple intuition. ...

The earliest formal general equilibrium model, published in 1962 by a University of Chicago economist, Arnold C. Harberger ... assumed a closed economy. In that model, the burden of the corporate income tax ultimately fell entirely on the owners of capital.

When the model was modified as an open economy in which capital in the taxed country can escape by flowing abroad to untaxed or lower-taxed countries, some or all of the burden of the corporate income tax shifted to labor. Under the assumption of perfect international capital mobility and perfect substitutability..., labor would ... bear the entire corporate income tax, because labor is the only immobile factor than cannot escape the tax.

Other modeling efforts since that time, or econometric estimates inspired by these models, have ranged between these extreme incidence models. Economists are divided on the issue. Some (including Gregory Mankiw) are persuaded that the corporate income tax ultimately falls mainly on labor... One can actually make a case for cutting the tax in the name of a more progressive income-tax structure... Other economists, including the authors of the surveys cited above..., are persuaded by the available empirical evidence ... that the burden of the corporate tax ultimately rests mainly on the owners of capital. ...

So ... why not abolish the tax altogether and instead tax human beings directly? The arguments against such a move are twofold.

First, even bringing in only 12 percent or so of total federal taxes, the corporate income tax represents the third-largest source of federal revenue and could not easily be replaced..., especially in these times of fiscal pressures.

Second, if the profits of corporations were not taxed, the corporate form of enterprise would become one more major tax shelter through which wealthy people could shield their income from taxation. That probably is the main reason why abolishing the corporate tax has never had any political traction, in the United States or abroad.

(There is quite a bit of additional detail in the original post, including links to papers summarizing what is known on this issue and a list of factors that affect the distribution of the tax between capital and labor.) Though there is uncertainty, when thinking about tax policy I would go with the view of those who are most familiar with the theory and evidence, i.e authors of the surveys, as opposed to more casual observers and assume that the burden is mainly on capital.