1. Bill Gentry concludes from a literature survey (pdf) that “labor bears a substantial burden from the corporate income tax.”
2. Work by Alison Felix in the US finds that:
Labor bears a substantial weight of the corporate tax. While this burden has fluctuated over time, the relationship between corporate taxes and wages has been consistently negative. In other words, higher corporate taxes are typically associated with lower wages.3. A study of European countries finds that, in the long-run, 92% of any rise in corporation tax falls upon wages.
This, I think, corroborates the view that employers’ NICs are in fact paid at least in part by workers: why should these be significantly different from corporation tax?
A big part of the mechanism here is the wage bargain. It‘s well-established that higher (pdf) profits lead to higher wages. This might be because of efficiency wage considerations; firms fear that if workers feel that they are being ripped off by high profits and low wages, they will shirk, so they pay more to motivate staff. Or it might be because more powerful workers threaten to quit if they don’t get a share of firms’ better fortunes. As a result, if firms get a tax break, workers will share in it. And conversely, if firms face a tax rise, workers will share the pain.
I don’t say all this to make a partisan political point; as I say higher employer NICs don‘t raise wage costs only because they lower wages - a point which hardly supports Labour's policy. I say it merely to emphasise the importance of the idea of tax incidence - that taxes don’t necessarily fall upon the people that they are formally levied upon. An inability to grasp this point is one of the features that distinguishes economists from non-economists.