The financial markets have begun to wake up to the fact that the Republican reforms to US corporate taxation will probably include important new “border adjustments” to the definitions of company revenues and costs. The basic idea is that US should shift to a “territorial” system, with corporations being taxed only on revenues and costs incurred within the US itself, and not on their worldwide aggregates, which is the principle behind the present system. 
A border tax was not explicitly part of the Trump platform before the Presidential Election. It was, however, included in the tax plan published last year by Paul Ryan in the House of Representatives, and Mr Trump has recently tweeted that companies that do not “make in USA” can expect to “pay big border tax”. That might be compatible with the Ryan plan, though it also might not be.
Although most other countries already operate “territorial” systems, the Republican plan includes other features that would make the new tax regime operate like a tariff on imports into the US, combined with a subsidy on many exports from the US, a combination that would have profound international economic consequences.
This is not just an obscure change to the details of America’s corporate tax code. It would be seen by trading partners as a protectionist measure that could disrupt world trade.
The direct effects of a border tax adjustment to the US corporate tax regime would be likely to raise American inflation, cut imports, boost exports and raise tax revenue, possibly by over $1.2 trillion over a decade. However, it would also raise the dollar’s exchange rate, which could offset or cancel out some of these other effects.
The impact on real GDP and employment would depend on how these effects panned out, and how the Federal Reserve reacted to the increase in inflation. It cannot be assumed that the effects would be beneficial. Recent estimates by Michael Gapen and Rob Martin at Barclays Capital suggest that the first year effects would be to raise US inflation by about 0.5-1 per cent, and to reduce real GDP by 1.0-1.5 per cent.
Given these economic effects, it is very doubtful whether this form of border tax, taken in isolation, would be good for the overall equity market, though other planned reforms to the corporate tax regime (including lower marginal tax rates, and full deduction of capital spending in the first year) certainly would be. Read more