I think the starting point is indeed transparency in reporting. I think that would take us a long way.
If I can cite it, the British accountancy profession has recently done a report on this. It concluded that in the end the best enforcement was transparency: that companies had to get themselves in a position whereby they could defend and justify publicly what they had done, and that if they couldn't justify it publicly, then they probably shouldn't be doing it. So if they took the transparency or corporate reporting showing whether there is a reasonable correspondence between the distribution of reported profits and the distribution of genuine economic activity, that is something that transparency could itself police.
If transparency isn't enough, then what are the alternatives? The alternatives are basically some system that overrides the company's own choice of how it divides up profits between jurisdictions and in the end assigns profits on some other basis. It's very hard to do that. It's very hard to get agreement on doing it.
Where I think you possibly could get agreement—and this is where the G-8 and G-20 are important—is in fact in this distinction between reasonable tax competition between, say, Canada and America, both centres of genuine economic activity, and abuse by tax tables, which are not standards of genuine economic activity but which come in with zero tax. The mutual zero tax is a situation that we very definitely want to avoid, whereas competition between centres of real economic activity in their tax rates is entirely legitimate and, indeed, healthy.
I think the full unitary taxation approach is not going to happen anytime soon, but I think there may be a halfway house in which first you get transparency, which polices things, and second, where there are blatant abuses, despite transparency, there is some agreement amongst the major centres of real economic activity that they will jointly police the abuse of the system that transfers profits to places that don't have real activity.