First, it is desirable to minimise international spillover effects. Friendly nations are considerate of each others’ interests and try to avoid doing harm to other countries’ trade flow's - not least because they want to see their own trade interests respected by other nations. Second, where policies have unintended side effects on markets and trade, they introduce extra costs to the economy in the form of use of resources that could be employed more productively elsewhere. In the political process, these extra costs are often not directly visible, but they are nevertheless real.