Evidently it was too much to hope that a debate on outsourcing and trade in American politics would be conducted at an elevated level. This week, Mitt Romney decided to fight mercantilism with mercantilism, and the result was not edifying.

The Obama campaign has for weeks been accusing Mr Romney of being a flint-hearted outsourcer while governing both Bain Capital and Massachusetts. Mr Romney first hit back by showcasing his rare talent for turning somewhat sensible sentiments into rhetorical own goals - "corporations are people"; "I'm not concerned about the very poor" - his campaign emphasising the distinction between outsourcing - companies buying in services or products rather than doing them inhouse - and offshoring - buying stuff from foreign companies or setting up operations abroad to serve the US market.

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When that accurate but semantic argument failed to get traction, the Republicans this week took what a Latin teacher from Mr Romney's prep school would have called the "tu quoque" riposte, making almost equally tenuous claims about Mr Obama's policies sending jobs abroad, complete with accusatory website.

What this debate needs is someone to stand up for imports. As my colleague Andrew Hill has pointed out, buying in services from abroad can make companies more efficient. At a macroeconomic level, if that means specialisation and productivity gains, it can mean more and better jobs. Exporters often need imports. The OECD recently published a cross-country study showing that sectors of the economy that bought in imported business services exported more of their output. Thus, as the authors put it, trade could support "a deepening virtuous circle of specialisation, productivity growth and movement up the quality ladder".

Unfortunately, these arguments are difficult both to conceptualise and to quantify. It is relatively easy to record companies moving functions abroad and tot up a scary job loss number, especially since supposedly displaced workers can get payouts from the federal Trade Adjustment Assistance programme for doing so. It is much harder to calculate the benefits for businesses of buying inputs in which foreign companies specialise.

When Apple this year released a study rebutting accusations that the company created too few jobs in America, it focused purely on the US parts of Apple's supply chain and its associated businesses: this many people earning money writing iPhone apps, that many people working in Apple retail stores. True enough, but even leaving aside the huge benefits to consumers, that is still only a fraction of the story.

The job-creating benefits of Apple's products accrue from every business activity that they facilitate: every graphic designer or architect who doesn't have to struggle with a PC, every media company that sells iPad subscriptions. Putting a jobs number on this is close to impossible - and, to be fair, would also mean netting off jobs eliminated by technological change. Anyone like to call up every graphic design firm in America, ask them to imagine a world without Macs (warning: there will be tears) and calculate how many employees they would have in it? Anyone care to estimate how much more expensive Macs would be to assemble in the US rather than abroad, and how that would affect the profits of every company in America that uses one?

Actually, we know as an extreme case what a country looks like that cuts itself off from cheap imported technology: Brazil tried it in the 1980s by protecting its own, woefully bad, computer industry, and it held back the modernisation of the rest of the economy.

Economies benefit from trade and international investment flowing both in and out, and that is a feature, not a bug. The rhetoric of the Obama administration rarely acknowledges this. Note for example its arbitrary target of doubling US exports over five years. As Karel De Gucht, the EU trade commissioner, pointed out, the White House had better be prepared for imports to shoot up as well.

Just because the true macroeconomic counterfactual to outsourcing is incredibly difficult to estimate doesn't mean we should use a misleading one. Modern economies involve constant creative and destructive churn. Occasional snapshots of carefully selected parts tell you very little indeed.