Casting around for a good reason to pass the Trans-Pacific Partnership into which Barack Obama’s administration has put so much lobbying effort, some economists have alighted upon the potential development benefits to the pact’s poorer members and particularly Vietnam.

The argument goes that Vietnam, which has largely been following a classic east Asian development model of export-led growth in manufacturing (and some agriculture such as rice and coffee), will benefit from the reductions in tariffs that will enable it to sell more garments into the vast US market.

There are, though, problems with this thesis, which apply more generally to any attempt to draw a substantial causal link between specific preferential trade deals and export-led growth.