The day before Thanksgiving, US president Donald Trump thanked the Kingdom of Saudi Arabia for making oil cheaper. “Like a big tax cut for America,” is how he described it. Fair enough. Janet Yellen, former chair of the Federal Reserve, has made the same argument. An American household spends about 4 percent of its monthly budget putting petrol in cars. When petrol gets cheaper, there’s more to spend on other things. Every president since Dwight Eisenhower would agree. Cheap oil is still great for American drivers. But it also discourages American oil firms from drilling new wells in West Texas shale. What Mr Trump does not seem to have grasped is that the country doesn’t respond to the price of oil the way it did even a decade ago. Sad drillers now have a bigger effect than happy motorists.

In 2008, the US produced about 150m barrels of oil a month. By August of this year, that had grown to 350m barrels. Oil producers adopted the technologies of horizontal drilling and hydraulic fracturing, opening up shale fields in Texas, Wyoming and North Dakota. It is as if the US has discovered and annexed a virgin petro-state. Crude imports have dropped by half and the country is once again the world’s single largest producer. Usually, consumption makes up the biggest contribution to growth in the US. It is a developed economy, full of people who buy stuff. For several years after the last recession, though, consumption growth gave way to private non-residential fixed investment — businesses buying new plants and machines.

Then, around 2015, people began buying stuff again. But businesses stopped doing so. Break out investments in the oil patch, however, and it becomes clear that that industry was dragging down all the others. The price of a barrel of oil had dropped from above $100 to below $50 within a year. Capital spending in America hadn’t collapsed. But capital spending in the Permian, Eagle Ford, Bakken and Niobrara shale fields had collapsed.