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The second big cause is an investment slump. Despite all the savings available to be invested, companies are holding back. Some have grown so large and monopoly-like that they don’t need to invest in new projects to make profits. Think about your internet provider: It may have terrible customer service, but you don’t have a lot of alternatives. The company doesn’t need to invest in new technology or employees to keep you as a customer.

Beside a lack of competition, the investment slump stems from what Summers calls the de-massification of the economy. Developers aren’t building as many malls and stores, because goods now go straight from warehouses to homes. Offices don’t need as much storage space. Cellphones have replaced not just desktop computers but also cameras, stereos, books and more. Many young people have decided they’re happy living in small apartments, without cars.

For all of these economic problems, there are promising solutions. But the United States is not giving those solutions a try.

The 2017 Trump tax law is a useful case study. It is a dreadful piece of economic policy — essentially a giant effort to aggravate income inequality. Tax cuts that benefit the wealthy most are huge and permanent. Tax cuts focused on everyone else are smaller and temporary.

But the law still pumped money into the economy last year, thanks largely to those temporary tax cuts for the middle class and poor. And guess what? G.D.P. growth finally met some forecasters’ expectations, as you can see from the first chart above. The economy expanded 2.9 percent in 2018. Unfortunately, the boost seems to have been temporary. In the first quarter of this year, growth has slowed markedly, probably to about 0.5 percent. It will most likely grow faster over the rest of 2019, but not 3 percent. Once again, economists have started downgrading their expectations.

A better policy response would start with a tax cut focused on the majority of Americans, not the wealthy. And there are many other ways to take on secular stagnation. When I spoke to Summers last week, he rattled off a list:

Infrastructure projects, to jump-start investment. The retirement of coal-fired power plants, which would also lead to new investment. Stronger safety-net programs, including Social Security, to reduce the savings glut. More aggressive antitrust policies, to combat monopolies. And a Federal Reserve that, at long last, stopped making the same mistake — of overestimating both growth and inflation.