Sometimes, called Trickle Down Economics, the theory was that if we put more money into the hands of those who supply the world with goods and services and jobs, the prosperity would spread to everyone at all strata of the economy.
Now we have the figures. The economy has grown. In other words, the GDP has grown, which is a measure of how many goods and services the economy has created.
And how has that enlarged economy been shaped? The growth -- the bulk of the economy's gains -- has gone to the people at the top, while everyone else has the same (income and wealth) as before or less, sometimes a lot less. More people are underemployed working in menial jobs. Standards of living have gone down for many, while wealth for the very top strata have multiplied by many times.
These facts prove it is a deeply flawed model.
And now, many of those proponents claim that disparity in income and wealth is not a problem. But doesn't that depend on the viewpoint of whomever you ask? If it's not a problem, then what, exactly, were they predicting would happen with Supply Side Economics? They claimed everyone would prosper. But that has, in fact, not happened. Something quite the opposite has happened. Are they for shared prosperity, or are they not? The observed fact is something else.