Should we be concerned about the strength of the US stock market? Absolutely, says financial historian Edward Chancellor, author of a definitive history of past speculative excesses, from the South Sea Bubble onwards, and a brilliant recent book, The Price of Time, about the folly of sustained low interest rates. An earlier version of this article first appeared on the online financial website, Reuters Breakingviews, and is reproduced with the author's permission.  

The US stock market has hit 50 new highs so far this year. Investors are now anticipating tax cuts and further fiscal largesse f...