What an economy really wants, after all, is not more investment per se but better investment. It wants capital to flow to companies that will create value - not in the form of a rising stock price but in the form of more goods for less cost, more jobs, and rising wages - by enhancing productivity.

A general principle of good taxation is that similar jobs, and similar kinds of compensation, should be taxed the same way: otherwise, the government is effectively subsidizing some jobs over others.

Lack of confidence, sometimes alternating with unrealistic dreams of heroic success, often leads to procrastination, and many studies suggest that procrastinators are self-handicappers: rather than risk failure, they prefer to create conditions that make success impossible, a reflex that of course creates a vicious cycle.

Bubbles and crashes are textbook examples of collective decision making gone wrong. In a bubble, all of the conditions that make groups intelligent - independence, diversity, private judgement-disappear.

Being unemployed is even more disastrous for individuals than you'd expect. Aside from the obvious harm - poverty, difficulty paying off debts - it seems to directly affect people's health, particularly that of older workers.

The history of the Internet is, in part, a series of opportunities missed: the major record labels let Apple take over the digital-music business; Blockbuster refused to buy Netflix for a mere fifty million dollars; Excite turned down the chance to acquire Google for less than a million dollars.

Lower oil prices won't, by themselves, topple the mullahs in Iran. But it's significant that, historically, when oil prices have been low, Iranian reformers have been ascendant and radicals relatively subdued, and vice versa when prices have been high.