Back at the start of 2004, America’s banks discovered that they could borrow money cheaply from Asia and lend it out in higher-yielding domestic mortgages while using sophisticated financial engineering to wall off and strictly control their risks – or so they thought. Over the next two years, annual US spending on residential construction roared upward, from $624 billion to $798 billion, the US unemployment rate dropped from 5.7% to 4.6%, and the economy grew at a 3.1% real annual rate.