For decades inflation was the bogeyman in rich countries. But now some economists reckon that deflation, or falling prices, may be a more serious threat—in America and Europe as well as Japan. That would be decidedly awkward, given the surge in borrowing by firms and households in recent years. Particularly worrying is the rise in borrowing by American households to finance purchases of houses, cars or luxury goods. Deflation would swell the real burden of these debts, forcing consumers to cut their spending. Deflation is not necessarily bad. If falling prices are caused by faster productivity growth, as happened in the late 19th century, then it can go hand in hand with robust growth. On the other hand, if deflation reflects a slump in demand and excess capacity, it can be dangerous, as it was in the 1930s, triggering a downward spiral of demand and prices. Today, both the good and bad sorts of deflation are at work. Some prices are falling because of productivity gains, thanks to information technology. But the weakness of profits suggests that most deflation is now bad, not good. Deflation is particularly harmful when an economy is awash with debt. Total private-sector debt is now much higher than when deflation was last experienced in the 1930s. Falling prices not only increase the real burden of debt, they also make it impossible for a central bank to deliver negative real interest rates, because nominal rates cannot go below zero. If deflation causes real debts to swell, debtors may have to cut spending and sell assets to meet their payments. This can unleash a vicious spiral of falling incomes, asset prices and rising real debt. Irving Fisher, an American economist, described this process in a famous article in 1993 entitled "The Debt-Deflation Theory of Great Depressions". He described how attempts by individuals to reduce their debt burden by cutting costs could paradoxically cause their debt burden to swell. Unable to increase prices to boost profits, firms have to cut costs, either by reducing labour costs and hence household income or by buying less from other firms. This is sensible for an individual firm, but it reduces demand in the economy, thwarting the desired improvement in profit, leading to another round of cuts and putting further downward pressure on prices.