After years of plenty America Inc is struggling to crank out more earnings

OVER THEGDP past 25 years America’s stockmarket has soared. Far from being built on thin air, this long bull run has rested on a boom in corporate profits. The worldwide earnings of all American firms, whether listed or not, have risen by 455% over this period and are now 35% above their long-term average relative to . America Inc mints $1bn every five hours.Globalisation, tepid wage rises, the ascent of tech and feeble competition made the bonanza possible. But as some of these forces ebb, the era of relentlessly expanding profits is under threat. Over the next few weeks America’s blue-chip companies will report their latest profit figures, which are expected to drop slightly (see section). Managers and investors need to be alert, especially given the growing number of firms with high debts that rely on bulging profits to stay afloat.