This month, the U.S. economy will have been growing for 121 months, the longest run since records began in 1854. Yet history suggests that expansions don’t last forever. What might cause a recession?

Unemployment is at a 50-year low and inflation is subdued and the household-debt-to-GDP ratio is declining. There are few signs of excesses in financial markets like the subprime mortgage lending of a decade ago, but stock markets and real estate prices are propped up by historically low interest rates and the debt of non-financial businesses is at an all-time high of 74% of GDP.

The economy is now largely dominated by the service sector but manufacturers are very dependent on complex and fragile global supply chains, leaving them vulnerable to unpredictable policy changes by governments.