As the third quarter of 2018 closed, the Standard Poor's 500 stock index — a barometer of the U.S. economy — closed the quarter at 2913.98 on Friday, as the unusually strong and long economic expansion powered stock prices to within 1% of its recently reached record high.

The S&P 500 rose 7.2% last quarter. It was the best quarter since the end of 2013, and the index was up about 9% since the start of the year.

Amid abnormal political polarization in the U.S., a crisis of trust in internet security, and nationalism rising worldwide while globalism was under attack, the bull market and economic boom recently turned nine years-old — the longest in U.S. history — raising worries about risk. What's driving stocks?