Ask Matt: What can kill the bull market?

Q: What can kill the bull market?

A: Bull markets take off when no one is looking. But they can often end when investors aren't paying attention.

Cheap stocks and an increase of participation in the market is what serves as the kindling to get a bull market started. But when stocks get pricey - and some investors might be looking to pull back a bit - that's when the bull market can find itself in trouble, says Jack Ablin of BMO Private Bank.

The combination of pricey stocks and a pullback in interest in buying them is a situation that's becoming more clear now, which is a warning sign for investors, Ablin says. The Standard & Poor's 500 isn't cheap anymore. The popular index of stock's valuation, based on price to revenue, is 25% higher than it's been over the long-term history, Ablin says. Additionally, the Federal Reserve is considering adjustments that could make investors have second thoughts about how much they have invested in the market.

Another concerning sign is that the market is largely being propped by a relatively small number of gigantic stocks, Ablin says. When investors get nervous, they increasingly "flock" to the smaller and smaller list of stocks that are working, like Google, he says. Quietly, large groups of stocks are already "rolling over." That's what ends a bull.

Paste BN markets reporter Matt Krantz answers a different reader question every weekday. To submit a question, e-mail Matt at mkrantz@usatoday.com or on Twitter @mattkrantz.