A crucial stock market signal just got its most bullish reading of the year

Miguel Riopa/AFP/Getty ImagesUS stocks may be hovering near record levels, but investors don't seem to be particularly worried about protecting the plentiful gains they've enjoyed.

At least that's the signal being sent by the options market, which is more unhedged on the S&P 500 than at any point this year.

The CBOE Equity Put/Call Ratio fell to 0.51 on Monday, the lowest since December 8, and 20% below the measure's current bull market average. A low reading implies that traders are making a small number of bearish bets, relative to wagers on an increase.

But while this gauge of investor worry is at a six-month low, it wasn't always so subdued. The ratio sat close to 1.0 at a five-month high as recently as mid-April.

Business Insider / Andy Kiersz, data from Bloomberg

So what conditions have recently shaped up to support the bullishness - or perhaps more accurately, lack of bearishness - being seen in the stock market right now?

Here's a breakdown of five themes supporting investor positivity, and suggesting that US stocks have further to run: