The Russell 2000 hasn’t seen this kind of winning streak since 2013.

After eight straight weeks of gains, history suggests small caps are flashing a big buy signal to all stocks.

“We took a look at future returns after these long weekly win streaks on the Russell 2000, sure enough small caps continued to do well. What’s more interesting, though, is the S&P 500 actually does a little bit better,” Ryan Detrick, senior market strategist at LPL Financial, told CNBC’s “Trading Nation” on Tuesday.

The past nine times the Russell 2000 had a winning streak that stretched over at least eight consecutive weeks, the S&P 500 posted an average 6.4 percent gain in the following six months. Over those same periods, the Russell 2000 added an average 4.3 percent.“Small-cap leadership is a good thing,” he added. “It could be carving the way potentially for this bull market to continue as the S&P’s second half of the year also does well.”

While the Russell 2000’s hot streak has cooled off this week, it had risen every single week since early May. The small-cap index has added 8 percent this year, while trade fears have dinged the S&P 500 and capped its year-to-date gains at nearly 2 percent.

Historical returns back up the bull case for the S&P 500, but Detrick still expects a bumpier path higher for the benchmark index.

“This is going to be anything but smooth. Last year spoiled us all. We expect more of a rocky ride. Two 10 percent corrections already this year on the S&P 500, went a couple years without one,” he said.

The underlying fundamentals, led by the best earnings growth since 2010, keep Detrick bullish on the S&P 500 for the second half of the year.

“When you have double-digit earnings growth, the S&P is higher every single time since 1990 (12 out of 12 times),” said Detrick. “Earnings drive long-term gains. We’ll still side with the bulls here led by earnings.”

The S&P 500 is expected to post 21 percent earnings growth for the full year as tax cuts passed in December give rise to companies’ bottom lines. The second-quarter earnings season, also expected to show off double-digit gains, will kick off with the big banks in mid-July.

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