Here's what happens to stocks when oil drops 50%

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We are in rare territory indeed on the oil front. West Texas Intermediate has dropped more than 50 percent in the last six months.

According to our partners at Kensho, this has happened only five times since 1980: 1987, 1991, 1999, 2002, and 2009.

Six months after those events, the S&P 500 was up four of the five times, with an average gain of 3.7 percent. Perhaps more importantly, WTI was positive six months later all five times, and was up 52 percent on average.

Read that carefully: All five times this has happened, WTI has been up six months later. And up 52 percent on average.

Read MoreBrent dips below $50 for first time since May 2009

That should give some comfort to those who have argued, as I have, that oil is more likely to be in the $50s or $60s in the second half of the year than it is to be in the $30s.

None of this answers the big questions: What's causing the drop?

I mean, nothing else is down 50 percent. This seems very specifically related to oil, not so much the global economy. The conventional explanation that this is a "supply issue"—with a small part of the drop due to lower global demand from a slower China and Europe—makes some sense on the surface.