That’s because the correlation between crude oil and stocks has not been stable over time. For several decades prior to 2000, for example, that correlation was negative, as you can see in the chart below. During those decades, higher oil prices in a given month led to below-average stock-market returns in the subsequent month.

Oil prices extended Monday's slide, with Brent crude futures down more than 3%, and are now back around where they were when fighting kicked off earlier this month. The cease-fire spurred a global stock-market rally,