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.

Crude oil prices slid more than seven percent Monday while Wall Street stocks advanced after markets shrugged off Iran's rebuttal to the US weekend attack on Iranian nuclear facilities.