Oil prices were rebounding on Wednesday morning, following a sharp selloff over the past two sessions.Brent crude futures, the international benchmark gained 0.9% to trade at $66.74 a barrel, and West Texas Intermediate U.

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.