With the Israel-Iran cease-fire holding, stocks hung near record levels on Wednesday. Major indexes finished mixed. The S&P 500 was unchanged and Nasdaq Composite edged up 0.3%, leaving both within 1% of their all-time highs.

Exxon stock and Chevron stock both added 0.3% ahead of the opening bell. American depositary receipts for British oil and gas giants BP and Shell were also rising. The moves came

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.