A little-known change to the tax code during Donald Trump's first presidency just might be the driving factor behind all those tech layoffs over the past few years.

As Quartz reports, a 2017 change to a 70-year-old tax law governing business spending didn't go into effect until 2022 — and three years after it finally did, we can see how harmful the move was.

Back in 1954, the Internal Revenue Service enacted Section 174, a rule that let companies deduct 100 percent of their research and development (R&D) spending, including salaries for the people involved in it. As Quartz notes, companies like Microsoft and Apple were built upon a tax system that rewarded R&D by allowing all such expenditures to count as write-offs — a system that Trump quietly dismantled at the start of his first stint

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