Executive leadership at major U.S. companies is less bipartisan than it used to be, according to a new paper in the National Bureau of Economic Research, which looked at party registration trends among executives at publicly listed companies since 2008.

So, why don’t Democrats and Republicans share the C-Suite as much as they used to?

Starting around 2015, it became less likely to see a Democratic CEO working alongside a Republican CFO.

“Executive teams have become more partisan. We like to match with other like-minded individuals,” said Elisabeth Kempf, a professor at the University of Chicago and a co-author of the paper. 

Kempf said shared political values at the top can be positive, since consensus may be easier to reach. But she also said there’s downsides when the company leadership all comes from the same part of the political spectrum.

“You might also maybe not hire somebody that’s actually the best fit for the job,” she said – and instead hire somebody who voted the way you did. 

Executives are also under more political scrutiny from consumers and employees, said Costas Panagopoulos at Northeastern University. 

“Average citizens are now looking to corporations to engage politically and apply pressure on politicians.” 

He doesn’t see that trend ending anytime soon.