A new study shows chief executive officers of major companies are more than twice as likely to donate to Republicans than to Democrats.

The study, conducted by professors at Harvard University and Tel-Aviv University, took 18 years to complete and followed the political contributions of more than 3,800 CEOs with companies listed in the S&P 500 stock index. It qualified a Republican donor as someone who donates at least two-thirds of contributions to the Republicans, and similarly for Democrats. Individuals who donated more evenly were classified as neutral.