CEO pay has been rising since the 1980s, with a sharp climb during the 1990s among large companies driven by hikes in incentives and restricted stock. By the turn of the millennium CEOs were making 120 times the pay of the average worker, whereas in the 1970s they were making about 30 times more, according to research by finance professor Carola Frydman and associate finance professor Dimitris Papanikolaou, both of Northwestern University's Kellogg School of Management. The researchers have theorized that the pay gap widens during periods of rapid technological innovation because it becomes more important to have a skilled CEO who can grow the company amid those shifts.