Quartz: Getting a job in academia is notoriously difficult. But the odds are especially bad for aspiring professors who didn’t earn their PhDs from a select few universities, according to a new study published in the journal Science Advances.

Researchers at Harvard and the University of Colorado, Boulder looked at full-time faculty in history and business departments in US colleges, and at computer science faculty in the US and Canada, between 2011 and 2013, co-author Aaron Clauset tells Quartz. They examined where the professors had earned their PhDs, and created a ranking system of the most prestigious schools in each subject, based on how successful their graduates were in finding jobs. They analyzed 16,316 assistant, associate and full professors across 242 schools.

Overall they found a fourth of the institutions accounted for about three fourths of tenure-track faculty. For example, 18 universities produce half of US and Canadian computer science professors, 16 universities produce half of US business professors, and eight universities account for half of US history professors. They chose these three fields to get a range, from humanities to scientific fields, and demonstrate that exclusive institutions dominated across the board, Clauset says. [Continue reading…]