To give you an idea, many young scientists (especially those in physical and life sciences) need to spend 10 to 15 years after college in training before they can secure a tenure-track position. Because science PhDs can’t get professorship without a portfolio of independent research and their research requires greater funding than that of humanities of social sciences, science PhDs must often do many, post-docs. Two is very common. After moving through the ranks of assistant and associate professor, the time between entering graduate school and becoming a Full Professor (of science) can be 17-20 year—and that's not even counting tenure!
"This widening at the bottom of the pyramid has been essential to maintain the publication output of PIs at the top, who face increasing competition to secure limited government funding..." says David Keays
, Group Leader at the Research Institute of Molecular Pathology Vienna. "As a consequence, it’s child’s play to get a PhD position but almost impossible to secure a faculty job. You might argue that this is natural selection at work, but I’m unconvinced it’s selecting for the best science.” Allison Schrager
notes that “the increase in post-docs or multiple post-docs before a professorship may reflect... that some science professors are using their students as cheap labor instead of training them to be independent researchers.”
According to American Association of University Professors
(AAUP), full time tenured and tenure-track professors are decreasing significantly over time. From 1975 to 2011, tenure-line faculty dropped from 57% of staff to just 30%, while contingent faculty (comprised of non-tenure-track and part-time faculty) grew from 43% to 70%.