The population being measured is academic psychologists.
In researching the pervasiveness of Gramscian memes it has become apparent to me that the spread of postmodernism and critical theory seems to have reached some sort of tipping point in the late 1980s. I graduated in 1982 and completed my masters in 1985. Some of the implications and ideas couched in postmodernism and critical theory were already circulating but they were by no means common or in the mainstream. Judging by the Group of 88 in the Duke Lacrosse Rape Hoax (from my post, The imbalance cannot last) by 2006 postmodernism and critical theory had taken over the following departments (their percentages being the percentage of professors in each department being willing to condemn and punish students without due process, merely based on their class and their race).
80% African and African-American StudiesOther sources seem to indicate a prevalence of postmodernism and critical theory among the psychology, sociology, communication and education departments as well.
72% Women's Studies
60% Cultural Anthropology
45% Romance Studies
31% Art & Art History
0% Biological Anthropology and Anatomy
0% Computer Science
0% Engineering, all departments in the entire school
0% Germanic Languages/Literature
0% Psychology and Neuroscience
0% Slavic and Eurasian Studies
The chart from Haidt's paper covers only psychology and is in terms of conservative/liberal or party affiliation but it provides a trend line which matches my experience. Gramscian memes are far more prevalent on the liberal end of the scales such that they can be taken as close substitutes. These Gramscian schools of thought, so contrary to American culture, did indeed reach their critical mass in select departments only in the late 1980s or early 1990s and beyond. It explains a lot. President Sullivan (Sociology) of UVA's instinctive inclination towards group punishment based on no evidence (in the recent UVA Gang Rape Hoax), Professor Susan J. Douglas (Communications) in her recent It's Okay to Hate Republicans screed, Sabrina Rubin Erdely (author of the UVA Gang Rape Hoax article) was class of 1994 with a major in English.
These purveyors of Gramscian memes seem to be the product of a relatively recent self-destructive turn in academia. Between MOOCs, reduction in education loans, declining university revenues, tough economic environments for graduates, (particularly those with low value degrees such as studies or sociology or communications), it is likely that the education system will purge itself of much of this cognitive pollution over the next couple of decades. The faster it happens, the better and the less damage done to the lives of young people led astray by naive enthusiasm for poisonous totalitarian utopias.