The mind rot thesis is the notion that if someone is exposed to some technology/ideology/paradigm X, that hampers - even cripples - their ability to study and understand some other technology/ideology/paradigm Y which might compete with X - even if Y is manifestly superior.
Perhaps most famously expressed by EwDijkstra in his famous quote about BasicLanguage:
It is practically impossible to teach good programming to students who have had a prior exposure to BASIC: as potential programmers they are mentally mutilated beyond hope of regeneration.
There are other EwDijkstraQuotes on other languages of the time, most notably CobolLanguage.
The MindRotThesis is frequently used by advocates of whatever Y is to explain why Y, despite being (allegedly) superior to X, is failing to displace X. As JavaIsTheNewCobol, JavaLanguage is a favorite target of this sort of argument.
Discussion:
One issue I have with the mind rot argument is that it is excessively pejorative; and it presupposes an explanation for why new technologies often fail to be adopted - when in many cases, well known economic effects will do. The MindRotThesis is also (if stated in absolute terms) easily disproved; many people do embrace new technologies and new ideas, even if they are unable to put them to immediate use. This wiki is full of such individuals.
OTOH, there are many people who are hostile, for whatever reasons, to new things that threaten the familiar. In the domain of technology, this can be explained on the basis of rational actor theory - why should I learn about capability-based OSes, or SelfLanguage, etc. - when doing so won't get me a better job? Furthermore, appeals to programmer altruism - arguments like "adapting Y should be done because it's good for the profession and/or the world" are often viewed with suspicion.
And that's where wikis come in handy. That was supposed to be the role of wikis. And the original wiki vision as it was confirmed to me by Ward. [Further comments moved to HasWikiFailed.] -- CC
The theory is in general false, because if it were true, there would be no way for such ideas to be replaced once they became popular. And we see every day that such ideas are replaced, either through education, or the dying off of their adherents, even if they are not mainstream. -- PeteHardie