There is an interesting discussion going on at Adrienne's blog over what is causing Leftists to embrace such obviously insane policy positions. Michael Savage, a talk-radio provocateur, coined an expression a few years ago that "liberalism is a mental disorder."
More accurately, we would suggest here that Leftist positions and behaviors are symptomatic of underlying psychological pathology. This is especially true among members of the Corporate Media. Everything from looking up to Satanist Sally Quinn as a role model to their repeated public tantrums and dissemination of the most ridiculous conspiracy theories, Media personnel are pretty clearly a mentally-unbalanced lot.
At the top levels of the Far Left: the Clintons, Podesta Brothers, and George Soros, for example; power and profit are the motivating factors. They've got good rackets going, fueled by payoffs from special interests and a compliant Media which aids and abets their criminal behaviors. But what about the rank-and-file Leftists?
We've asked the same question in a more lighthearted vein when discussing Trump Derangement Syndrome. The day after Trump was elected, an Ohio man immolated himself in a children's playground using Trump Campaign paraphernalia as fuel. The most recent manifestation is the Mayor of Oakland who's made herself effectively a human shield defending MS-13 thugs from arrest and deportation. In between those times, numerous Leftists have wound up in jails, psych-wards, the unemployment line, hospitals, or even dead because they lost it in public and became a danger to themselves and others.
So we argue that it's too broad a statement to claim that Liberalism is a mental disorder. Some are Leftists---especially younger people---who've been politically and socially educated by the degenerate Media and the public schools. They haven't had enough life experience to see through the tissue of deception veiling their mind's eye. Some Liberals are so because they (rightfully) fear the political power of the Alt-RINOs without realizing that those clods are a tiny---but loud---minority whose power is exaggerated by the MSM.
But for those Leftists who should know better, some deeper mental problem seems to be a driving factor. Mass-communications have made it possible for people teetering on the brink of mental collapse to build support-networks with others just as far gone as they are. Then they project their collective neuroses onto society-at-large or onto some political, religious, or ethnic group.
Thus, to answer Adrienne's question: there's no single answer. Radicalism always attracts the morally degenerate, the ignorant, the fearful, and the loonies. They make up the backbone of the postmodern so-called 'Progressives' just as earlier generations of them turned to Jacobinism, Marxism, Bolshevism, Naziism, etc. They typically aren't too dangerous until they achieve positions of actual power. Our job in a Democracy is to prevent that from happening---and exposing them for what they are is the best safeguard.