Oversocialized and low self-esteem:

7 But what is leftism? During the first half of the 20th century leftism could have been practically identified with socialism. Today the movement is fragmented and it is not clear who can properly be called a leftist. When we speak of leftists in this article we have in mind mainly socialists, collectivists, “politically correct” types, feminists, gay and disability activists, animal rights activists and the like... What we are trying to get at in discussing leftism is not so much movement or an ideology as a psychological type, or rather a collection of related types.

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9 The two psychological tendencies that underlie modern leftism we call “feelings of inferiority” and “oversocialization.” Feelings of inferiority are characteristic of modern leftism as a whole, while oversocialization is characteristic only of a certain segment of modern leftism; but this segment is highly influential.

Another (parsimonious) theory is the "spiteful mutant" hypothesis out forth by Dutton and others that argue harmful mutations have built up in Western populations in modern times because infant mortality is so low (used to be like 50% in 1800s, now >1%). Since the brain is 88% of the genome (or affected by 88% of genesee, something like that), mutations of the body are also probably reflect mutations the mind (and we do have data showing leftists suffer from more mental disorders) These mutations cause them to behave in maladaptive ways that are detrimental to group fitness - e.g., hating their in-group, hating masculine attributes, loving invaders, etc. This causes a significant reduction in group fitness