Humans are built to be hypocritical, i.e., to give lip service and soft thought to high ideals, while mostly acting to achieve low practical personal ends. We manage this disconnect both by being stupid, and so not noticing our hypocrisy, and by being insincere, and so caring less when we notice.
Now human characteristics vary quite a bit, and so some folks are both unusually smart and unusually conscientious about their ideals. More than most people, these folks notice their hypocrisy, and try to avoid it. And since far ideals tend toward incoherence and impracticality, this has led smart sincere folks to invent a wide range of “ideologies” to substitute for their jumbled intuitions, with matching actions that range far from the norm.
The chance to show sincerity and smarts via our ideals makes it more important that one’s far ideals fit with a coherent and well-thought out ideology, than to be accurate relative to some external standard. So humans are relatively unconcerned to discover they have wildly divergent ideologies; they accept that they disagree. While a middle average opinion might be more accurate on average, it would less sparkle with the shine of clear clever sincere thought. In addition, divergence lets folks show loyalty to particular groups.
This smart sincere syndrome less afflicted our distant ancestors because fuzzy far feelings rarely lead to clear inescapable conclusions. While far mode is good for creative thinking, it usually leaves plausible excuses for rejecting conclusions that one does not like. But the more recent invention of near-mode-based math/logical style analysis, applicable to far abstract problems, has made it easier for humans to notice and avoid inconsistencies. So today, the smart sincere syndrome especially afflicts many folks with high math ability.
Now a modest dose of smart sincerity, limited by time, topic or temperament, is a good sign, as it indicates the positive qualities of intelligence and conscientiousness, qualities most any organization can put to good use. So everyone wants to seem ideological to some degree. And even a large dose of smart sincerity, if bundled with complements such as beauty, stamina, or charisma, can bring success as a “movement” or spiritual leader. But without such complements, an overdose of smart sincerity tends toward evolutionary failure, typically achieving less success relative to ability.
Today, a common solution to this dilemma is libertarian axiomatics, a simple coherent ideology supporting most, but hardly all, ordinary practical actions. Another common solution is to embrace a particular successful person, profession, or institution as the key to achieving global ideals; full loyalty and support of such a thing may, if reciprocated, help one achieve standard measures of success.
However, pity the simply smart sincere, who try make sense of their inherited incoherent impractical far ideals, via more coherent if idiosyncratic ideologies, that encourage unusual, and usually unadaptive, behavior. Stories told of their dramatic bids for ideal consistency may be their main legacy from this our dream-time era.
Added 17Jan: Rob Wiblin says terrorists fit this pattern.