To speak in the language of the political spectrum as currently (and arbitrarily) defined: leftism results from indoctrination, while rightism can be considered either a natural precondition or a response to over-indoctrination, reflecting through and beyond it. Indoctrination, please note, is not necessarily meant here in a negative sense. Simply put, a leftist worldview is never a priori; it must be learned (think college-educated vs. non-college-educated voting trends, etc.). Once fully digested, however, its logic stretches out into unbounded reason, seemingly self-evident in its perpetuity. This worldview, once obtained, is hard to relinquish. Why leave a hard-won safe haven to wander conceptual deserts with no end in sight? One can understand the temptation to settle down and temper one’s intellectual restlessness here. As they say, many such cases.
Indoctrination, in fact, can be defined as the middle ground between an instinctive and educated state—a vast plain of necessary but not sufficient education. The art of becoming oneself is better described as a returning to oneself. We begin our educational journeys (and I mean this in the broadest possible life sense, not just in regard to formal schooling) by departing from our true (but yet undiscovered) selves. The hope is that through this forced march away from ourselves and toward alien yet revered lights, we may rediscover a truth always within us, merely lacking illumination.
(My basic conjecture: the very uneducated (a better word, perhaps, is instinctive) and the very intelligent tend to find themselves on one side; the moderately educated (who would leave off that qualifying adverb in their self-assessment) tend to find themselves on the other. Strange bedfellows, perhaps, but I think this trend generally holds—and not only in the political sphere.)
Looking at history, what do people incline toward if left to their own devices? Tribalism, for one. A moderate dose of leftism can (and usually should) be prescribed to counteract tribalism in its extreme; take too much, however, and you destroy (or disband) the tribe itself along with its tribalism.
Religion, too, can be observed through this lens. Consider a specific sect: to be born and raised understanding the world in such a way is a clear case of indoctrination. But so too is a completely secular, materialistic understanding of the world. Looking at any people in history before modernity and its resultant globalism, religions differed greatly, but religion itself was universally the rule, no matter where one went. We are born believing in God(s); the nature of this God, as we come to understand it, however, is a form of indoctrination—at best, a mild one.
Yet a wholly materialistic view of the universe, of human existence, and of the idea that its meaning (or lack thereof) must and can only be discovered through materialistic inquiry—oh, to believe this requires an even greater degree of indoctrination than belief in any particular God or set of doctrines. Humans are born believers to the extent that they are born helpless, which is to say, through and through. The materialists who walk among us, scoffing at the Gods, believing themselves to be the beacons of common-sense clear-headedness, are themselves the Kool-Aid-gargling street preachers—when viewed from a broader historical perspective, both past and future.