Is it possible to be Buddhist by nature, or only by indoctrination? Lately I've been wondering if I (or someone, any person) is religious, agnostic, etc., by nature.
Many evolutionary psychologists, eg. Justin L Barrett and Ara Norenzayan, do argue that religion is a natural phenomenon. The argument are quite complex. Barrett for example argues that our brain contains various modules or devices. For example the Agent Detection Device helps us distinguish agents that are capable of directed motion, from objects which simply follow the laws of physics. 

Having detected an agent another module which Barrett calls Theory of Mind, describes the object in terms of it's desires, motivations, and goals in order to help anticipate how it might act. 

However if we mistake a stick for a snake we don't lose much. If mistake a snake for a stick we may die. So better to err on the side of seeing agents. But this means we tend to see agents everywhere. Plus our Theory of Mind Device means that we tent of think of agents as being like us - as having a human-like mind. 

Barrett adds another element - the Minimally Counter-intuitive concept. If something we think we see is complete counter-intuitive we won't believe it. If it is completely intuitive, we'll believe it, but hardly even notice that we do. But something that is mostly intuitive but counter-intuitive in small number of ways we not only tend believe it, but to find it interesting and memorable. 

In addition many experiences, such as the classic out-of-body experiences, make it very easy for people to believe in mind separate from body. This is more or less the default position for most people. This idea is discussed by Thomas Metzinger, in his 2005 article 'Out-of-Body Experiences as the Origin of the Concept of a "Soul".' Mind &amp; Matter Vol. 3(1), pp. 57–84; I have written a precise for my blog: 

All combined we get invisible agents as partially counter-intuitive, but interesting and memorable propositions. For many people these take the form of spirits. Spirits are common throughout the Asian Buddhist world. and some become gods. Of course in Buddhism we have both, but tend to down-play this. Still it's possible to apply these ideas to more abstract concepts like karma as well. 

Most of the work of these scholars is focussed on theistic religion. I'm beginning to apply their ideas to Buddhism to see if it can add to our understanding of it. See for example my recent essay on Barrett's work in relation to belief in karma and rebirth: Why Are Karma and Rebirth (Still) Plausible (for Many People)?

Karma is a very important invisible agent that ensures a just world by making sure that people get the rebirth they have earned. Many people of my acquaintance find the twin ideas of karma and rebirth entirely plausible and likely even. 

So it seems there is a good argument for saying that most people are religious by nature, but that some are not. It's entirely possible that someone could come to the same conclusions as a Buddhist about the world. Indeed these evolutionary approaches seem to be saying that it is atheism and materialism that require a more detailed explanation. Not believing in the supernatural is almost unknown in traditional societies, and yet it is quite widespread in the West, especially outside of America.