Polytheism and war

The argument was made by other social scientists earlier than that. (I can't recall which one exactly, but I'd hazard Max Weber, Carl Jung, or one of eithers' followers.) The gist of the argument goes something like this:

In a polytheistic culture, people tend to be more tolerant towards each other's (religious) beliefs, and thus don't normally fight over who is right or wrong. And indeed, one can observe that where Greeks and Romans feuded against one another from time to time over taking power, they were relatively tolerant against other religions and religious sects. (They made an exception for monotheistic religions, but this was for different reasons. In broad superficial strokes, the Jews refused to pay taxes and the Christians refused to do their military service).

By contrast, in a monotheistic environment, people tend to believe that there is only One way (that is: their way), and they tend to be willing to go to war over it. Internally because of this or that heresy; externally towards other religions with who they disagree with (read: crusading the Muslims down South).

There arguably is some superficial truth to that, in the sense that the main two monotheistic religions (Christians and Muslims) exercised proselytism and had no qualms with going to war externally against heathens or internally against heretics to drive their point home at sword tip.

The two also had no qualms with going to war between their peers as well, of course, and actual religious/sacred conflicts proper between the various groups were not that many in comparison.

Still there's a dearth of wars over "They worship Zeus rather than Jupiter". Whereas there's no shortage of crusades during the middle ages, early colonial conquests done in the name of God, religious wars during the Reformation, etc. I'm less familiar with the Arab world's history but I'd hazard that there were a few Sunni vs Shia related civil wars. And there has been a few recent wars between Christians and Muslims if one is willing to accept the Islamic terrorist/Western extreme right narrative.

A related argument, as an aside, is that nationalism and ideology are just new monotheistic religions - they also require some degree of faith and dogma - that are masquerading as different things under different names. And sure enough, people have got to war in the past two centuries or so over nationalism and ideology - including World Wars, no less.

Leaving those two aside, there are two other examples in history.

The third major monotheistic religion (Jews) didn't engage in proselytism at all, and has always made do (however it could) with whichever other (majority) religion they cohabited with and certainly haven't gone to war over it - they were persecuted, occasionally rebelled, but never went to war over religion. (The Islamic terrorist narrative would have it that they did fight over religion in recent times, but Israel would argue in return that they simply fought for their survival.)

Zoroastrians - the last major group - weren't proselytes either insofar as I'm aware, and seldom if ever fought against neighbors over religious grounds either.

The point in those last two examples is this: while there is some element of truth in the notion that monotheistic religiosity yields more religious or sacred wars, it actually only applies to groups that are proselytes.

[Sorry for the lack of references or sources. I'll try to find a moment later to add a few.]

James Hillman, a post-Jungian psychologist who introduced the idea/concept of seeing psychology as a variety of religious experience - on the concept of religious polytheism and its relation to psychology (Wikipedia entry - Hillman).

Instead of seeing Hillman's argument that polytheism reduces war (literally), it is better to view Hillman's argument as tolerance of other beliefs and culture and this NYT (1995) explains how it relates to psychology:

Hillman wants society to turn away from its obsession with the unanchored altitudes of spirit, and come back down to earth, to the soul. He also asks us to reject the judgmental thinking of monotheism and return our psyches to the polytheism exemplified by ancient Greece.

David Hume, a Scottish philosopher and historian, said this in his 1757 dissertation, “The Natural History of Religion":

While one sole object of devotion is acknowleged, the worship of other deities is regarded as absurd and impious. Nay, this unity of object seems naturally to require the unity of faith and ceremonies, and furnishes designing men with a pretence for representing their adversaries as profane, and the objects of divine as well as human vengeance. For as each sect is positive that its own faith and worship are entirely acceptable to the deity, and as no one can conceive that the same being should be pleased with different and opposite rites and principles, the several sects fall naturally into animosity, and mutually discharge on each other that sacred zeal and rancour, the most furious and implacable of all human passions.

Finally, this book, God Against The Gods: The History of the War Between Monotheism and Polytheism, also made essentially a similiar argument (i.e. intolerance of monotheism, etc).

A historian's view on psychology and religion that is (possibly) consistent with Hillman:

• Mercea Eliade, a Romanian historian of religion, fiction writer, philosopher, and professor at the University of Chicago

In Encyclopedia of Psychology and Religion (Springer, 2010), p. 69:

In his writings on religion (Patterns of Comparative Religion 1958; The Myth of the Eternal Return 1954; Shamanism: Archaic Techniques of Ecstasy 1964) he recognized a basic division between traditional religions such as the archaic cults of Asia, Europe, and America, and the historical religions of Judaism, Christianity, and Islam. The chief element in the former is the depreciation of history and the rejection of the profane, mundane world, combined with an emphasis on actions and things that repeat and restore transcendental models. Only those things that participate in and reflect the eternal archetypes through which cosmos came out of chaos are real in this outlook. The mode of expression in this model is in consequence repetitive.

Post-archaic or historical religions such as Judaism, Christianity and Islam tend to see a discontinuity between God and the world and to locate the sacred not in the cosmos but beyond it. These hold to linear views of history in the belief that the meaning for humankind is worked out in historical process which is seen to have a purposeful plan. For this reason, the historical religions have been monotheistic and exclusivist in their theologies.