There are three theories as to how similar myth constructs are found throughout the world. The first is the theory of Diffusion, or the belief that a myth complex originated in one time and place and spread outwards through human contact. As Joseph Campbell explains, “The archaeology and ethnography of the past half-century have made it clear that the ancient civilizations of the Old World–those of Egypt, Mesopotamia, Crete and Greece, India and China–derived from a single base, and that this community of origin suffices to explain the homologous forms of their mythological and ritual structures” (Primitive Mythology, 202). While this has been established for the Old World, similarities between its myth-ritual complexes and those found in the Americas are not explained through diffusion. Most American scholars reject the idea of cross cultural contact between hemispheres before the melting of the last Ice Age. Campbell continues, “With respect, to the New World there is still raging a violent, and even cantankerous scholarly conflict of opinions” (Primitive Mythology 203).
As a counterpoint to the theory of diffusion, some scholars believe in Convergence, as explained again by Campbell in his Atlas of World Mythology, “anthropologists now commonly hypothesize an alternative explanation covered by the mystical term convergence, denoting an independent, apparently accidental development of similarities between separate cultures “(The Sacrifice 18-9). Scholars arguing for convergence suggest that pure environmental factors may explain the creation of similar myth constructs around the world. They note that plants and animals that migrate into a new region take on different and specific characteristics of other plants or animals in the same environmental region. If this can happen to plants and animals, why not to human thinking? These parallels, however, are not homologous. Overlaying biological functions onto metaphysical productions by analogy is not a sufficient methodology for separating similarities in myth-ritual systems between the hemispheres.
A third theory explaining similarities between myths around the world is called Parallelism or Inventionism. Campbell defines this theory as “a term denoting the independent development of similar elements or traits in several cultures from a common element” (The Sacrifice 28). More than accidental coincidence or sole environmental factors, Parallelism between myths is a product of the autonomous creation of images within the psyche. “Myth, like a dream, is an expression of the human imagination thus grounded in realities of the psyche and, like a dream, reflecting equally the influences and necessitites of a specific social environment, . . . which, in turn, is linked to a landscape” (The Sacrifice 28).
For Campbell, these three theories were not antonymous to each other, but co-existed. All three processes were in play. While Joseph Campbell promoted the psychological theory of myth, he turned out to be a die-hard diffusionist. Campbell makes his diffusionist arguments throughout his published works. Perhaps the best place to read his thoughts on the subject come from his Primitive Mythology (pages 202-15); Historical Atlas of World Mythology, Vol. 2, Part 1, The Sacrifice (pages 14-30); and the entire book The Flight of the Wild Gander.
In his Primitive Mythology, for example, Campbell cites Leo Frobenius, who convincingly argued that the planting villages of equatorial America were extensions from a Polynesian cultural zone. Thus, the hunting culture in early America “which had been carried into the continent from north-eastern Siberia, across [the] Bering Strait, and spread downward vertically from Alaska to Cape Horn–must have been struck horizontally by sea voyagers from Polynesia and cut through, as by a wedge” (204-5). And citing Frobenius, “In out study of Oceania it can be shown that a bridge existed, and not a chasm, between America and Asia. It would be a contradiction to all the laws of local culture of Oceania for us to assume that the Polynesians called a halt and turned back at Easter Island. And from Hawaii, furthermore, an often traveled bridge of wind currents leads to the Northwest Coast” (qtd. in Primitive Mythology 205).
Campbell wrote this fifty years ago. The dynamics of the diffusion debate have not changed in that time. European and South American scholars are more open to diffusion processes, while North American scholars remain entrenched against it. All similarities between the Old World and the New are explained away by this latter group through environmental or mental processes. It does not appear that this academic entrenchment will end anytime soon.
As for me, I agree with Campbell. All three processes—Diffusion, Convergence, and Parallelism—are part of the network of complex myth-ritual systems around the world. While the environment and psyche contribute to similarities in cultural products, the principle of Occam’s Razor often encourages diffusionist explanations. The entrenchment of academia against diffusionism is really just a high wall standing between us and our lack of knowledge of ancient history.