change at a much faster rate than the physical environment, which had previously been paramount in steering the course of evolution. No correlation between brain size and language size is likely to be straightforward, but it would be surprising if there were no correlation at all.
Culture in the modern era is evolving at a much faster rate than at any previous stage. The 20th century, in particular, has seen dramatic changes in the human (largely human made) environment. Even if the rates of biological and social evolution were ever coordinated in the way our model proposes, it is quite possible that they have become uncoordinated in the modern era. If we include acquisition of the conventions of written language as part of language acquisition, modern language acquisition takes longer than the time to puberty (see Miller & Weinert, 1998). This may be an instance of cultural evolution racing along so fast that it is now impossible for biological evolution to adapt.
Our model depicts a self-feeding spiral of language size responding to increases in speed of acquisition and speed of acquisition in turn responding to increased language size. Where will it end? It cannot go on for ever. At some stage, considerations external to the closed system of our model will exert an influence. Language acquisition cannot be speeded up indefinitely; there must be some cost. Languages will not expand indefinitely in size; there must be some principle of diminishing returns for increased size. It is possible that the evolution of human language in the modern age has reached the point where such external factors of cost and benefit, which we have not modeled, have come into play.
This work was supported by two fellowships at the Collegium Budapest Institute for Advanced Study, and by Economic and Social Research Council research grant R000326551. We thank our colleagues at the Collegium Budapest, Eörs Szathmary and Axel Kowald for their helpful input.
Baldwin, J. M. ( 1896). A new factor in evolution. American Naturalist, 30, 441-451.
Boyd, R., & Richerson, P. J. ( 1985). Culture and the evolutionary process. Chicago: University of Chicago Press.
Cavalli-Sforza, L. L., & Bodmer, W. F. ( 1971). The genetics of human populations. San Francisco: Freeman.