Second Language Acquisition and the Critical Period Hypothesis

James R. Hurford Simon Kirby University of Edinburgh

Species evolve, very slowly, through selection of genes that give rise to phenotypes well adapted¹ to their environments. The cultures, including the languages, of human communities evolve much faster, maintaining at least a minimum level of adaptedness to the external, noncultural environment. In the phylogenetic evolution of species, the transmission of information across generations is via copying of molecules, and innovation is by mutation and sexual recombination. In cultural evolution, the transmission of information across generations is by learning, and innovation is by sporadic invention or borrowing from other cultures. This much is the foundational bedrock of evolutionary theory.

But things get more complicated; there can be gene-culture coevolution.² Prior to the rise of culture, the physical environment is the only force shaping biological evolution from outside the organism, and cultures themselves are clearly constrained by the evolved biological characteristics of their members. But cultures become part of the external environment and influence the course of biological evolution. For example, altruistic cultures with developed medical knowledge reduce the cost to the individual of carrying genes disposing to certain

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