Natural selection of the critical period for language acquisition

The language acquisition period in humans lasts about 13 years. After puber ty it becomes increasingly difficult to learn a language. We explain this p henomenon by using an evolutionary framework. We present a dynamical system describing competition between language acquisition devices, which differ in the length of the learning period. There are two selective forces: that play a role in determining the critical learning period: (i) having a longe r learning period increases the accuracy of language acquisition; (ii) lear ning is associated with certain costs that affect fitness. As a result, the re exists a limited learning period which is evolutionarily stable. This re sult is obtained analytically by means of a Nash equilibrium analysis of la nguage acquisition devices. Interestingly the evolutionarily stable learnin g period does not maximize the average fitness of the population.