HOW DO SENSITIVE PERIODS ARISE AND WHAT ARE THEY FOR

The general issue of sensitive periods in development is discussed. In the particular case of imprinting, onset of sensitivity is influenced by a specific change in state. Sensitivity ends because imprinting narrows preferences to familiar objects. The first preferences to be established are in part protected by responses such as escape from novel objects. Since these protective devices can be overcome, existence of a descriptive sensitive period can be reconciled with modification of the preferences later in development. When filial and sexual imprinting normally occur at different stages of development, the different evolutionary pressures operating on the timing mechanisms can be attributed to quite different biological roles in kin recognition. Lessons rather than general principles are drawn from the studies reviewed here. © 1979.