... believed the imprinting was irreversible and occurred during a genetically determined time period, which he called the 'critical period'. However, many other researchers have shown that young birds can imprint after the critical period if kept in isolation, or in an unstimulating environment. Researcher, Slukin (1965) coined the term 'sensitive period' and Dworetzy in 1981 defined the 'critical' or 'sensitive' period as '....times in our life when we are genetically primed to respond to certain influences, when at other times those influences would have little or no effect' (in Gross, 1987). Lorenz and other researchers induced goslings to imprint on many different objects including boxes, balls and watering cans. A further animal study involving eight separated monkeys, conducted by H.F.Harlow in 1959, produced surprising results. In each case the mother monkey was replaced in a heated enclosure by two wire 'surrogate mothers', one covered with a soft cloth, the other ...

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