 Non-occurrence of structural types as negative evidence:  Suppose that a child's grammar predicted that a certain string is  part of the target language. Suppose further that that string never  appears in the data, even when the context seems appropriate.  Proponents of the unlearning problem say that non-occurrence cannot  constitute negative evidence — maybe Dad simply always chooses  to say The girl who is in the jumping castle is Kayley's  daughter, isn't she? rather than the auxiliary-fronted version,  Is the girl who is in the jumping castle Kayley's daughter?  If so, it would be a mistake for the child to conclude on the basis  of this information that the latter string is ungrammatical.