  In response, some non-nativists have argued that UG-based theories  are not in fact good theories of language acquisition. Tomasello  (2003: 182ff.), for instance, identifies two major areas of  difficulty for UG-based theories, such as the  principles-and-parameters approach. First, there is the  ‘linking’ problem, deriving from the fact of linguistic  diversity: almost no UG-based accounts explain how children link the  highly abstract categories of UG to their instantiations in the  particular language they happen to be   learning.[10]  His example is the category ‘Head,’ In order to set the  ‘Head parameter,’ a child needs to be able to identify  which words in the stream of noise she is hearing are in fact clausal heads. But heads “do not come with identifying tags on them in  particular languages; they share no perceptual features in common  across languages, and so their means of identification cannot be  specified in [UG]” (Tomasello 2003:183). Second, there is the  problem of developmental change, also emphasized by Sokolov and Snow, 1991. It is difficult to see how UG-based approaches can account for  the fact that children's linguistic performance seems to emerge  piecemeal over time, rather than emerging in adult-like form all at  once, as the parameter-setting model suggests it   should.[11]  In response, generativists have appealed to such notions as  ‘maturational factors’ or ‘performance  factors.’ But, Tomasello argues, such measures are ad hoc in  the absence of a detailed specification of what these maturational or performance factors are, and how they give rise to children's actual  performance.