2002 , 2005a , 2005b , 2006 ). For discussion of Chomsky’s idea that language is a perfect system of optimal design, see Lappin, Levine and Johnson ( 2000a , 2000b , 2001 ), Holmberg ( 2000a ), Piattelli-Palmarini ( 2000 ), Reuland ( 2000 , 2001b ), Roberts ( 2000 , 2001a ), Uriagereka ( 2000 , 2001 ), and Freidin and Vergnaud ( 2001 ). For further discussion of the Innateness Hypothesis outlined in } 1.4 , see Lightfoot ( 1999 ), Anderson and Lightfoot ( 2002 ), Antony and Hornstein ( 2003 ), Givo´n ( 2002 ), Hauser, Chomsky and Fitch ( 2002 ) and Fitch, Hauser and Chomsky ( 2005 ); for a more critical view, see Everett ( 2005 , 2006 ) and Sampson ( 2005 ), and for a reply to such criticism, see Chomsky’s contri- butions to Antony and Hornstein ( 2003 ). For a textbook summary of perceptual evidence that very young infants may be sensitive to syntactic structure, see Lust ( 2006 , } 9.2.1 ). For evaluation of the idea that children learn languages in spite of receiving degenerate input , see Pullum and Scholz ( 2002 ), Thomas ( 2002 ), Sampson ( 2002 ), Fodor and Crowther ( 2002 ), Lasnik and Uriagereka ( 2002 ), Legate and Yang ( 2002 ), Crain and Pietroski ( 2002 ), Scholz and Pullum ( 2002 ), Lewis and Elman ( 2002 ), and Gualmini and Crain ( 2005 ). For discus- sion of the critical period in language acquisition, see Lenneberg ( 1967 ), Hurford ( 1991 ) and Smith ( 1998 , 2004 ); on Genie, see Curtiss ( 1977 ) and Rymer ( 1993 ). On evidence of a double dissociation between linguistic and cognitive abilities, see Clahsen ( 2007 ). The idea outlined in } 1.5 that grammars incorporate a set of UG principles is developed in Chomsky ( 1981 ). The Locality Principle sketched in the same section has its historical roots in a number of related principles, including the Relativised Minimality Principle of Rizzi ( 1990 ), the Shortest Move principle of Chomsky ( 1995 ), and the Attract Closest Condition of Richards ( 1997 ). The idea that grammatical differences between languages can be reduced to a small number of parameters is developed in Chomsky ( 1981 ). A complication glossed over in the text discussion of the Null Subject Parameter is posed by languages in which only some finite verb forms can have null subjects: see Vainikka and Levy ( 1999 ) and the collection of papers in Jaeggli and Safir ( 1989 ) for illustration and discussion. The discussion of the Wh-Parameter in the main text is simplified by ignoring the complication that some languages allow more than one wh-expression to be fronted in wh-questions (see Bosˇkovic´ 2002a , Grohmann 2006 and Sura´nyi