The Westend Lectures on Brain and Cognition with William Idsardi
How is Phonology different (from Syntax)?

One way in which phonology might be different from syntax (Bromberger & Halle 1989) is in its
formal computational properties; that is, the computational or logical complexity of phonological
patterns might be strictly less than that of the patterns observed in syntax (Heinz & Idsardi 2011,
2013, Berwick & Chomsky 2016). Hauser, Chomsky & Fitch 2002 (HCF) propose that recursion
is the distinguishing characteristic of syntax; this proposal is consistent with the observation that
syntax exhibits patterns beyond context-free power (Chomsky 1956, 1957, 1959, 1963, Joshi,
Vijay-Shanker & Weir 1990), whereas all known phonological patterns are strictly sub-regular
(Heinz & Rogers 2013). A toy example of a recursive Merge system (Chomsky 1995, 2000, 2007,
2008; Uriagereka 1998, 2008, 2012) is developed with non-recursive phonological and semantic
representations (see also Pietroski 2011, 2012, 2017). Some proposed examples of recursive
structures in phonology are examined and criticized, and an alternative analysis is suggested, in
which lexical Merge is distinguished from general Merge. Finally, some evolutionary speculations
are considered, and phonology is suggested as the appropriate language analogy for birdsong.