Editing Fis phenomenon (section)

=== Invariant order of phonological development ===
A 1941 study by Roman Jakobson hypothesized that children who speak English follow a basic phonological order when acquiring their language's feature distinctions and, more strongly, that some elements of this order are sequential with respect to others, ''i.e.'', that for some distinctions, a child cannot fully acquire that distinction unless the child has already learned one or more some specific other distinctions. In a 1948 study, Schvachkin hypothesized that Russian-speaking children develop phonetic distinctions in an invariant order. A table is then shown where the “hushing” vs “hissing” sibilants were second to last on the order of acquisition.<ref>Moore, Timothy E. Moore (1973). ''Cognitive Development and the Acquisition of Language''. Harcourt Brace Jovanovich. pp. 215, 216, 217.</ref> Given that children are still refining and working on their phonetic production skills, it might be the case that these features are not produced accurately because previous ones still need work.