  It has been claimed that within a language, morphologically irregular words
are more likely to be phonotactically simple and morphologically regular words
are more likely to be phonotactically complex. This inverse correlation has
been demonstrated in English for a small sample of words, but has yet to be
shown for a larger sample of languages. Furthermore, frequency and word length
are known to influence both phonotactic complexity and morphological
irregularity, and they may be confounding factors in this relationship.
Therefore, we examine the relationships between all pairs of these four
variables both to assess the robustness of previous findings using improved
methodology and as a step towards understanding the underlying causal
relationship. Using information-theoretic measures of phonotactic complexity
and morphological irregularity (Pimentel et al., 2020; Wu et al., 2019) on 25
languages from UniMorph, we find that there is evidence of a positive
relationship between morphological irregularity and phonotactic complexity
within languages on average, although the direction varies within individual
languages. We also find weak evidence of a negative relationship between word
length and morphological irregularity that had not been previously identified,
and that some existing findings about the relationships between these four
variables are not as robust as previously thought.
