Phonological variation in child-directed speech is modulated by lexical frequency
Eon-Suk Ko1 & Jongho Jun21
Chosun University, 2 Seoul National University
How does a child discover the underlying form of a morpheme under phonological variation?
We address this question by investigating the production of Korean noun stem-final
obstruents /th/, /ch/, and /ph/ before a vowel-initial suffix (Ko 1989; Kang 2003; Jun 2010) inchild-directed speech (CDS) compared to adult-directed speech (ADS). The accommodation
hypothesis of CDS (Furguson, 1964; Kuhl et al., 1997; Fernald, 2000) might predict a higher
ratio of canonical forms in CDS than their variants. Another factor influencing phonologicalvariation, though largely overlooked in CDS research, is lexical frequency (Bybee, 2003;
Gahl, 2008), which would predict a greater proportion of canonical forms in high frequency
words in morpho-phonological phenomena. The present study conducts two experiments
comparing the proportion of the canonical vs. variant forms ([s], [ch], [t], [th], [c], [p]), andperforms analyses to tease apart the effect of speech register and lexical frequency. Our
results show that CDS contains a higher ratio of canonical forms, but that it is a fortuitous
outcome driven by the adoption of high frequency word items in CDS rather than reflecting
linguistic accommodation by caregivers.
In Experiment 1, 12 Korean mothers performed word-teaching tasks to their infants(M=0;10.3) and another adult using a picture book. The target words for CDS (k’och ‘flower’,
path ‘field’, suph ‘woods’) and ADS (koch ‘lynx’, sath ‘reed mat’, seoph ‘brushwood’) were firstread and then explained. The target phonological alternations were extracted based ontranscription. We fitted a mixed effect logistic regression model with the realization of theCANONICALITY as the dependent variable (canonical=1, variant=0). Fixed effects were
REGISTER (CDS, ADS), CODA (/ph, ch, th/), VOWEL (/i, ɨ/), and STYLE (reading, spontaneous).
We also included by-item and by-subject random intercepts. The coefficient of REGISTER, ourmain interest, suggests that the rate of canonical form realization is higher in CDS than in ADS.
The register effect, however, is confounded with lexical frequency since the target words in
CDS are common words whereas those in ADS are rare, which calls for a follow-up experiment.
In Experiment 2, 12 college students produced 24 types of high- and low-frequency
target nouns as determined by set thresholds in three tasks: reading, explaining semantic
minimal pairs, and the map tasks (Anderson et al., 1991). A mixed effect logistic regression
model was fitted to the phonological alternations with the same dependent and randomvariables as Exp 1, and the fixed effects including FREQUENCY (low, high), TASK (reading,minimal pair, map), SYLLABLE COUNT (monosyllabic, disyllabic), CODA (ph, ch, th) and VOWEL(i, ɨ). The coefficient of FREQUENCY, our main interest, suggests that high-frequency words are
more likely to be pronounced with the canonical form.
We conducted a further analysis comparing the proportion of canonical forms in theproduction of the two monosyllabic high frequency words, i.e. /k’oth/ ‘flower’ and /path/ ‘field’,
which are common target words across the CDS condition of Exp 1 and ADS in Exp 2. A mixedeffect logistic regression was fitted with same dependent and random variables as Exp1 andExp2, and the fixed effects REGISTER (cds, ads), TASK (reading, spontaneous), VOWEL (i, ɨ, e),and CODA (ph, ch, th) (Table 3). The results show that the main effect of REGISTER is not
significant. Further, its negative coefficient (-0.348) indicates a lower rate of canonical forms
in CDS, contrary to what would be expected by the accommodation hypothesis.
Our results indicate that CDS does provide children with a simplified and enhancedphonological model, but that caregivers’ use of a higher ratio of canonical forms is essentiallya consequence of adopting high frequency word items in CDS. Reported lack of phonologicalenhancement in CDS in previous research (e.g. Buckler et al., 2018) could be attributed to theperceptual difficulty of disambiguating the canonical form from its variant (e.g.

gree[n]~gree[m] beans) even if a higher ratio of canonical forms had been provided. Our study
thus highlights the importance of considering the usage factors and morpho-phonologicalphenomena in investigating the function and nature of CDS (Cychosz et al., 2021).

Lexical frequency(b) High frequency words resist
change in morpho-phonological rules(Exp 2).

(a) CDS uses high frequency
lexical items (AdditionalAnalysis).

Register (Caregivers’
use of CDS)(c) CDS phonology is less variable than ADS (Exp 1).
.
(c’) The effect of CDS on phonological variation loses
significance when the frequency effect is controlled for(Additional Analysis).

Phonologicalvariation in morphophonological rules

Figure 1. Mediation model. (a) represents the effect of register on lexical frequency (Additional Analysis,Lexical Frequency in CDS and ADS), (b) represents the effect of lexical frequency on phonological variation inmorpho-phonological rules (Exp 2), (c) represents the direct effect of register on phonological variation (Exp 1),(c’) represents the indirect effect of register on phonological variation, mediated by the frequency effect(Additional Analysis: Monosyllabic High Frequency Words in CDS and ADS, Common Lexical Items across
CDS and ADS).
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