Presentations

Abstract

The relationship between lexical and grammatical knowledge in young children is
impressively strong. Indeed, the correlation between productive vocabulary and
grammar (r = .84) is larger than that between productive and receptive vocabulary (r =
.63) when measured with the commonly used Communicative Development
Inventories (CDIs). This correlation fits cleanly with usage-based theories of language,
which assume no clear distinction between the lexicon and grammar (Tomasello,
2003). However, it could also reflect separate systems that are mutually causally
related (mutualism); initially uncorrelated domains can gradually become so correlated
as to be statistically indistinguishable when they are mutually causally related (Van der
Maas et al 2006). Disentangling these accounts is complicated by the non-linear
relationship between true and measured grammatical/lexical knowledge, which is not
accounted for in traditional regression-based approaches. Here we present a new
approach to disentangling these accounts which overcomes these measurement
challenges. We examined the dimensional structure of item-level data from CDI data
on Wordbank (Frank et al. 2017) using item-response theory and the DETECT method
(Stout et al. 1996). We first considered all non-longitudinal data from the American
English subsample of Wordbank. A DETECT analysis found evidence of moderate
multidimensionality with vocabulary and grammar items clustering separately, contra
some usage-based accounts which assume no distinction between grammatical and
lexical knowledge. Given that mutualism predicts that two domains become
increasingly correlated with age, we next ran a similar analysis in separate sets of
younger (~18 months) and older (~28 months) children. These data were
unidimensional at 18 months and multidimensional at 28 months. In sum, our results did not strongly support either account described above and are most consistent with
an initially integrated lexico-grammatical system that becomes decoupled between the second and third year.

Abstract

Infant-directed speech (IDS) is typically slower, higher-pitched with greater pitch
modulation and larger vowel space than adult-directed speech (ADS) (Saint-Georges et
al., 2013). IDS may aid development of infant attention (Senju & Csibra, 2008), emotion
(Fernald, 1992) and language (Golinkoff et al., 2015), though IDS quantity (Cristia et al.,
2019) and acoustic features vary across languages and cultures (Moser et al., 2020).
One proposed source of cross-cultural variability is the time that caregivers have infant
body-contact (Falk, 2004). However, most studies involve small samples from WEIRD
populations, so cultural variability is poorly estimated. We focused on free play and
mother-infant interactions in Uganda and the UK to assess cross-cultural differences in
IDS quantity and acoustic features and test the body-contact hypothesis. In Study 1,
we calculated the proportion of free play mothers spent producing IDS and/or were in
body contact with their infant (3-9 months). In Study 2 we recorded mothers speaking
to their infant (3-6 months) and an adult experimenter, including naming objects to
elicit the corner vowels /i, u, a/. We extracted mean pitch, pitch modulation, speech
rate and vowel space measures. In contrast to the body-contact hypothesis, mothers
in Uganda and the UK produced comparable amounts of IDS, despite Ugandan mothers
spending significantly more time in body contact with their infant. Study 2 showed that
IDS was higher in mean pitch and pitch modulation than ADS in both Uganda and the
UK, but this difference was more pronounced in the UK. Speech rate for IDS was
significantly slower than ADS in Uganda, but not the UK. We found no evidence of
group level vowel-hyper articulation in either population. We discuss possible drivers
of this cultural variation in acoustic features of IDS and highlight the importance of
future work probing downstream effects of this variation on infant behaviour.