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.
