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Prosody in Infant-Directed Speech Is Similar Across Western and Traditional Cultures

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When speaking to infants, adults typically alter the acoustic properties of their speech in a variety of ways compared with how they speak to other adults; for example, they use higher pitch, increased pitch range, more pitch variability, and slower speech rate. Research shows that these vocal changes happen similarly across industrialized populations, but no studies have carefully examined basic acoustic properties of infant-directed (ID) speech in traditional societies. Moreover, some scholars have suggested that ID speech is culturally specific and does not exist in some small-scale societies. We examined fundamental frequency (F0) production and speech rate in mothers speaking to both infants and adults in three cultures: Fijians, Kenyans, and North Americans. In all three cultures, speakers used higher F0 when speaking to infants relative to when speaking to other adults, and they also used significantly greater F0 variation and fewer syllables per second. Previous research has found that American mothers tend to use higher pitch than do mothers from other cultures, but when maternal education was controlled in the current study, we did not find a significant difference in average pitch across our three populations. This is the first research systematically comparing spontaneous ID and adult-directed speech prosody between Western and traditional societies, and it is consistent with a large body of evidence showing similar acoustic patterns in ID speech across industrialized populations.
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Prosody in Infant-Directed Speech Is
Similar Across Western and Traditional
Cultures
Tanya L. Broesch a b & Gregory A. Bryant c
a Simon Fraser University , Canada
b Emory University
c University of California , Los Angeles
Accepted author version posted online: 18 Oct 2013.Published
online: 16 May 2014.
To cite this article: Tanya L. Broesch & Gregory A. Bryant (2013): Prosody in Infant-Directed Speech
Is Similar Across Western and Traditional Cultures, Journal of Cognition and Development, DOI:
10.1080/15248372.2013.833923
To link to this article: http://dx.doi.org/10.1080/15248372.2013.833923
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Prosody in Infant-Directed Speech Is Similar
Across Western and Traditional Cultures
Tanya L. Broesch
Simon Fraser University, Canada and Emory University
Gregory A. Bryant
University of California, Los Angeles
When speaking to infants, adults typically alter the acoustic properties of their speech in a variety of
ways compared with how they speak to other adults; for example, they use higher pitch, increased
pitch range, more pitch variability, and slower speech rate. Research shows that these vocal changes
happen similarly across industrialized populations, but no studies have carefully examined basic
acoustic properties of infant-directed (ID) speech in traditional societies. Moreover, some scholars
have suggested that ID speech is culturally specific and does not exist in some small-scale societies.
We examined fundamental frequency (F
0
) production and speech rate in mothers speaking to both
infants and adults in three cultures: Fijians, Kenyans, and North Americans. In all three cultures,
speakers used higher F
0
when speaking to infants relative to when speaking to other adults, and they
also used significantly greater F
0
variation and fewer syllables per second. Previous research has
found that American mothers tend to use higher pitch than do mothers from other cultures, but when
maternal education was controlled in the current study, we did not find a significant difference in
average pitch across our three populations. This is the first research systematically comparing
spontaneous ID and adult-directed speech prosody between Western and traditional societies, and
it is consistent with a large body of evidence showing similar acoustic patterns in ID speech across
industrialized populations.
Communicating with infants presents adults with a pragmatic challenge. Because babies have
limited communicative competence, adults must adjust many of their behaviors to accommodate
babies’ needs. There are a variety of documented behaviors that caregivers perform to accomplish
this. One common strategy is to modify the voice in a way that not only gets a baby’s attention,
but also helps direct behavior in a desired way. Compared with how adults speak to other adults,
parents often alter their speech patterns, specifically by adjusting prosodic characteristics such as
vocal pitch (making their voice go up and down in tone), amplitude (voice loudness), and speech
rate. Adults also use a variety of other techniques, such as simplifying their speech, using diminu-
tives, and singing (Ferguson, 1977; Fernald, 1992).
Tanya L. Broesch is currently at Simon Fraser University but was a student at Emory University during data
collection.
Correspondence should be sent to Tanya L. Broesch, Ph.D., Department of Psychology, Simon Fraser University,
8888 University Drive, Burnaby, BC, Canada V5A 1S6. E-mail: tanya_broesch@sfu.ca
JOURNAL OF COGNITION AND DEVELOPMENT, 0(0):1–13
Copyright #2014 Taylor & Francis Group, LLC
ISSN: 1524-8372 print=1532-7647 online
DOI: 10.1080/15248372.2013.833923
Downloaded by [Simon Fraser University] at 20:06 16 January 2015
Cross-cultural and cross-linguistic observations with mothers reveal that similar vocal
modifications transcend cultural and linguistic boundaries, suggesting that the tendency to
modify infant-directed (ID) speech is a species-specific adaptation designed to facilitate effective
mother–infant communication. However, although simplified speech patterns have been
observed in a number of cultures around the world (Ferguson, 1964), the acoustic properties
of ID speech have not been studied systematically in small-scale, traditional societies.
Documenting the existence of particular speech modifications across diverse cultures is impor-
tant in understanding whether this behavior represents a more general human communication
phenomenon or if it is culturally specific. In the present study, we recorded and acoustically
analyzed ID speech and adult-directed (AD) speech in three disparate cultures—rural Fijians,
the Bukusu of Kenya, and middle-class Americans—and confirmed our expectation that ID
speech manifests similarly across quite different cultural groups.
ID speech can be broken down into two basic elements: simplified speech register and
acoustic modification of speech. Simplified speech register has been previously documented
in both industrialized and nonindustrialized cultures, but the acoustic properties of speech have
been reported in only large urban and industrialized societies (Ferguson, 1964, 1977). The goal
of this study was to determine if mothers in traditional societies alter their speech acoustically
when speaking to an infant compared with when speaking to an adult. Additionally, it has been
proposed that acoustic modification is more typical when addressing prelinguistic infants,
whereas simplified speech is more common when addressing older infants and young children
(Fernald, 1992). Therefore, it is important to measure acoustic features of speech when address-
ing infants to determine if vocal changes are similar across diverse societies.
Simplified speech registers to infants and young children have been observed in many
European, Asian, and African cultures and are often assumed to be a human universal (Ferguson,
1964, 1977; Fernald, 1992). Evidence for ID speech exists in urban centers worldwide, such as
Australia, Japan, Thailand, Syria, the Middle East, India, Russia, France, Bangkok, Germany,
Mexico, the United States, Canada, and Great Britain, among others (Ferguson, 1964, 1977;
Fernald, 1992; Fernald et al., 1989; Kitamura, Thanavishuth, Burnham, & Luksaneeyanawin,
2002). Ferguson (1977) summarized evidence for adult vocal modification in 15 different
languages and 23 different societies and reported a difference in ‘‘speech register’’ by adults
when addressing children, which can refer to a change in any linguistic feature of speech, such
as vocabulary or syntax. In addition, work by Watson-Gegeo and Gegeo (1986) suggested that
Kwara’ae (Melanesian group of the Solomon Islands) caregivers modified their speech register
in similar ways to ID speech found in other societies. Although these researchers reported
linguistic variation (simplification of speech) when talking to young children, they did not pro-
vide any systematic analysis of the speech in that particular culture. Their evidence consisted of
self-reports, natural observations, and interviews about ID speech.
Some of the most detailed ethnographic observations and linguistic analyses of caregiver–
infant interactions in small-scale indigenous societies have come from work in Western Samoa
(Ochs, 1986; Schieffelin & Ochs, 1986). These researchers reported many differences from
American children in the way toddlers produced language, as well as differences in the ways par-
ents spoke to their children. Interestingly, Western Samoan caregivers were reported to not ‘‘com-
municate’’ with their infants in the typical way that is observed and described in the West (Ochs,
1982). In particular, observations of Americans indicate that mothers speak directly to infants
(even a few days old) and engage in face-to-face interaction, often interpreting movements and
2BROESCH AND BRYANT
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behaviors as intentional. This is in sharp contrast to what is observed in Western Samoan
parent–child interactions where infants are not spoken to for social engagement and their beha-
viors are not treated as social acts. Infants are sung to and cooed over for distraction rather than
as a conversational partner (Ochs, 1982). However, no acoustic analysis of speech has been
reported from their work in Western Samoa, leaving open the question of whether these mothers
modify their speech in ways similar to mothers in Western cultures.
Recent research examining the perception of ID speech in traditional societies suggests that
Western ID speech is highly recognizable by speakers regardless of the language they speak
(Bryant & Barrett, 2007; Bryant, Lie´nard, & Barrett, 2012). Bryant and Barrett (2007) recorded
American mothers producing ID and AD speech in four intention categories (prohibition, approval,
comfort, and attention), and the recorded tokens had all the typical acoustic features found
in ID speech across many languages. They then presented these recordings to Shuar
hunter-horticulturalists in Amazonian Ecuador and found that they were able to distinguish ID from
AD speech as well as reliably identify the intention categories of both kinds of speech, with an
advantage in ID speech. This work showed that even when presented with speech in a language
they do not understand, adult listeners can recognize important aspects of speakers’ informative
intentions, and ID speech is particularly well suited for the job. Bryant and Barrett (2007) attributed
this cross-cultural recognition success to the form–function relationship between physical
properties of signals and senders’ communicative intentions. This relationship underlies the
structure of signaling systems not only across human cultures, but across mammalian species as
well (E. S. Morton, 1977; Owren & Rendall, 2001). Because ID speech is often used with preverbal
babies, it is the sound that matters most, and so we should expect principles of animal signaling to
be operating that apply independent of language and culture. Not only should prosodically marked
ID speech be a common strategy across most if not all cultures, but the nature of the marking should
be quite similar as a function of the common communicative goals of speakers to infants.
To date, almost no research has examined closely the acoustic features of ID speech in
traditional societies. One study looking at three Quiche mothers found no evidence of systemati-
cally higher pitch when these women were speaking to their infants, but the results are difficult to
interpret given the extremely small sample size and post-hoc nature of the speech extraction
(Pye, 1986; Ratner & Pye, 1984). There is, however, strong evidence for acoustic correlates of
different affective communicative contexts across several languages in industrialized populations.
For example, Fernald and colleagues (1989) recorded mothers speaking to their infants in
German, French, Italian, and Mandarin Chinese and acoustically analyzed the prosodic contours
of the speech. They reported that different communicative contexts elicited different prosodic
forms. For example, prohibitive utterances tended to have abrupt amplitude onsets and lower
overall pitch, but approval utterances often had higher average pitch, and greater pitch variation,
including a prominent fundamental frequency (F
0
) rise–fall contour. The acoustic properties of a
mother comforting, prohibiting, approving, or getting the attention of a young child were distinct
across these intention categories and were similar across languages (Fernald, 1989).
To better understand the cultural similarities and variations of ID speech, we explored maternal
speech to infants and adults in three culturally distinct populations. Given the vast amount of eth-
nographic literature indicating that mothers modify their speech register when addressing infants,
as well as the highly similar mappings between acoustic structure and communicative function,
we expected the acoustic properties of ID speech to be similar across all societies, with mothers
increasing their average F
0
,F
0
variation, and F
0
range when speaking to infants compared with
INFANT-DIRECTED SPEECH ACROSS CULTURES 3
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when speaking to adults, as well as mothers speaking more slowly to infants than they speak to
other adults.
METHOD
Participants and Location
Forty-three mothers were recorded producing both ID and AD speech. These paired samples
included 12 mothers from Fiji, 3 from Kenya, and 28 from the United States. The pitch profiles
of the Kenyan and Fijian mothers were highly similar, and due to the small Kenyan sample (3),
they were combined to comprise a non-Western group. The mean age of the mothers was 31;3
(SD ¼73 months >range ¼21;0–45;0) and there was no significant difference between Western
(M
age
¼32;8; SD ¼59 months; range ¼24;0–42;0) and non-Western mothers (M
age
¼29;5;
SD ¼86 months; range ¼21;0–45;0), t(24) ¼1.4, p¼.174. We collected maternal age infor-
mation for only 26 participants (11 non-Western, 15 Western) out of 43, due to experimenter error
(Western) and mothers not knowing their age (non-Western). During ID speech, mothers spoke to
their own infants, and the average age of these children was 8.3 months (SD ¼3.6 months;
range ¼2–16 months). There was no significant age difference between the non-Western infants
(M
age
¼8.6 months; SD ¼4.5 months; range ¼2–16 months) and Western infants (M
age
¼8.1
months; SD ¼2.9 months; range ¼2–13 months), t(38) ¼0.43, p¼.67. Infant age information
was missing for 3 infants in the Western group. The amount of formal education attained
by all mothers was 13;11 (SD ¼50 months; range ¼7;0–22;0), and there was a significant
difference between non-Western mothers (9;6; SD ¼24 months; range ¼7;0–12;0) and Western
mothers (16;10; SD ¼2;1; range ¼14;0–22;0), t(23) ¼8.82, p<.001. We collected education
information for 25 participants (10 non-Western, 15 Western).
An additional 30 participants (26 non-Western, 4 Western) were analyzed for ID speech only,
as we did not have recordings of AD speech for them. The mean age of these mothers was 24;6
(SD ¼77 months; range ¼17;0–41;0). The mean age of the infants was 8.5 months (SD ¼3.9
months; range ¼2–15 months). We analyzed these 30 mothers together with the 43 who were
recorded producing both ID and AD speech, comprising a total sample of 73 for a separate
analysis of ID speech acoustic features.
Fiji. The study was conducted in two distinct locations in Fiji: the Yasawa Island Group,
located in the Northwestern group of the Fiji Islands, and the Lau Island Group, located in the
Southeastern group of the Fiji Islands. These two locations are similar in that they both rely on
subsistence agriculture and marine foraging and fishing for their livelihood (Henrich, 2004;
Sahlins, 1962). Each village has a population of less than 150, and there is no secondary school
on either of the island groups. From the Fijian mainland of Viti Levu, the villages on Yasawa
Island are 1 to 2 days of travel by boat, with air access. The villages of the Lau Island group
are 5 days by boat, with no other access (no air strip). These Fijian villages were largely devoid
of Western media influences, as neither island had access to television or newspapers at the time
of this study, with the exception of the medical station in the village of Tovu, Lau, which
occasionally allowed public viewings of sports games on specific days.
Early childhood practices are similar in the two regions as children are cared for primarily
by mothers in the first few years of life with help from other female villagers, but without
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significant alloparenting (H. Morton, 1996). Typically, mothers receive help in household
domestic chores while tending to infants in the first 3 to 6 months of life. In addition, it is
not uncommon for older siblings or cousins to help out with childcare, but they rarely do in
the 1st year of life. All mothers were the self-declared primary caregivers for all of our
infant–mother dyads in this study. All participants were recruited by word of mouth after the
village elders granted consent to conduct the study in these regions.
Kenya. The study was conducted in the Eastern region of Kenya near the rift valley in the
Bungoma district. Mothers were recruited and tested in Chemwa Village, a small village with
a population of 1,220 that is composed of 198 households and is located near a large town center
(Bungoma town). Access to electricity is limited so households experience little influence of
Western media (no television or magazines). Media exposure comes in the form of national
(Kenyan) newspapers and local radio. Television and Western media exist in Bungoma town,
which is about 1 hr away by bicycle taxi. The daily life of mothers consists of childrearing and
income generation activities, as well as participation in community organizations. Most house-
holds in this region rely on small-income stores and labor on sugarcane and maize crops for a
daily wage. The socialization goals and parenting practices of mothers in this region have been
identified as distinct from Western styles of parenting in the 1st year of life, with an emphasis
on nurturing and protecting rather than psychosocial development (LeVine, 1994). All mothers
in this sample were self-declared primary caregivers.
United States. The study was conducted in a psychology laboratory in Atlanta, GA. All
mothers were contacted through a database composed of families solicited through a variety of
recruitment methods such as mailings, local day-care centers, and birthing centers. All mothers
were self-declared primary caregivers.
Recording Procedure
Mothers were video-recorded digitally (Sony DCR-SR45, Dolby Digital AC-3 audio
compression, 448 kbps, 20 Hz–20 kHz frequency response) interacting with their infants as part
of a study examining mother–infant interactions and the early social environment across cultures.
Prior to recording, informed consent was obtained by a native speaker in each of the locations.
After consent was obtained, mothers and their infants were brought to the testing location and were
seated in a quiet corner of the room or outdoor area. They were asked to interact naturally with
their infant, with the goal of keeping the infant content for 10 min (see the Appendix for text of
the instructions). Both the mother and infant were seated on the floor, with the infant facing the
mother and within arm’s reach. The mothers were asked not to pick up the infant, but touching
was allowed at the mothers’ discretion. They were also told that if the infant cried, we would stop
the camera, but if the infant fussed, they could signal to us to stop and we would only use the first
few minutes of video, which would be sufficient for the study. Mothers were not explicitly
instructed to talk or play with their infants, but instead were told to ‘‘do whatever they preferred’’
as long as the infant stayed within view of the camera. To obtain AD speech, mothers were asked
questions at the beginning and end of their infant interactions. They were asked general descriptive
questions such as the age and sex of the infant, as well as questions about their thoughts of the
interaction.
INFANT-DIRECTED SPEECH ACROSS CULTURES 5
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Audio Extraction
The first 10 s of uninterrupted vocalization directed toward the infant (and toward the adult for
AD speech) was extracted for acoustic analysis. We defined a vocalization as any utterance or
sound coming from the adult while engaging with the infant (or adult), but we did not include
sounds derived from only lip movements (e.g., speech raspberries). These vocalizations were
extracted (i-movie software) and exported as uncompressed wav audio files (44.1 kHz, 16 bit,
mono). The recording context was originally designed to capture episodes of free interaction
between mothers and infants, so some mothers had only brief episodes of AD speech. In cases
where AD speech was less than 10 s in duration, we captured any instance of AD speech
(occasionally occurring during multiple vocalizations). Background and infant noise was not
edited out at this stage, but if the infant vocalized consistently for more than 3 s, it was
considered an interruption and the next 10-s instance was captured instead.
Acoustic Analysis
Due to the spontaneous nature of the recordings, many sounds were present in addition to the
mothers vocalizing and had to be edited out. All parts of the recordings that included nontarget
vocalizations (e.g., other people talking, crying babies, overlapping speech, animals, etc.) were
removed prior to acoustic analysis, and some recorded clips were not able to be analyzed (n¼5)
due to excessive nontarget vocalizations=sounds. In the ID speech clips, 75%of the original
recordings were retained on average. For AD speech recordings, clips were shorter and more
variable in length; however, 76%of the recordings were retained in the analyses, comparable
to the ID speech samples.
Edited audio clips were analyzed using Praat Version 5.2.21 (Boersma & Weenink, 2011).
We measured F
0
,F
0
variability (F
0
SD), and minimum and maximum F
0
values using an
autocorrelation method. Octave errors and other analytical errors were removed by hand or were
fixed through pitch-setting adjustment. Default pitch settings suggested by Praat were used for
women (120–600 Hz) but changes to these settings were done on a case-by-case basis after
visual inspection of the F
0
values and never exceeded 20 Hz adjustment in the lower limit
and 60 Hz in the higher limit. F
0
SD and F
0
range were used as measures of pitch variation.
F
0
range was measured by subtracting the minimum F
0
from the maximum F
0
value for each
speaker. All F
0
values were converted to semitones (relative to 50 Hz) for statistical analyses.
Speech rate analysis was conducted on the edited clips of ID and AD recordings using a Praat
script that detects syllable nuclei automatically (De Jong & Wempe, 2009) and generates
syllables-per-second output. All files were low-pass filtered (4 kHz) to remove extraneous noise
that could affect intensity and voicing measurements used in the script algorithm.
RESULTS
We measured vocal pitch (F
0
) in mothers interacting with infants in a free interaction context and
speaking with another adult, and we expected that mothers would use higher pitch and greater
pitch range in ID speech compared with AD speech. Moreover, we expected that this difference
in speech type would be similar across cultural groups. A repeated-measures multivariate analysis
6BROESCH AND BRYANT
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of variance (MANOVA) was conducted, with speech type (ID or AD) as a within-subjects factor,
culture (Western and non-Western) as a between-subjects factor, and semitone conversions of
mean F
0
,F
0
SD, and F
0
range as dependent variables. There was a main effect of speech type,
F(3, 39) ¼4.46, p<.01, partial g
2
¼.26, for mean F
0
and F
0
SD but not F
0
range, and there
was no Culture Speech Type interaction F(3, 39) ¼2.28, p>.05. Both Western and
non-Western mothers used higher F
0
when speaking to infants than when speaking to adults,
F(1, 41) ¼11.28, p<.01, partial g
2
¼.22. Both groups also had greater F
0
variation (F
0
SD),
F(1, 41) ¼5.00, p<.05, partial g
2
¼.12. There was no significant increase in F
0
range
(max F
0
– min F
0
), F(1, 41) ¼2.74, p¼.11, partial g
2
¼.06. Western and non-Western mothers
did not differ significantly on any of the pitch dimensions, other than a marginal difference
in overall F
0
in ID speech, with Western mothers using higher overall F
0
,F(1, 41) ¼3.43,
p¼.07, partial g
2
¼.08. Recorded clips varied in the percentage of usable sound for acoustic
analysis, and this was included in the model as a covariate but was not significant (F<1). See
Table 1 for means and standard deviations.
TABLE 1
Means and Standard Deviations of Three Pitch Measurements and Speech Rate in Infant-Directed and
Adult-Directed Speech Across Non-Western and Western Cultural Groups (N¼43)
Speech type
Non-Western (n ¼15) Western (n ¼28)
Mean F
0
F
0
SD F
0
Range Syl=s Mean F
0
F
0
SD F
0
Range Syl=s
Infant-directed 252 (35.2) 55 (22.4) 243 (83.2) 3.09 (0.75) 287 (57.4) 68 (24.0) 302 (91.7) 3.09 (0.94)
Adult-directed 231 (39.7) 32 (22.9) 121 (100.0) 4.08 (0.65) 234 (37.5) 50 (21.5) 224 (107.0) 3.96 (0.74)
Note. All F
0
values are in Hertz. Standard deviations are in parentheses. Syl=s¼syllables per second.
FIGURE 1 Mean fundamental frequency (F
0
) by speech type (infant-directed speech [IDS]=adult-directed speech
[ADS]) and culture. (Color figure available online.)
INFANT-DIRECTED SPEECH ACROSS CULTURES 7
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We also measured speech rate with the expectation that mothers would speak more slowly
when addressing infants than when speaking to another adult. A repeated-measures MANOVA
was conducted, with speech type (ID or AD) as a within-subjects factor, culture (Western and
non-Western) as a between-subjects factor, and speech rate (number of syllables per second)
FIGURE 2 Standard deviation of fundamental frequency (F
0
) by speech type (infant-directed speech [IDS]=adult-
directed speech [ADS]) and culture. (Color figure available online.)
FIGURE 3 Mean speech rate (syllables=second) by speech type (infant-directed speech [IDS]=adult-directed speech
[ADS]) and culture. (Color figure available online.)
8BROESCH AND BRYANT
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as the dependent variable. There was a significant effect of speech type, F(1, 41) ¼27.07,
p<.001, partial g
2
¼.40, with both Western and non-Western mothers using a slower speech
rate when speaking to infants (M¼3.09, SD ¼0.87) than when speaking to adults (M¼4.00,
SD ¼0.71), but there was not a significant effect for culture, F(1, 41) ¼0.11, p¼.74, partial
g
2
¼.03, nor was there a significant interaction of culture and speech rate, F(1, 41) ¼0.12,
p¼.73, partial g
2
¼.003. See Figures 1 through 3 for graphs of the results.
Average pitch was marginally different across cultures, so to further explore this, we exam-
ined 30 additional mothers (26 Western, 4 non-Western) who were recorded producing only ID
speech (i.e., no AD speech data available). We conducted a separate analysis of variance includ-
ing all 73 mothers with culture (Western or non-Western) as a between-subjects factor and mean
F
0
as the dependent measure. In this model, Western mothers produced significantly higher
mean F
0
than did non-Western mothers, F(1, 71) ¼9.38, p¼.003, d¼0.80. We had education
data for a subset of this sample (n¼39), and in this subset, the mean F
0
difference remained
significant, F(1, 37) ¼6.06, p¼.02, d¼0.92, but when mothers’ education was controlled as
a covariate, this effect was eliminated, F(1, 36) ¼0.866, p¼.36.
DISCUSSION
Communicating effectively with infants is a challenge shared by caregivers worldwide. Although
there are documented differences across cultures in the ways people interact with preverbal
babies, we should expect certain patterns to emerge independent of language and culture.
Research on the prosodic features of ID speech has shown that particular features of vocalizations
arise consistently across quite different languages and cultures. But most studies of ID speech
have been done using industrialized populations with common exposure to Western media.
We measured a perceptually salient acoustic dimension of ID speech (F
0
perceived as vocal pitch)
in mothers from two non-Western, traditional societies, and found that pitch changes in ID speech
relative to AD speech were very similar in these groups, as well as other languages in which
acoustic data have been reported (Fernald et al., 1989). Mothers in all three cultures increased
their vocal pitch when talking to babies and increased their pitch variability relative to when they
spoke to another adult. Additionally, we found that mothers in all cultures slowed their speaking
rate to infants relative to speaking to another adult, which is also consistent with previous work
examining ID speech across different languages. This research constitutes the first systematic,
basic acoustic analysis of ID speech from any traditional society and supports the hypothesis that
ID speech prosody manifests similarly across disparate cultures.
Because of close relationships between acoustic variables in vocal signals and their communi-
cative functions, we should expect a great deal of cross-cultural similarity, and even cross-species
similarity, in how form follows function (Bryant & Barrett, 2007; Cosmides, 1983; Fernald, 1992;
E. S. Morton, 1977; Owren & Rendall, 2001). ID speech in humans is a rather unique communi-
cative behavior among primates, but at least one other species, squirrel monkeys (Saimiri
boliuiensis peruuiensis), also alter their vocalizations in a context-dependent manner when
signaling to immature offspring (Biben, Symmes, & Bernhards, 1989). In our closest living
relatives, the chimpanzees, some modified gestures have been observed in mothers with their
infants, but no vocalizations resembling human ID speech have been documented (Falk,
2004). Falk (2004) argued that an increased juvenile dependency period led to selection pressure
INFANT-DIRECTED SPEECH ACROSS CULTURES 9
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on mothers to be particularly skilled at maintaining close proximity to their infants through
gestures and vocalizations.
ID speech is a reliably developing trait in humans that helps solve adaptive problems asso-
ciated with effective communication with preverbal infants. We should expect cultural evol-
utionary processes to result in some differences across groups in the manifestation of any
communicative behavior. But studies of vocal emotion across cultures have revealed a great deal
of structural similarities independent of language, making many emotional vocalizations univer-
sally recognizable (e.g., Bryant & Barrett, 2007, 2008; Pell, Monetta, Paulmann, & Kotz, 2009;
Sauter, Eisner, Ekman, & Scott, 2010; Scherer, Banse, & Wallbott, 2001). The same regularities
that occur in vocal emotions across cultures also likely occur in ID speech given the close ties
between these phenomena (Fernald, 1992; Trainor, Austin, & Desjardins, 2000). Increases in
pitch and pitch variability in ID speech likely function to get infants’ attention and communicate
emotional intentions in a maximally effective way (Fernald, 1992; Werker & McLeod, 1989),
though some later developmental linguistic functions might be served as well, possibly with
trade-offs in effectiveness as the signals change over time (Fernald, 1992). A variety of factors
is at work in the shaping of maternal vocal strategies with infants, including developmental
issues and the local ecology. Although the current research does not speak to the specific role
of ID speech in cognitive development, it does support the notion that certain aspects of ID
speech function similarly across cultures given the accumulating evidence that the relative
changes from AD speech manifest quite similarly in all cultures studied to date. The current
study provides some evidence that mothers’ education can play a role in how mothers talk to
babies. We found that in a subsample of mothers for whom we had education information, cul-
tural differences in average pitch in ID speech were eliminated once the level of education was
controlled. These data suggest that mothers with higher education might use higher overall pitch
in ID speech, but not AD speech. The issue of how education, age, and culture interact with ID
speech certainly warrants further examination.
In cultures where caretakers rarely speak directly to infants, we should expect other nonvocal
strategies to fulfill common communicative goals such as getting an infant’s attention or com-
municating emotional intentions. But when caretakers are speaking to infants, similar forms will
likely emerge as a function of similar communicative goals across cultures. There still remains
the possibility for trade-offs between communicative strategies in particular contexts (e.g., Reilly
& Bellugi, 1996) or across languages. There is also evidence for functional trade-offs in prosody
between affective and linguistic forms that could have communicative importance in ID speech
(McRoberts, Studdert-Kennedy, & Shankweiler, 1995). For example, when affective prosody
is in organizational conflict with prosodic indicators of syntactic structure, speakers’ prosodic
strategies will vary depending on the age of the target child and the relationship between the
speaker and child (Kempe, Schaeffler, & Thoresen, 2010).
For decades, linguists and anthropologists have reported the richness of ID speech in great
detail—from alterations in the prosody to adjusting the content and vocabulary in an effort to
accommodate infants. Here we only explore two dimensions of a quite complex behavior. The nat-
ure of the physical recordings did not permit additional analyses of other potentially differentiating
acoustic variables such as breathiness, harmonics-to-noise ratio, or cepstral peak prominence that
correlate with perceptible voice quality changes. Future work should explore the different
communicative contexts in which ID speech is used and how context changes the various
physical properties of the signal. Studies have shown across languages that similarities across
10 BROESCH AND BRYANT
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communicative contexts do exist (e.g., Fernald, 1992), but no work has examined specific infant
behavioral responses. A continuing effort of analyzing ID speech in indigenous cultures will help
further illustrate the apparent universality of the structural features of this way of speaking to
infants, and these data, combined with more nuanced work examining the communicative func-
tions over developmental time, will help us understand this important aspect of mother–infant
interaction.
ACKNOWLEDGMENTS
We thank all of the mothers and their infants for their participation, as well as the research
assistants in Fiji, Kenya, and the Emory Infant and Child Lab. We also thank Nivja de Jong
and Jennifer Widerhorn for assistance with the acoustic analysis. Portions of this work were
presented at the 23rd Annual Meeting of the Human Behavior and Evolution Society in
Montpellier, France.
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APPENDIX
Script of Instructions for Mothers Provided Prior to Video Recording
ENGLISH
We will be videotaping you interacting with your infant. Please just do whatever you
normally do with your infant to keep him or her engaged. We will be videotaping and ask that
you continue to interact with your child=infant for 10 min. Please do not lift up your infant
at anytime as we have the camera on the infant. Please do not use any props. Thank you
very much.
12 BROESCH AND BRYANT
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SWAHILI
Tutakuwa tukichukua video yako ukihusiana na mtoto wako. Tafadhali fanya jinsi wewe
hufanya kwa kawaida ili kumfanya angalau awe anafanya jambo, yani, asisubae. Tutakuwa
tukichukua video na tunakuhimiza kuendelea kucheza na mtotowako kwa dakika 10. Tafadhali
usimuinue mwanako kwasababu tumeangalisha kamera kwake. Tafahdhali usitumievihimili
vyovyote. Asante sana.
FIJIAN=BAUAN:
O keitou na tabaka tiko na nomudrau veimaliwai kei na levumu. Keitou na qai kerea kevaka o
rawa ni o cakava ga na ka o dau cakava e na veisiga me dau vakawelei koya kina. Keitou na
tabaka me 10 na miniti na nomudrau tiko vata. Keitou kerea talega me o kakua ni o keveta
na luvemu ka ni na tabaki tiko. E kerei talega me o kakua ni vakyagataka e dua na vakawele.
Vinaka vakalevu.
INFANT-DIRECTED SPEECH ACROSS CULTURES 13
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