Fundamental frequency of infants’ and parents’ utterances
in longitudinal recordings
Shigeaki Amano, Tomohiro Nakatani, and Tadahisa Kondo
NTT Communication Science Laboratories, NTT Corporation, 2-4 Hikari-dai, Seika-cho,Souraku-gun, Kyoto 6190237, Japan

共Received 27 February 2005; revised 24 September 2005; accepted 5 December 2005兲
The fundamental frequencies 共F0兲 of daily life utterances of Japanese infants and their parents fromthe infant’s birth until about 5 years of age were longitudinally analyzed. The analysis revealed thatan infant’s F0 mean decreases as a function of month of age. It also showed that within- andbetween-utterance variability in infant F0 is different before and after the onset of two-wordutterances, probably reflecting the difference between linguistic and nonlinguistic utterances.
Parents’ F0 mean is high in infant-directed speech 共IDS兲 before the onset of two-word utterances,
but it gradually decreases and reaches almost the same value as in adult-directed speech after the
onset of two-word utterances. The between-utterance variability of parents’ F0 in IDS is large before
the onset of two-word utterances and it subsequently becomes smaller. It is suggested that thesechanges of parents’ F0 are closely related to the feasibility of communication between infants andparents. © 2006 Acoustical Society of America. 关DOI: 10.1121/1.2161443兴
PACS number共s兲: 43.70.Ep, 43.70.Gr 关AL兴I. INTRODUCTION
A. Infant F0

An infant’s speech development can be studied using
several approaches. One involves analyzing infant utterances
acoustically to reveal the developmental changes that occur
with age. Among the acoustic characteristics of utterances,
we focused on the fundamental frequency 共F0兲. Several variables have been used in previous research to represent the F0change that occurs during speech development. These variables include mean 共Robb and Saxman, 1985兲, mode 共Lauferand Horii, 1977兲, median 共Laufer and Horii, 1977兲, range
共Keating and Buhr, 1978兲, maximum 共Kent and Bauer,
1985兲, minimum 共Fairbanks, 1942兲, and standard deviation
共Robb and Saxman, 1985兲. The number of percentage of occurrences has also been used to determine the developmental
changes in, for example, rising, falling, and flat F0 patterns
共Robb, Saxman, and Grant, 1989兲. Of these variables, weused the mean and standard deviation of F0 in this study,
because the mean has frequently been used in previous studies and the standard deviation would be a better and morerobust indicator than the range to represent F0 variability 共cf.
Masataka, 2002兲.
Kent 共1976兲 reviewed the developmental change in the
F0 mean from birth to adulthood by collecting the results ofcross-sectional studies. The infant F0 is high at birth andgradually becomes lower as the infant grows up. The rate at
which F0 decreases is higher before 3 years of age than after
it. A difference appears in the F0 of boys and girls after theadolescent voice change of boys at 11 or 12 years of age. As
Kent 共1976兲 admitted, it was a tentative view of F0 changes
with the limited data available at that time. The actual F0mean and its developmental change might differ from that
presented in his review.
More recently, some studies have reported the precise
F0 mean and its developmental change. For example, Bennet
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Pages: 1636–1647

共1983兲 reported the developmental change in F0 in the 7–11
age range based on 3 years of longitudinal recordings. She
reported
that
the
F0
mean
decreases
about
12 Hz per 12 months. Hollien, Green, and Massey 共1994兲
studied the F0 mean of boys in their early teens around theadolescent period with 5 years of longitudinal recordings.
According to their study, the F0 mean for 34 boys was
233.1 Hz before adolescence, 173.8 Hz during adolescence,and 121.6 Hz after adolescence. This drop in the F0 mean
was also shown by the Lee, Potamianos, and Narayanan
共1999兲 cross-sectional study for 436 children in the 5–17 age
range. They reported that pubertal F0 change starts about age
12 and ends about age 15 for male children. They also reported that F0 significantly decreases in the 7–12 age range
for male and female children but not after age of 12 forfemale children.
The F0 mean was also reported for younger ages. For
example, Robb, Saxman, and Grant 共1989兲 longitudinally recorded the utterances of seven infants in the 8 – 26 month age
range on 12 occasions over 13 months. They investigated theF0 of monosyllabic and bisyllabic utterances but they foundno significant difference between months for either type ofutterance. The F0 mean was 396 Hz for monosyllables and399 Hz for bisyllables. McRoberts and Best 共1997兲 studied
the F0 mean of one infant of 3 – 17 months under three conditions, infant alone, infant with mother, and infant with father. The F0 was not dependent on the conditions butchanged significantly with age. That is, there was a generaltendency for the F0 mean to decrease month by month. It
was 407 Hz at 7 months, 330 Hz at 15 months, and 326 Hz
at 17 months.
However, longitudinal studies in infant and early childhood were conducted over much shorter periods than thosein late childhood. The longest study undertaken with infants
and in early childhood is the Robb, Saxman, and Grant
共1989兲 study consisting of observations over 1.5 years. As

0001-4966/2006/119共3兲/1636/12/$22.50© 2006 Acoustical Society of America

regards late childhood, a 3-year research project was undertaken by Bennet 共1983兲 and a 5-year study was undertaken
by Hollien, Green, and Massey 共1994兲. As a consequence,
few longitudinal data have been provided for F0 from birth
to about 5 or 6 years old. If data for this range are provided
and combined with results such as those of the Lee, Potamianos, and Narayanan 共1999兲 study in 5–17 age range, we canobtain a consecutive view of the developmental change in F0
from birth to young adulthood.
As regards F0 variability, previous studies used two
kinds of standard deviation 共SD兲. One is the within-utterance
SD of F0. This is an average of the SD of F0 in each utterance. This variable represents F0 variability within one utterance. The other is the between-utterance SD of F0. This is
the SD of the F0 mean. This variable represents the variability of the F0 mean among utterances.
Prescott 共1975兲 used these two kinds of SD to analyze
the F0 of infants. He found that the within-utterance SD of
F0 observed longitudinally increases from 28 Hz at
0 months to 53 Hz at 6 – 9 months for the same four infants.
Moreover, cross-sectional observations revealed that thewithin-utterance SD of F0 also increases from 30 Hz at
1 month to 53 Hz at 6 – 8 months with 10 infants for eachmonth. On the other hand, he found no significant increase or
decrease in the between-utterance SD of F0 in either case.
Shepard and Lane 共1968兲 reported that the within- andbetween-utterance variability of F0 were both unchanged ina longitudinal observation of two infants from0 – 4.5 months. Robb, Saxman, and Grant 共1989兲 showed thatthe between-utterance SD of F0 did not change during anapproximately 1-year longitudinal observation of infants in
the 8 – 26 month age range. Robb and Saxman 共1985兲 conducted cross-sectional research with 14 infants. They reported that the between-utterance SD in F0 is much higher at
11– 16 months than at 17– 25 months.
These studies offer some information about the developmental change in F0 variability. However, they do not provide sufficient data to depict the developmental change in F0
variability from birth to early childhood because the observation period is too short. In addition, they provided contradictory results in certain cases. For example, Prescott 共1975兲
reported an increase in the within-utterance F0 variability
but Shepard and Lane 共1968兲 did not. These problems should
be resolved by investigating F0 data in early childhood.
B. Parents F0

We also investigate another developmental aspect of F0,
namely “infant-directed speech 共IDS兲,” which is spoken to
an infant by a caretaker such as a parent. IDS is sometimes
called motherese 共Shute and Wheldall, 1989兲, maternal
speech 共Penman et al., 1983兲, infant directed talk 共Werker
and McLeod, 1989兲, or baby talk 共Singh, Morgan, and Best,
2002兲. IDS is different from “adult-directed speech 共ADS兲”
in some respects. For example, it has a higher F0 共McRoberts and Best, 1997兲, a wider F0 range 共Fernald et al., 1989兲,a shorter utterance duration 共Fernald and Simon, 1984兲,
longer pauses 共Grieser and Kuhl, 1988兲, and an exaggerated
formant frequency 共Kuhl et al., 1997兲. IDS with these charJ. Acoust. Soc. Am., Vol. 119, No. 3, March 2006

acteristics attracts an infant more than ADS 共Cooper et al.,
1997; Werker and McLeod, 1989兲. And this preference probably helps an infant to acquire its native language 共HirshPasek et al., 1987兲.
Within the characteristics of IDS, F0 seems to be an
important factor in terms of an infant’s preference. The workof Spence and Freeman 共1996兲 supports this idea. They compared an infant’s preference as regards low-pass filtered IDS
and whispered IDS. The low-pass filtered IDS contains F0
information but the whispered IDS does not. Infants preferred the low-pass filtered IDS to the whispered IDS, suggesting that F0 is an important factor in IDS. Additional
support for this idea was obtained by Fernald and Kuhl
共1987兲. They reported that infants prefer synthesized IDS
with F0 modulation to that with amplitude modulation. Thissuggests that F0 in IDS is an outstanding feature for infants.
Many of the characteristics of IDS are said to be foundin almost every language, and it is suggested that these characteristics might be universal 共e.g., Grieser and Kuhl, 1988兲.
But there are some exceptions. For instance, Ratner and Pye
共1984兲 reported that a higher F0 in IDS is not observed in
Quiche Mayan, which is spoken in the western highlands ofGuatemala. In addition, the extent of F0 change seems todiffer across languages. For example, mothers of AmericanEnglish native speakers tend to use a much more exaggerated
F0 in IDS than mothers who speak other languages 共e.g.,
Fernald et al., 1989; Papousek, Papousek, and Symmes,
1991兲. Similarly, mothers of Australian-English native speakers tend to use a more exaggerated F0 in IDS than mothersof Thai native speakers 共Kitamura et al., 2002兲. Shute andWheldall 共1989兲 reported that mothers of British-English native speakers use a high F0 in IDS, but that the increase in
F0 is less than for mothers of American-English native
speakers.
As for Japanese, IDS seems to have the same characteristics as other languages. For example, Niwano and Sugai
共2003兲 reported that mother and father use a higher F0 in
terms of mean, maximum, minimum, and range in IDS to
infants at 3, 5, and 7 months of age. Masataka 共1992兲
showed that there is a higher mean and a wider range of F0
in IDS than in ADS especially when infants do not respondto the IDS. Masataka 共2002兲 reported that the F0 of IDS toan 11-month-old infant is higher than that of ADS. He also
found that the F0 increase in IDS is larger in a female withchildren 共i.e., mother兲 than a female without children 共i.e.,
nonmother兲.
Several factors affect the F0 in IDS. One relates to people’s experience with their own babies as shown by Masataka 共2002兲. Other factors are, for example, an infant’s facial
expressions 共Reissland, Shepherd, and Cowie, 2002兲 and thecradling side 共Reissland, 2000兲. Although there may be manyother factors including language difference, previous studies
almost always provide evidence that F0 of IDS is high.
However, it is not clear how F0 in IDS changes with
infant age. There have only been a few studies on F0changes in IDS with age 共e.g., Remick, 1976; WarrenLeubecker and Bohannon, 1984兲. For example, Remick
共1976兲 studied mothers’ F0 in IDS to their infants whoseages ranged between 16 and 30 months with an approxiAmano, Nakatani, and Kondo: F0 of infants and parents

1637mately 2-month interval. Although she showed the F0change in IDS as a function of an infant’s age, the observed
F0 change might be an artifact of individual differences, because she used a cross-sectional method. Recently, Kitamura
et al. 共2002兲 observed the F0 change in IDS with a longitudinal method for infants from 0 to 12 months old. They
showed that the F0 in IDS is higher than in ADS and that itchanges with infant age. However, their observation period
was less than 1 year. It is still unknown how F0 in IDSchanges over longer periods. A greater change may be found
if the F0 in IDS is observed with infants for more than
1 year. In particular, long term observation would enable us
to discover when the special characteristics of F0 in IDS
disappear and become the same as in ADS. Little attention
has been paid to this point as regards Japanese or other languages. It was simply assumed that IDS has some special
characteristics in relation to a prevabal infant 共e.g., Sachs,
1977兲.
It is likely that F0 changes in IDS are closely related tothe speech development of infants, and that a large F0change occurs in IDS when the infant starts to use “language.” We assume that this change would occur at the onsetof two-word utterances, because then parents can communicate more easily, and probably more normally, with theirinfant using “language.” This idea is supported by the factthat Japanese mother-child conversation style becomes adultlike around the two-word utterance period 共Kajikawa,
Amano, and Kondo, 2004兲. The onset of one-word utterances
is another possible change point. However, we assume thatparents have a lot of difficulty communicating with theirinfant at the onset of one-word utterances, and would not
speak to the infant in the same way as to adults. Based on
this idea, we analyzed the F0 in IDS before and after the
onset of two-word utterances. The mean and standard deviation were used for the analysis as with infant F0. The infant’s
F0 was also analyzed before and after the onset of two-wordutterances, because this supplies additional information
about the F0 change in infant utterances, which might relate
to the F0 change in the IDS of parents.
In summary, we try to clarify the developmental changes
that take place in the F0 mean and its variability in infants’and parents’ utterances before and after the onset of twoword utterances. It would provide systematic information on
F0 development with age from 0 to 5 years old, which has
not been well studied in previous research.
II. F0 ANALYSIS

To observe the developmental change in F0, we conducted two F0 analyses of the utterances of an infant and its
parents. The first was intended to show that the F0 of infantutterances decreases month by month. It was also intended toshow the developmental change in the F0 variability of infant utterances in terms of between- and within-utterance
SD. More specifically, we compared these values before andafter the onset of two-word utterances.
The second F0 analysis concerned the fact that Japaneseparents’ F0 is different in IDS and ADS. We intended toshow that the F0 mean in IDS is high before the onset of an
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J. Acoust. Soc. Am., Vol. 119, No. 3, March 2006

infant’s two-word utterances and it has almost the samevalue as ADS after it. We also intended to show developmental change in F0 variability in IDS with regard to betweenand within-utterance SD. More specifically, we comparedthese values before and after the onset of two-word utterances and in ADS.
A. Infant F0
1. Data

An infant speech database 共Amano, Kato, and Kondo,
2002兲 was used as the data population. The database contains
speech files 共16-bit quantization and a 16-kHz sampling frequency兲 of natural utterances of five pairs of Japanese infants
and parents during daily life at home. The utterances weredigitally recorded approximately monthly for about 1 hourfrom birth to 5 years old. Infants B, C, and D were selectedfrom the infant speech database, because the number of registered utterances was larger for these infants than the others.
All the infants were born and raised in Tokyo or its
suburbs. They were all of normal height and weight at birth,and had no symptoms of disorder with respect to speech
perception or speech production during the recording period.
Although Infants C and D are siblings, we regarded theirdata as independent because their recordings were conducted
separately.
The utterances that satisfied the following conditions
were extracted from the database and used for analysis:
共i兲
共ii兲
共iii兲

The background noise should be low so that utterances are clear.
There should be no overlap with other utterances sothat F0 value is clear.
Utterances with certain particular characteristics must
be excluded in order to focus on normal utterances.
Such characteristics include hiccupping, coughing,sneezing, yawning, singing, reading aloud, and number counting.

Table I shows the months during which infant utteranceswere extracted. The mean value of the number of extracted
utterances per month was 432 共SD= 312兲 for Infant B, 406
共SD= 173兲 for Infant C, and 486 共SD= 243兲 for Infant D.
2. Procedure

A voiced/unvoiced part was specified in the extracted
utterances by using the dominance spectrum method, called“DominAnce Spectrum based Harmonics extraction
共DASH兲” 共Nakatani, Irino, and Zolfaghari, 2003; Nakataniand Irino, 2004兲. F0 was estimated for the voiced part byusing the “Ripple Enhanced Power Spectrum 共REPS兲”
method 共Nakatani and Irino, 2002, 2004兲. A 42 ms Hanningwindow with 1 ms window shift was used for both methods.
Although these methods perform very well in terms ofvoiced/unvoiced 共V/UV兲 classification and F0 estimation,
there are occasional errors. To reduce the errors as much as
possible, trained operators checked and corrected the F0 values and V/UV labels by investigating the superimposed F0
and V/UV information segment by segment on a spectrogram.
Amano, Nakatani, and Kondo: F0 of infants and parents

TABLE I. Months containing infant’s utterances in infant speech database.

Infant ID
B

C

D

Month
0, 1, 2, 3, 4, 5, 6, 8, 9, 10,
11, 12, 13, 14, 15, 16, 17, 18, 19, 20,
21, 22, 23, 24, 25, 26, 27, 28,
30, 31, 33, 34, 35, 36, 37, 39, 40,
42, 43, 44, 45, 48, 51, 52, 53
0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,
11, 12, 13, 14, 15, 16, 17, 18, 19, 20,
21, 22, 23, 24, 25, 27, 30,
31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
42, 43, 44, 45, 46, 47, 48, 49, 50,
53, 55, 56, 59, 60
0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,
11, 12, 13, 14, 15, 16, 17, 18, 19, 20,
21, 22, 23, 24, 26, 27, 28, 29, 30,
31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
41, 43, 44, 45, 46, 47, 48, 49,
51, 52, 55, 56, 59

Number ofmonths45

52

53

To exclude unusual F0 values, we used those between
20 and 1500 Hz. Although these lower and higher limits
were decided empirically, they are appropriate for covering
the F0 range of an infant utterance. The suitability of thelimits is supported by, for example, Robb and Saxman
共1985兲. They reported that the F0 mean was 357 Hz, the SD
was 105 Hz, and the range of the mean was 164– 1366 Hz
for the utterances of 14 infants between the ages of 11 and25 months.
F0 mean, SD 共within兲 and SD 共between兲 were calculated
from the F0 value for the voiced segment. F0 mean is theaverage of the mean F0 value of each utterance. SD 共within兲
is the average of standard deviation of the F0 value of eachutterance. SD 共between兲 is the standard deviation of the F0mean value among utterances. SD 共within兲 represents the F0
variability within one utterance, whereas SD 共between兲 represents the variability of the F0 mean value among utterances.
To examine the developmental change in infant’ F0, regression analysis was conducted for the F0 mean as a dependent variable 共y兲 with infant’s age in months as an independent variable 共x兲. The regression method was the best linear
unbiased estimation.
We undertook analyses of variance for the two SDs tocompare their values before and after the onset of two-wordutterances. The onset of two-word utterances was decided by
two Japanese adults by listening to recordings of infants’utterances. The definition of a two-word utterance in thisstudy is an utterance that contains two content words whose
pronunciation is clear enough to be recognized by both Japanese adults.
3. Results and discussion

F0 mean in infant utterances: Figure 1 shows the F0mean of infant utterances as a function of month. The regressions of the F0 mean were y = 368− 0.65x 关r2 = 0.109,
F共1 , 43兲 = 5.27, p ⬍ 0.05兴 for Infant B, y = 373− 1.53x 关r2
J. Acoust. Soc. Am., Vol. 119, No. 3, March 2006

FIG. 1. F0 mean and SD of infant utterances as a function of the infant’s
age in months. The vertical dotted line represents the onset of two-wordutterances for each infant.

= 0.687, F共1 , 50兲 = 109.67, p ⬍ 0.001兴 for Infant C, and y= 384− 1.11x 关r2 = 0.555, F共1 , 51兲 = 63.59, p ⬍ 0.001兴 for Infant D. The coefficient of x of the regression was significant
for all infants. 关t共43兲 = −2.30, p ⬍ 0.05, two-tailed, for Infant
B; t共50兲 = −10.47, p ⬍ 0.001, two-tailed, for Infant C; t共51兲
= −7.97, p ⬍ 0.001, two-tailed, for Infant D兴. When the data
was pooled for all the infants, the regression of the F0 mean
was y = 385− 1.33x 关r2 = 0.400; F共1 , 148兲 = 98.75, p ⬍ 0.001兴.
The coefficient of x of the regression was significant
关t共148兲 = −9.94, p ⬍ 0.001, two-tailed兴.
These negative and significant coefficients indicate that
F0 has a tendency to decrease month by month both for anindividual infant and infants as a whole. This tendency is
consistent with the results of previous studies 共e.g., Bennet,
1983; Eguchi and Hirsh, 1969; Kent and Murray, 1982;
McRoberts and Best, 1997兲. In addition, the rate of F0 deAmano, Nakatani, and Kondo: F0 of infants and parents

1639

TABLE II. SD 共between兲 and SD 共within兲 of F0 of infant utterances before/after onset of two-word utterances.
SD 共between兲 共Hz兲SD 共within兲 共Hz兲

Infant ID

BeforeAfter

Infant ID

B

C

D

Mean

B

C

D

Mean

103.9
66.1

106.6
58.3

86.5
65.2

98.6
63.0

37.2
41.9

38.1
40.6

30.7
43.3

35.2
42.0

crease is similar to that reported in some previous studies.
From the regression of the pooled data in our study, we obtain an F0 decrease rate of 16 Hz per 12 months for infantsof 0 – 5 years old. This F0 decrease rate is close to the result
reported by Bennet 共1983兲. She showed that the F0 decrease
rate is about 12 Hz per 12 months for children in the
7 – 11 year age range.
However, some previous studies have reported different
F0 decrease rates. For example, when we calculate the F0
decrease rate from the results of Eguchi and Hirsh 共1969兲, it
is 8.9 Hz per 12 months for 3 – 6 year-old children. This ismuch smaller than our result. In addition, their data indicatesthat the decrease rate is almost zero at age 7 – 9 years. Although this difference might be an artifact of the crosssectional method that Eguchi and Hirsh 共1969兲 used, it ispossible that the F0 decrease rate changes with age in childhood. At least for boys, the F0 value drops very sharply
during the period of adolescent voice change between about
12 and 15 years of age 共Hollien, Green, and Massey, 1994;
Lee, Potamianos, and Narayanan, 1999兲. However, we currently have insufficient evidence about the change in the F0
decrease rate prior to the period of adolescent voice change.
We may only say from our data that F0 has a general tendency to decrease month by month and that the decrease rateis constant from 0 – 5 years of age.
Assuming that F0 is about 300– 400 Hz for infants, theF0 decrease rate per 12 months is about 3.0%–4.0% in Bennet’s 共1983兲 study, about 2.2%–3.0% in the Eguchi and Hirsh
共1969兲 study, and about 1.9%–6.1% in the current study. Because these percentages are very small, it would be difficult
to detect the tendency of the F0 decrease when speechsamples are collected over a short period. This might be one
reason why some previous studies failed to find any significant tendency in the F0 decrease. For example, in Prescott’s
共1975兲 study, no consistent increase or decrease tendency
was observed for F0 mean between 0 months and6 – 9 months either in a longitudinal same-infant group or ina cross-sectional different-infant group. Robb, Saxman, andGrant 共1989兲 conducted longitudinal recordings 12 times
over 13 months with seven children 8 – 26 months of age, butthey did not find any significant F0 difference among
months. Robb and Saxman 共1985兲 also failed to find any
significant F0 decrease when using the cross-sectional
method with 14 infants 11– 25 months of age. The observation period was less than 1.5 years in all these studies. It issuggested that 2 or more years of observation would be necessary to obtain a significant tendency for the F0 decrease.
As regards F0 changes during the first 12 months, Shep1640

J. Acoust. Soc. Am., Vol. 119, No. 3, March 2006

ard and Lane 共1968兲 reported that the F0 mean decreased
between 0 and about 1 month and then increased and became
stable at 2 – 4.5 months. On the other hand, Fairbanks 共1942兲
showed that the F0 mean increases at 0 – 4 months and thenbecomes almost constant at 5 – 9 months. Kent and Murray
共1982兲 reported that the F0 mean of infants is 445 Hz at
3 months, 450 Hz at 6 months, and 415 Hz at 9 months.
These short-period F0 changes are not confirmed by our F0
analysis in this study, because the F0 mean fluctuates frommonth to month and its pattern differs among infants 共Fig. 1兲.
For example, the F0 mean decreases at 0 – 4 months for Infants B and D, but it increases for Infant C. Such individual
differences were also observed by Laufer and Horii 共1977兲 inlongitudinal F0 observations of four infants over the first
24 weeks. As they pointed out, individual F0 fluctuation depends on many factors including infant’s activity level, utterance skill, and physical and social environment. These factors may differ among studies. Therefore, it would be
difficult to verify the true F0 change over a short period. To
reveal the F0 change over a short period, further research isnecessary with precisely controlled conditions as regards theinfant and his/her environment.
F0 variability in infant utterances: Figure 1 shows SD
共between兲 and SD 共within兲 of F0 in infant utterances as afunction of month. Table II shows the averaged SD 共between兲 and SD 共within兲 before and after the onset of twoword utterances. We performed an analysis of variance with
one factor of period with two levels 共before vs after the onsetof two-word utterances兲. An infant factor was included in a
residual in the analysis. The factor of period was significant
for SD 共between兲 关F共1 , 146兲 = 122.3, p ⬍ 0.001兴. This indicates that SD 共between兲 is larger before the onset of twoword utterances than after it. Another analysis of varianceshowed that the factor of period was also significant for SD
共within兲 关F共1 , 146兲 = 26.8, p ⬍ 0.001兴. This indicates that SD
共within兲 is smaller before the onset of two-word utterances
than after it.
These results mean that infant F0’s are very different
from each other between utterances before the onset of twoword utterances, but this difference is reduced after the onsetof two-word utterances. On the other hand, infant F0 does
not change very much within one utterance before the onsetof two-word utterances, but it becomes more variable within
one utterance after the onset of two-word utterances.
The developmental change in F0 variability probably
reflects an infant’s ability to speak a language. Infants do not
speak a language when they are very young. During that
Amano, Nakatani, and Kondo: F0 of infants and parents

period, it is highly probable that infants do not have theability to control F0 precisely. This inability to control F0
would mean that infants could not start an utterance with aparticular F0. At the same time, they could not quickly increase or decrease the F0 within one utterance. These characteristics are reflected in the large between-utterance variability and the small within-utterance variability before theonset of two-word utterances. It might be said that infant F0
is physically driven with some randomness, rather than intentionally, before the onset of two-word utterances.
As infants grow up, they acquire the ability to control
F0 precisely. This acquisition of F0 control is critical withrespect to spoken language, because the F0 pattern conveys amessage in communication. For example, interrogative andaffirmative information is expressed by the F0 pattern. In
addition, the F0 pattern covers part of the lexical informationin Japanese 共Kubozono, 1993兲. With precise F0 control, infants can intentionally start their utterances with a fairly constant F0. And they can quickly increase or decrease the F0
within one utterance to create a complex F0 pattern whencommunicating with other human beings, mainly with theirparents. These characteristics are reflected in the decrease inbetween-utterance variability and the increase in withinutterance variability after the onset of two-word utterances.
Robb and Saxman 共1985兲 obtained very similar results
to ours regarding SD 共between兲, although they used thecross-sectional method with 14 infants. They showed that SD
共between兲 is large 共63– 238 Hz兲 at 11– 16 months and it becomes small 共53– 84 Hz兲 at 17– 25 months. Because the onset of two-word utterances corresponds to 17– 19 months in
our study, their results agree with ours in that SD 共between兲
is large before the onset of two-word utterances and subsequently becomes small. In addition, our SD 共between兲 values
before and after the onset of two-word utterances are in asimilar range to theirs. These correspondences support ourclaim that the between-utterance F0 variability is large before the onset of two-word utterances and subsequently
small.
However, some studies appear to disagree with our result. For example, Robb, Saxman, and Grant 共1989兲 showedthat SD 共between兲 does not change between 8 and 26 monthsof age. This means that the between-utterance F0 variability
is constant before and after the onset of two-word utterances
共about 17– 19 months兲. However, their study is methodologically different from ours. They observed only one- and twosyllable utterances, whereas we observed almost all types ofutterance including nonlinguistic utterances. Before the onsetof two-word utterances, there are other types of utterance inaddition to one- and two-syllable utterances, because nonlinguistic utterances cannot always be categorized as “n-syllable” utterances. Such uncategorized utterances areprobably dominant and they have a greater variety of F0
values than one- and two-syllable utterances. If the uncategorized utterances are excluded, the between-utterance F0 variability would be greatly reduced. In addition, after the onsetof two-word utterances, the uncategorized utterances would
become less common, and linguistic utterances such as oneand two-syllable utterances would become dominant. Because the one- and two-syllable utterances would have less
J. Acoust. Soc. Am., Vol. 119, No. 3, March 2006

F0 variability than the uncategorized utterances, thebetween-utterance F0 variability is small after the onset of
two-word utterances. As a consequence, the betweenutterance F0 variability would be small both before and after
the onset of two-word utterances, and no difference would beobserved between them in the Robb, Saxman, and Grant
共1989兲 study. Therefore, the Robb, Saxman, and Grant
共1989兲 results would not necessarily contradict our results.
Rather, their different results are probably caused by the limits of their utterance observation.
As for the within-utterance F0 variability, Prescott
共1975兲 reported that SD 共within兲 is 28 Hz at 0 months and
53 Hz at 6 – 9 months. Our SD 共within兲 before the onset of
two-word utterances 共35.2 Hz兲 falls in this range. Laufer andHorii 共1976兲 reported that SD 共within兲 is 27.7– 38.8 Hz at
0 – 5.5 months. Our SD 共within兲 again falls in their range.
These coincidences with previous studies support the validityof our results for within-utterance F0 variability before theonset of two-word utterances. However, because their observation periods were much shorter than ours, the support islimited. No other studies have investigated the withinutterance F0 variability after 10 months of age. Specifically,no studies have investigated the difference in the withinutterance F0 variability before and after the onset of twoword utterances. Further study is needed to check the validity of our results for within-utterance F0 variability.
One of the claims of Prescott 共1975兲 is that SD 共within兲
increases from 0 to 9 months. However, when we conducted
regression analysis with our data between 0 and 9 months
with SD 共within兲 as a dependent variable y and month as a
independent variable x, the regression coefficient of x was
not significant either for an individual infant or for infants as
a whole. Therefore, Prescott’s 共1975兲 claim as regards theincrease in within-utterance F0 variability is not confirmedby the current study. Nor is it in agreement with the results ofLaufer and Horii 共1976兲 or Shepard and Lane 共1968兲. One of
the reasons for this disagreement would be that Prescott
共1975兲 observed only an infant’s cry, whereas the current
study, Laufer and Horii 共1976兲, and Shepard and Lane 共1968兲
observed an infant’s utterances including cry and noncry. It is
possible that the within-utterance F0 variability of cry exhibits a different developmental change from that of noncry.
However, the observed increase might be an artifact of fluctuation because Prescott 共1975兲 used only two sample points
共0 – 1 months and 6 – 9 months兲. It would be better to use
more sample points along with infant age to observe any
developmental change.
B. Parents F0
1. Data

The parents of Infants B, C, and D were selected foranalysis. Infants C and D are siblings and this might causesome artifacts to appear in the data. However, because therecordings were conducted separately for Infants C and D,
such artifacts would be very small. Therefore we treat themas having different parents allowing for the possibility ofartifacts.
The months containing parents’ utterances in the dataAmano, Nakatani, and Kondo: F0 of infants and parents

1641

TABLE III. Months containing parents’s utterances in infant speech database.
Number ofmonthsInfant ID

ParentMonth

B

Father0, 2, 3, 4, 5, 6, 8, 9, 10, 11, 12, 13, 15, 16, 19, 20,
21, 22, 23, 24, 25, 27, 28, 30, 31, 33, 34, 37, 39, 40,
42, 43, 44, 48, 51, 53
0, 1, 2, 3, 4, 5, 6, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17,
18, 19, 20,
21, 22, 23, 24, 25, 26, 27, 28, 30, 31, 33, 34, 35, 36,
37, 39, 40,
44, 45, 48, 51, 52, 53
0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17,
18, 19, 20,
21, 22, 23, 24, 25, 27, 30, 31, 32, 33, 34, 35, 36, 37,
38, 39, 40,
42, 43, 44, 45, 46, 47, 48, 49, 50, 53, 55, 56, 59, 60
0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17,
18, 19, 20,
21, 22, 23, 24, 25, 27, 30, 31, 32, 33, 34, 35, 36, 37,
38, 39, 45
0, 1, 2, 3, 4, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17,
18, 19, 20,
21, 22, 23, 24, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35,
36, 37, 38, 39, 40,
41, 43, 44, 45, 46, 47, 48, 49, 51, 52, 55, 56 59
0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 11, 12, 13, 14, 15, 16, 17, 18,
19, 20,21, 22, 27, 28, 29, 31, 32, 33, 34, 39,
44, 48, 52, 59,

MotherC

FatherMotherD

FatherMother

36

43

52

38

52

35

250 Hz for a father and 500 Hz for a mother. These limits
were empirically set to exclude unusual F0 values. But they
are probably sufficient to cover the F0 range of parents’utterances, because Terasawa, Kakita, and Hirano 共1984兲 reported that the F0 mean of Japanese males is 122 Hz and thestandard deviation is 19.9 Hz, whereas the F0 mean of Japanese females is 241 Hz and the standard deviation is20.0 Hz. Moreover, F0 is about 110– 140 Hz for male adultsbase are shown in Table III. The parents’ utterances wereextracted from the database under the same conditions as
those used for the infants. Their IDS and ADS were separately extracted from the database. The mean number of extracted utterances is shown in Table IV.
2. Procedure

F0 values were obtained by the same procedure as forthe infant utterances except that the upper F0 limit was

TABLE IV. Mean and SD 共in parentheses兲 of the number of parents’ utterances, and the number of months used
for analysis before/after onset of two-word utterances.
Mean of number ofutterances per month

Numberof months

Infant ID

FatherIDS

BeforeAfter

ADS

MotherIDS

Before
After

ADS

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J. Acoust. Soc. Am., Vol. 119, No. 3, March 2006

Infant ID

B

C

D

B

C

D

Mean

69
共66兲
147
共127兲
11
共13兲

85
共67兲
171
共161兲
6
共5兲

200
共111兲
348
共173兲
3
共2兲

14

16

16

15.3

22

36

36

31.3

29

26

14

23.0

125
共101兲
72
共72兲
10
共13兲

257
共186兲
367
共202兲
5
共5兲

161
共94兲
116
共94兲
3
共3兲

17

16

16

16.3

26

22

19

22.3

31

27

7

21.7

Amano, Nakatani, and Kondo: F0 of infants and parents

FIG. 2. F0 mean and SD of infant-directed speech by parents. The vertical dotted line represents the onset of two-word utterances for each infant. The
horizontal dotted line represents F0 mean in the ADS of each parent.

and 190– 230 Hz for female adults according to the summaryof previous F0 studies provided by Hollien, Hollien, and deJong 共1997兲. The mean and SD of F0 in IDS were calculated
for each month. On the other hand, the mean and SD of F0
in ADS were calculated for all the months. This is because
ADS characteristics most probably do not change with an
infant’s age, and because the number of ADS utterances
共Table IV兲 is too small to provide reliable mean and SD
values for each month.
3. Results and discussion

F0 mean in parent utterances: Figure 2 shows the F0mean of infant-directed speech uttered by the parents of Infants B, C, and D as a function of month. The F0 mean in
IDS before the onset of two-word utterances tends to behigher than after it. Table V shows the F0 mean in IDS
before and after the onset of two-word utterances and in
ADS. As shown in Table V, the father’s F0 mean before theonset of two-word utterances tends to be 10– 20 Hz higher
than that in ADS. Similarly, the mothers’ F0 mean before theonset of two-word utterances tends to be 10– 40 Hz higher
than that in ADS. However, there seems to be no differencein F0 between ADS and IDS after the onset of two-wordutterances.
To confirm these tendencies, we conducted an analysis
of variance of F0 with one factor with three levels 共IDS
J. Acoust. Soc. Am., Vol. 119, No. 3, March 2006

before the onset of two-word utterances, IDS after the onsetof two-word utterances, and ADS兲. The father and motherwere analyzed separately because the gender difference isobvious in an adult’s F0 共e.g., Hollien, Hollien, and de Jong,
1997兲. A factor of the individual parent was included in a
residual.
For the father, there was a significant differences in theF0 among the three levels 关F共2 , 204兲 = 28.19, p ⬍ 0.001兴. An
HSD test revealed that IDS before the onset of two-wordutterances is significantly different from ADS 共p ⬍ 0.05兲 andfrom IDS after the onset of two-word utterances 共p ⬍ 0.05兲.
No significant difference was found between IDS after the
onset of two-word utterances and ADS. Similar results wereobtained for the mother. There was a significant difference
among these three levels 关F共2 , 176兲 = 38.76, p ⬍ 0.001兴. The
HSD test revealed that IDS before the onset of two-wordutterances is significantly different from ADS 共p ⬍ 0.05兲 andfrom IDS after the onset of two-word utterances 共p ⬍ 0.05兲.
No significant difference was found between IDS after the
onset of two-word utterances and ADS.
These results indicate three points. The first is that, forboth father and mother, F0 mean is higher in IDS before theonset of two-word utterances than ADS. That is, Japaneseparents use a higher F0 when speaking to their infant than toother adults. This point agrees with previous research forJapanese 共e.g., Fernald et al., 1989; Masataka, 1992, 2002;
Amano, Nakatani, and Kondo: F0 of infants and parents

1643

TABLE V. F0 mean before and after onset of two-word utterances in IDS and in ADS.
F0 mean 共Hz兲
Infant ID

Father

IDS

B

C

D

Mean

Before
After

157
147
143

158
140
141

147
137
133

154
141
140

Before
After

298
264
254

281
246
250

261
249
246

280
254
251

ADS
MotherIDS
ADS

Niwano and Sugai, 2003兲 and with other languages 共e.g.,
Fernald and Simon, 1984; Grieser and Kuhl, 1988; Jacobson
et al., 1983; McRoberts and Best, 1997; Stern et al., 1983兲.
The second point revealed by the current results is that
the F0 mean in IDS is higher before the onset of two-wordutterances than after it. That is, parents speak to their infants
with a high F0 before the onset of two-word utterances, but
not subsequently. Few studies have focused on this point andso little data is available. One exception is the work of Jacobson et al., 共1983兲. According to their data, there is nodifference in the F0 mean of IDS for infants at 4 – 8 months
共corresponding to the period before the onset of two-wordutterances兲 and at 22– 30 months 共corresponding to the period after the onset of two-word utterances兲. The F0 mean ishigh in both cases, and this is not in agreement with our
results.
The reason for this disagreement might be that our parents talked to their infant in a very natural situation in theirhome, whereas the participants in the previous study talked
to their infant in an experimental room, and the conversation
included the sentences required by the experimenter. This
would constitute a somewhat unnatural situation. Their participants probably tried to attract infant’s attention. Thismight have raised the participants’ F0 when they spoke toinfants not only of 4 – 8 months but also of 22– 30 months.
This possibility is partly supported by Shute and Wheldall
共1989兲. They showed that the F0 mean in IDS is higher infree talk than when sentences are read. Because the intention
to attract infant’s attention would be greater in free talk thanwhen reading sentences, their results suggest a relationshipbetween intention and F0 mean. This possibility is also supported by Katz, Cohn, and Moore 共1996兲. They indicated that
the F0 mean changes according to the pragmatic category of
IDS such as attention, approval, and comfort.
The third point is that the F0 mean in IDS reaches almost the same value as in ADS after the onset of two-wordutterances. That is, parents speak to their infants with thealmost same F0 value as when speaking to adults after infants start to produce two-word utterances.
This third point probably results from the fact that parents can normally communicate with their infants in thesame way as with adults when the infants start to speak language. In other words, the development of an infant’s communication ability stops parents using a high F0 in IDS. If
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J. Acoust. Soc. Am., Vol. 119, No. 3, March 2006

we use a high F0 when we speak to adults, it conveys aspecial meaning such as anger or surprise. Using a high F0 isnormally inappropriate when we communicate with adults.
When we want to communicate normally with an infant who
can speak, a high F0 would also be inappropriate. For thisreason, parents stop using a high F0 in IDS and use the same
F0 to an infant when they start to speak two-word utteranceswith which the infant can communicate with his/her parents.
It is suggested that the degree of difficulty or impossibility ofcommunication is one of the main factors inducing the highF0 in IDS. This suggestion is supported by the fact that theF0 mean is high in speech to elder people 共Masataka, 2002兲
and pets 共Burnham, Kitamura, and Vollmer-Conna, 2002兲
where communication is difficult or impossible.
In Fig. 2, the F0 mean in IDS before the onset of twoword utterances seems to decrease as a function of month.
When we performed a regression analysis of our data before
the onset of two-word utterances with F0 mean as a dependent variable y and month as an independent variable x, theregression coefficient of x was negative and significantly different from zero for four out of six parents 关t共14兲 = −3.04,
p ⬍ 0.01, two-tailed, for Infant B’s mother; t共12兲 = −2.85, p⬍ 0.05, two-tailed, for Infant C’s father; t共13兲 = −3.16, p⬍ 0.01, two-tailed, for Infant C’s mother; and t共13兲 = −6.15,
p ⬍ 0.001, two-tailed, for Infant D’s father兴. The regressionof the four parents was y = 316− 2.34x 关r2 = 0.381, F共1 , 15兲= 9.24, p ⬍ 0.01兴 for Infant B’s mother, y = 164− 0.91x 关r2= 0.384, F共1 , 13兲 = 8.10, p ⬍ 0.05兴 for Infant C’s father, y= 293− 2.35x 关r2 = 0.416, F共1 , 14兲 = 9.96, p ⬍ 0.01兴 for Infant
C’s mother, and y = 154− 0.93x 关r2 = 0.730, F共1 , 14兲 = 37.9,
p ⬍ 0.001兴 for Infant D’s father. When regression analysis
was conducted for all the fathers and mothers together, theregression coefficient of x was also negative and significantly
different from zero 关t共42兲 = −2.15, p ⬍ 0.05, two-tailed, forthe fathers; t共46兲 = −2.27, p ⬍ 0.05, two-tailed, for the mothers兴. The regression was y = 157− 0.54x 关r2 = 0.097, F共1 , 43兲
= 4.63, p ⬍ 0.05兴 for the fathers; and y = 282− 1.28x 关r2= 0.099, F共1 , 47兲 = 5.15, p ⬍ 0.05兴 for the mothers. Therefore,
we can say that the F0 mean of IDS gradually decreases
month by month before the onset of two-word utterances.
Finally it reaches the same value as ADS as described earlier
in this section.
Previous studies partly support this change of F0 mean
Amano, Nakatani, and Kondo: F0 of infants and parents

TABLE VI. SD 共between兲 and SD 共within兲 of F0 before and after onset of two-word utterances in IDS and in ADS.
SD 共between兲 共Hz兲SD 共within兲 共Hz兲

Infant ID

Father

IDS

B

C

D

Mean

B

C

D

Mean

Before
After

25.2
25.7
22.5

22.1
15.8
11.4

22.1
16.5
7.49

23.1
18.3
15.3

15.7
19.4
16.7

14.5
14.4
15.4

14.2
13.9
12.2

14.7
15.4
15.3

Before
After

61.7
44.0
29.1

58.6
37.8
23.9

40.7
35.8
11.3

53.8
39.6
25.0

34.2
34.5
37.1

31.8
27.2
28.9

23.7
27.1
17.9

30.0
30.0
31.6

ADS
MotherIDS
ADS

Infant ID

in IDS before the onset of two-word utterances. For instance,in the McRoberts and Best 共1997兲 longitudinal observationof IDS at 3, 7, 10, 15, and 17 months of age, the F0 mean of
the mother tends to decrease linearly as the infant’s age increases. This result coincides with our observations. However, the F0 mean of the father does not show a clear tendency to decrease. That is, the father’s IDS changes
nonlinearly as the infant’s age increases.
On the other hand, some studies 共e.g., Reissland, 1998;
Niwano and Sugai, 2003兲 did not show this tendency for theF0 mean to decrease. In particular, Kitamura et al. 共2002兲
reported a linear increase accompanied by an inverted U
shape trend in the F0 mean of IDS. However, their trend
analysis might be less reliable than ours, because their observation period 共0 – 12 months兲 is shorter than ours
共0 – 19 months兲 and they used fewer observation points
共5 months兲 than us 共14– 17 months, see Table IV兲. Moreover, because the high F0 in IDS must fall to a normal levelat some infant age, it is unreasonable to expect the trend toconstitute a global increase. However, a local increase would
be possible. The Kitamura et al. 共2002兲 result can be interpreted as a combination of a local increase and a global
decrease, because such a combination can realize an inverted
U shape trend, as with their results.
We might have missed finding such a local trend in our
analysis, because the F0 mean fluctuates as seen in Fig. 2.
The problem is to determine whether such a local trend really exists, and if so, at which age it exists. These remain
open questions. However, currently we can say that the F0mean exhibits a global tendency to decrease as the infant’s
age increases in months before the onset of two-word utterances.
F0 variability in parent utterances: Figure 2 shows SD
共between兲 and SD 共within兲 of infant-directed speech by theparents of Infants B, C, and D as a function of month. Table
VI shows SD 共between兲 and SD 共within兲 of the F0 in IDS
before and after the onset of two-word utterances and in
ADS. In Table VI, SD 共between兲 in IDS before the onset of
two-word utterances tends to be larger than that in IDS after
the onset of two-word utterances. And SD 共between兲 in IDS
after the onset of two-word utterances tends to be larger thanthat in ADS. These tendencies were observed for both father
and mother. On the other hand, SD 共within兲 seems to have no
obvious tendency.
J. Acoust. Soc. Am., Vol. 119, No. 3, March 2006

To test these tendencies, we conducted a one-factor
analysis of variance for SD with three levels 共IDS before theonset of two-word utterances, IDS after the onset of twoword utterances, and ADS兲. The father and mother were analyzed separately as in the analysis of the F0 mean in thepreceding section. A factor of individual parent was included
in a residual.
For the father, the main factor was significant for SD
共between兲 关F共2 , 206兲 = 14.46, p ⬍ 0.001兴. An HSD test revealed that every pair of levels was significantly different forSD 共between兲 共p ⬍ 0.05兲. However, the main factor was not
significant for SD 共within兲. Similar results were obtained forthe mother. The main factor was significant for SD 共between兲关F共2 , 178兲 = 42.63, p ⬍ 0.001兴. An HSD test revealed that every pair of levels was significantly different for SD 共between兲 共p ⬍ 0.05兲. However, the main factor was not significant for SD 共within兲.
The results reveal four points about F0 variability inparents’ utterances. The first is that the between-utterance F0variability is larger in IDS than in ADS both before and after
the onset of two-word utterances. The second is that the
between-utterance F0 variability in IDS is larger before theonset of two-word utterances than after it. The third is thatthe within-utterance F0 variability in IDS is the same as in
ADS both before and after the onset of two-word utterances.
The fourth is that the within-utterance F0 variability in IDS
shows no difference between before and after the onset of
two-word utterances.
There have been very few previous studies of F0 variability in IDS, and so little comparable data is available. One
exception is the data provided by Jacobson et al. 共1983兲.
They showed that SD 共within兲 in IDS for infants at
4 – 8 months is almost the same as that for infants at
22– 30 months. Their results are consistent with the fourth
point, because 4 – 8 months and 22– 30 months, respectively,
correspond to before and after the onset of two-word utterances. The other three points have not been previously studied and are presented here for the first time.
The four points mentioned above indicate that the extent
of F0 in IDS and ADS is almost the same within one utterance. Moreover, the F0 in IDS and ADS does not changevery much within one utterance. However, the F0 varies
greatly between utterances. This is because the whole F0 of
Amano, Nakatani, and Kondo: F0 of infants and parents

1645

an utterance is shifted upwards or downwards. This betweenutterance F0 variability in IDS decreases after the onset of
two-word utterances. One of the causes of this decrease is
the difficulty or impossibility of communication as mentioned for the F0 mean in the preceding section. However,unlike the F0 mean, the between-utterance F0 variability in
IDS does not reach the same value as that in ADS. The
between-utterance F0 variation in IDS is larger than that in
ADS even after the onset of two-word utterances. Thismeans that parents speak to infants with a variety of F0
values between utterances even after the onset of two-wordutterances at least up to 5 years old. This characteristic of F0
variability might be observed only for Japanese. Further
studies in other languages are necessary in this regard.
III. GENERAL DISCUSSION

The F0 characteristics of Japanese infants and parents
were investigated using an infant speech database. The F0
characteristics were analyzed for a large number of utterances over a very long observation period by monitoring thesame infants and parents. This study not only confirmed
some of the results of previous research on the F0 characteristics of infants and parents but also revealed new facts about
the developmental change in F0 with months of age and its
relationship to the onset of two-word utterances in terms ofmean and between- and within-utterance variability.
As regards an infant’s F0, the analysis showed that 共1兲
the mean has a tendency to decrease month by month, 共2兲 thebetween-utterance variability before the onset of two-wordutterances is larger than that after the onset of two-wordutterances, and 共3兲 the within-utterance variability before theonset of two-word utterances is smaller than that after the
onset of two-word utterances. This difference in variability
probably reflects the characteristics of linguistic and nonlinguistic utterances of infants.
With respect to the parents’ F0, the analysis revealed
that 共1兲 the mean is higher in IDS before the onset of twoword utterances than in ADS, 共2兲 the mean in IDS is higher
before the onset of two-word utterances than after it, 共3兲 themean in IDS becomes almost the same as in ADS after the
onset of two-word utterances, and 共4兲 the mean in IDS
gradually decreases month by month before the onset of twoword utterances. In addition, in terms of parents’ F0 variability, the analysis revealed that 共1兲 the between-utterance variability is larger in IDS than in ADS both before and after the
onset of two-word utterances, 共2兲 the between-utterance variability in IDS is larger before the onset of two-word utterances than after it, 共3兲 the within-utterance variability in IDS
is the same as in ADS both before and after the onset of
two-word utterances, and 共4兲 the within-utterance variability
in IDS is the same before and after the onset of two-wordutterances.
These results show that, both for infants and parents, theF0 characteristics change significantly before and after the
onset of two-word utterances. We suggest that mutual communication capability between infants and parents is closely
related to this F0 change. In other words, one of the factors
in the F0 change is the feasibility of communication between
infant and parents.
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J. Acoust. Soc. Am., Vol. 119, No. 3, March 2006

It should be noted that the change does not occur suddenly. This change is gradual as can be seen in Figs. 1 and 2.
This is probably because an infant’s language development
proceeds gradually, and the parents’ changes in utterance
proceed at a corresponding pace.
The input provided by a parent’s utterance plays an important role in an infant’s language development. In otherwords, parents’ vocal interactions facilitate an infant’s language development. The current results say nothing about
this interaction, because the utterances of infants and parents
were independently analyzed. Only the global tendency of
the utterances is shown in this study. However, if we were to
analyze a successive pair of utterances from infants and parents as McRoberts and Best 共1997兲 did, some evidence of
this interaction could be obtained. Although McRoberts andBest 共1997兲 did not find any evidence of accommodation orimitation between successive utterances from infants andparents in terms of F0 mean, it might be found were we to
use our huge and long-period utterance data. In addition,accommodation or imitation might be found in terms of SD.
These points will be investigated in a future study.
Although this study provides clear results in relation tothe onset of two-word utterances, we should be aware ofcertain limits for generalization. First, the results are basedon a small number of participants. Moreover, the parents ofInfants C and D are the same people. Because of this, theresults might only reflect the particular individual characteristics of these participants rather than general characteristics.
Second, the results are based on the free utterances of theparticipants in daily life. The utterances were not well controlled in terms of content, intention, environment, or otherfactors. This might result in fluctuations in the data as seen in
Figs. 1 and 2. Such fluctuations might hide some true F0characteristics. Finally, the data were divided into two parts
and analyzed regarding the onset of two-word utterances.
However, the onset of two-word utterances is arbitrarily set
as a boundary. Although the results are fairly clear with thisboundary and reasonable interpretations are possible, no statistical or mathematical evidence was provided for the use of
the onset of two-word utterances as the boundary. Someother developmental point, such as the onset of one-wordutterances, might provide a more valid boundary.
Further research is necessary to overcome these deficiencies. As for the first point mentioned above, increasing
the number of utterances will enable us to perform a more
precise and reliable analysis. For that purpose, we shall continue to develop the database to obtain more utterances.
However, the maximum number of participants in the database is five, and this might be insufficient to ensure validity.
Cross-sectional research should be conducted to confirm theresults obtained in this work. As for the second point, utterances are categorized according to their content, grammatical
type, and pragmatic type. If we analyzed each category, thefluctuation in F0 would be reduced and more reliable results
would be obtained. As for the final point, a more precise
statistical analysis is necessary to check the validity of theonset of two-word utterances as a boundary. The boundary
can be identified by fitting the F0 change to a second-orderfunction or a compound of flat-and decrease-first-order funcAmano, Nakatani, and Kondo: F0 of infants and parents

tions. Another way would be to conduct short longitudinalobservations with many participants over a given period withthe boundary candidates. For example, half-monthly observations from 10 to 25 months would be sufficient to checkthe validity of using the onset of two-word utterances as theboundary.
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