Table 1.

Normative milestones in the development of vowel perception and production.

Age	Perception feature	Production feature
In utero	Exposure to an ambient language affects aspects of speech perception after birth (see examples below).	n/a
1–2 days	Neonates prefer vowels from the native language as opposed to vowels from a foreign language (Moon et al., 2013).	Vowellike sounds are produced in cry and in comfort vocalizations (humming and cooing). Four basic cries have been identified within the first month. Following the birth cry, these are basic cry, pain cry, and temper cry (Petrovich-Bartell et al., 1982).
2 months	n/a	Production of vowels in early vocalizations in all languages. Especially in the first month, vocalizations may take the form of quasiresonant nuclei (lacking the full resonant quality of vowels). Fully resonant nuclei appear between 2 and 4 months.
3–5 months	Vowel prosody emerges? “Cry” vs. “fuss” perceived by mothers based on peak intensity, F2, and F1:F2 ratio (Petrovich-Bartell et al., 1982).	In infants who imitate adult vowels, the vowels /i u ɑ/ become more tightly clustered from 3 to 5 months (Kuhl & Meltzoff, 1996).
6 months	Discrimination of extrinsic vowel durations in some infants (Eilers et al., 1984). Generalization of vowel exemplars from adult male to adult female or child (i.e., speaker normalization; Kuhl, 1983). Discrimination of spectrally dissimilar vowels (Kuhl, 1979).	Onset of canonical babbling based largely on consonant + vowel (CV) syllables. FSL common in babbling but may disappear only to reappear later (U-shaped developmental curve; Nathani et al., 2003).
10 months	Development of the duration distinction for PVCV (Ko et al., 2009).	Typical onset of jargon babbling, which has characteristics of intonation, rhythm, and pausing carried largely by vowels. Vowel formants in infant babbling take on characteristics of the ambient language (de Boysson-Bardies et al., 1989). 10-month-olds prefer vowels of normal duration over stretched vowels (Kitamura & Notley, 2009).
12 months	Discrimination of tense vs. lax vowels. Theories such as NLM (1993), NLM-e (Kuhl et al., 2008), and PRIMIR (Werker & Curtin, 2005) assert that experience-based perceptual reorganization occurs in the first year, leading to decreased sensitivity to nonnative contrasts and increased sensitivity to native contrasts. Infants are sensitive to mispronunciations of vowels in familiar words by as early as 14 months of age (Mani et al., 2012).	The corner vowels [i u ɑ] appear around this age or shortly after (Davis & MacNeilage, 1990; Selby et al., 2000; Templin, 1957). These vowels correspond to the natural referent vowels (Polka & Bohn, 2011), and they appear to establish the basic vowel triangle of articulation and acoustics. These vowels also meet the dual criteria of dispersion and focalization. Language-specific rhythm begins to emerge at 12 months (Post & Payne, 2018).
24 months	18–24 months—what was perceived in the 7-month time frame regarding native language gives rise to phonetic categorization leading to language learning (Werker & Hensch, 2015).	Production of diphthongs in most children. Vowel duration is adjusted for tense–lax distinction and PVCV (Ko, 2007).
36 months	n/a	PVCV generally present (Krause, 1982; Raphael et al., 1980). Vowel inherent spectral change may be present (Assmann & Katz, 2000).
4 years	n/a	Production of all nonhrotic vowels in most children and production of rhotic vowels by many but not all children. Onset of sexual dimorphism of vowel formants, with boys having lower formant frequencies than girls (Kent & Vorperian, 2018; Vorperian & Kent, 2007).
5 years	Adultlike perception categorization of PVCV (category boundary and category separation; Lehman & Sharf, 1989).	Rhythmic patterning in stress-timed languages such as English is still being refined (Post & Payne, 2018).
8 years	n/a	Postvocalic voicing—category boundary and category separation in production were adultlike by 8 years of age. Supralaryngeal anatomy is now mature (de Boer & Fitch, 2010).
10 years	Adultlike perceptual consistency of PVCV (Lehman & Sharf, 1989).	Variability in production still greater than in adults (Lehman & Sharf, 1989).
11–15 years	n/a	Voice transition in both sexes, larger change in boys (Berger et al., 2019; Maturo et al., 2012).

Note. FSL = final syllable lengthening; PVCV = postvocalic consonant voicing; NLM = Native Language Magnet Model; NLM-e = Native Language Magnet Model–Expanded; PRIMIR = Processing Rich Information from Multidimensional Interactive Representations; n/a = not applicable.