New research published in PLOS One suggests that the vocalizations babies make in their first year of life follow specific patterns, challenging the long-held belief that these early sounds are random. By analyzing all-day home recordings of infants, researchers found that babies actively explore different types of sounds, even when not prompted by adults. These early vocalizations often occur in clusters, hinting at an organized process of vocal development that may be crucial for later speech and communication.
While much research has focused on how social interaction and imitation of caregivers contribute to language learning, less attention has been given to babies’ own vocal exploration. The researchers wanted to investigate whether babies practice making different kinds of sounds independently of adult interaction, and if these sounds occur in patterns rather than randomly. By understanding how and why babies produce these sounds, scientists hope to uncover foundational processes that support language acquisition, as well as develop new ways to track healthy vocal development in early infancy.
“Infants’ endogenous vocalizations and their exploration of different vocal types serve as foundations for subsequent language development, and the exploration surely influences social interaction and imitation. Despite its significance, exploratory practice of vocal types has received very little attention in the literature until our recent papers, particularly when contrasted with the literature’s emphasis on social interaction and imitation,” said study author Hyunjoo Yoo, an assistant professor at The University of Alabama and director of the Alabama Baby Communication (ABC) Lab.
“Moreover, our understanding of vocal development during the precanonical stage, which occurs before infants produce canonical babbling syllables such as ‘ma’ or ‘ba’ (typically emerging around 5–7 months), remains limited. Given the theoretical and clinical importance of spontaneous vocal production and exploration, coupled with the scarcity of research on this topic, we embarked on an extensive empirical investigation into vocal type exploration and practice.”
The study involved 130 typically developing infants, who were monitored using day-long audio recordings captured in their homes. These infants were part of a larger study on vocal development conducted by the Marcus Autism Center in collaboration with Emory University. The researchers collected 1,154 recordings, representing an average of 8.9 recordings per infant. Each recording was randomly sampled to analyze 21 five-minute segments, allowing the researchers to observe vocalizations at different times of the day.
The key sounds the researchers focused on were what they called “protophones,” which include non-linguistic vocalizations that babies make in their first year. These protophones were divided into three main types: vocants (vowel-like sounds), squeals (high-pitched sounds), and growls (low-pitched or harsh sounds). Coders, trained to identify these categories, systematically analyzed the recordings to detect patterns of clustering—that is, periods where one type of sound, such as squeals, occurred repeatedly in a short span of time. They also tracked whether these patterns changed as the infants aged.
The researchers discovered that infant vocalizations are far from random. Instead, they found evidence of “clustering”—the repeated occurrence of specific sound types within particular time periods. Approximately 40% of the infants’ recordings showed significant clustering of squeals or growls, meaning that babies produced these sounds in noticeable bursts rather than distributing them evenly across the day.
Interestingly, the study found that this clustering happened whether or not adults were actively interacting with the child. This suggests that infants are practicing vocalizing independently, engaging in what the researchers called “vocal play.” This play may serve as a form of exploration, allowing infants to experiment with and refine their ability to control their voices.
“Our findings reveal that infants engage in practice with various vocal types from the earliest months of life, indicating their efforts to consolidate specific dimensions of vocalizations,” Yoo told PsyPost. “Through endogenous practice with these categories, infants learn to produce distinct vocal types, laying a foundation for further language development.”
The results showed that clustering patterns varied with age. While researchers had initially expected the most clustering to occur around 3–4 months, a period often thought of as the beginning of vocal play, they found that clustering occurred at all ages.
“Based on prior literature about vocal play, we expected either the emergence or the highest proportion of vocal type clustering to occur around 3–4 months,” Yoo explained. “However, we discovered that this age range did not exhibit the highest proportion of significant clustering. Interestingly, our data indicated that clustering occurred across all age groups (including the very youngest group), with some increase in rates from 5 months onward. Nonetheless, due to the limitation of our simplified coding scheme, we are not inclined to view this pattern of age results as offering the final word about vocal category clustering.”
The highest rates of clustering, particularly for squeals, occurred after 5 months. Growls, on the other hand, showed more consistent clustering across all ages. This might suggest different developmental timelines for controlling high-pitched versus low-pitched sounds, although more research is needed to confirm this.
“We were surprised by the distinct developmental trajectories of squeals and growls,” Yoo said. “Notably, there were relatively stable significant proportions of growl clustering across the six age groups, contrasting with higher proportions of squeal clustering beyond 5 months of age. We speculated that this discrepancy could be linked to the development of phonatory control, as squeals are produced in high pitch, potentially requiring particularly advanced vocal fold control compared to low-pitched growls. However, these speculations warrant further investigation into the development of vocal fold control.”
Despite its valuable insights, the study has some limitations. The coding system used to categorize vocalizations simplified the infants’ sounds into three broad categories (vocants, squeals, and growls), but infant vocalizations are more complex than this. More detailed analysis of subcategories of sounds—such as different types of squeals or growls—could provide a deeper understanding of how infants develop control over their voices.
This line of research has the potential to yield significant practical applications, particularly in identifying early markers for communication disorders.
“One of the primary reasons for our pursuit of the question of clustering is that it could turn out to be a useful marker in screening for disorders of communication,” Yoo explained. “Recently, we studied clustering in typically developing infants and infants later diagnosed with autism spectrum disorder (ASD). We found that both typically developing infants and infants later diagnosed with ASD showed clustering patterns in their first year, suggesting that all the infants in this study appeared to be practicing vocal categories. The results provide support for the idea that vocal play and clustering of vocal types may be a fundamental property of human development, laying a deep foundation for later speech and language development.”
“Based on this finding, we could envision two follow-up studies. First, a follow-up study would be an investigation of vocal category exploration in developmentally delayed infants to see if there is a difference. Second, we would compare American and Korean-learning infants on the degree to which they cluster infant vocal categories. We have already explored this topic with a small portion of the data (11 infants from each language) and found no cross-language difference, as clustering of vocal types may be a fundamental property of human development. However, it would be more interesting and reliable to see this conclusion with large-scale data.”
The study, “Infant vocal category exploration as a foundation for speech development,” was authored by Hyunjoo Yoo, Pumpki Lei Su, Gordon Ramsay, Helen L. Long, Edina R. Bene, and D. Kimbrough Oller.
