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 The Ins and Outs of Baby Talk
Linda Polka and Yufang Ruan
   It is usually no secret when there is a baby in the room. Infants attract our attention, and we immediately and instinctively change our speech when we engage with them. “Baby talk” fills the air. This distinct speech register, also known as motherese or more formally as infant-directed speech (IDS), has been observed across diverse languages and cultures. Babies demonstrate a clear preference for IDS. The strong endorsement of IDS by infants continues to fuel the curiosity of scientists, clinicians, and caregivers about this common speech form and how it shapes infant development.
In the research world, “infant” is often defined as a child under 2 years of age. In recent years, scientific interest in IDS has increased dramatically. Figure 1 shows that the number of publications on the topic of IDS and citations of this work increased markedly since 2006. In 2019, the Acoustical Society of America sponsored two well-attended special sessions devoted to IDS (see, pages 1728-1731 and 1763-1767). IDS is undeniably a hot topic.
Although producing IDS with a baby is a simple and natural task, researchers have worked long and hard to describe the distinct acoustic properties of IDS. This article shows that a great deal of progress has been made. Some acoustic properties of IDS, including aspects of vocal pitch and rhythm, are now well-established. Other acoustic properties that pertain to the vocal resonances of the speech signal, are less well understood and are currently a focus of intense research attention and debate. Explaining exactly how specific properties of IDS impact infant development is another challenge that continues to drive research activity. In this article, we also outline some of the knowledge gaps that are energizing research- ers to reach for a deeper understanding of the unique acoustic properties of IDS and to explore how IDS is con- nected with infant speech. As we learn more about IDS and why babies thrive on it, we are also finding ways to leverage this knowledge to promote infant development.
Infants Prefer Infant-Directed Speech
IDS has captivated scientists precisely because it is so effec- tive in enticing infant attention. Across many studies in
  Figure 1. Publications (blue) and citations (orange) of papers on infant-directed speech (IDS) from 1990 to 2020. From the Web of Science.
 26 Acoustics Today • Spring 2021 | Volume 17, issue 1
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