Many studies have found that infant-directed (ID) speech has higher pitch,
has more exaggerated pitch contours, has a larger pitch range, has a slower
tempo, and is more rhythmic than typical adult-directed (AD) speech. We sh
ow that the ID speech style reflects free vocal expression of emotion to in
fants, in comparison with more inhibited expression of emotion in typical A
D speech. When AD speech does express emotion, the same acoustic features a
re used as in ID speech. We recorded ID and AD samples of speech expressing
love-comfort, fear, and surprise. The emotions were equally discriminable
in the ID and AD samples. Acoustic analyses showed few differences between
the ID and AD samples, but robust differences across the emotions. We concl
ude that ID prosody itself is not special. What is special is the widesprea
d expression of emotion to infants in comparison with the more inhibited ex
pression of emotion in typical adult interactions.