A computer programme was trained to spot the difference and it could then find it – in less than one second – in mums speaking other languages.
The difference was found in 12 non-English speakers communicating in languages including Hebrew, Mandarin and Cantonese.
Dr Piazza told the BBC: “There is wide-ranging research showing infants learn better from infant-directed versus adult-directed speech.
“Specifically they can segment words into syllables better and they can learn novel words better and that probably encompasses these timbre features.”
The study did not look at dads or grandparents, but the researchers anticipate similar timbre adjustments.
Prof Jenny Saffran, from the University of Wisconsin-Madison, commented: “This is the first study to ask whether [mothers] also change the timbre of their voice, manipulating the kinds of features that differentiate musical instruments from one another.
“This is fascinating because clearly speakers are not aware of changing their timbre, and this new study shows that it is a highly reliable feature of the way we speak to babies.”