Babies have a rudimentary grasp of maths long before they can walk or talk, according to new research.
By the age of seven months infants have an abstract sense of numbers and are able to match the number of voices they hear with the number of faces they see.
The research could be useful in devising methods for teaching basic maths skills to the very young, say researchers in the US.
The study is published in Proceedings of the National Academy of Sciences.
Look and listen
Adults can easily recognise the numerical equivalence between two objects they see and two sounds they hear.
This is also the case for some animals, such as the monkey, but until now there has been conflicting evidence about the ability of very young babies to do this.
Kerry Jordan and Elizabeth Brannon of Duke University, Durham, North Carolina, played a video of two or three adult women strangers simultaneously saying the word "look" to babies aged seven months.
The videos were displayed on two monitors positioned side-by-side as the babies sat on a parent's lap. Audio tracks, synchronised with both videos, were played through a hidden speaker.
On average, the infants spent a significantly greater proportion of time looking at the display that matched the number of voices they heard to the number of faces they saw.
"Our results demonstrate that by seven months of age, infants can represent the equivalence between the number of voices they hear and the number of faces they see," the scientists wrote.
"The parallel between infants' and rhesus monkeys' performance on the task is particularly striking."
The research suggests that there is a shared system between infants before they learn to talk and non-verbal animals for representing numbers.
Understanding more about this system could be useful in devising methods for teaching basic maths skills to the very young.
"The study asks important questions about numerical abilities in infancy," Dr Anna Franklin of the Surrey Baby Lab, Department of Psychology, University of Surrey, UK, told the BBC News website.
"The findings support the argument that young infants are capable of a wide range of mental operations and that infants are smarter than we think."