Babies as young as three months old can distinguish sad from neutral sounding voices, according to a study.
Parts of their brains tasked with processing emotion light up more when the babies hear sad voices, found researchers at two London universities.
When looking at three to seven-month-olds, they also found they were more attuned to listening to human sounds like coughing or yawning, than familiar non-human sounds like those made by toys or water.
Evelyne Mercure of University College London said: "Our results suggest that the infant temporal cortex is more mature than previously reported.
"It is a rare demonstration that specialised areas exist in the brain very early in development."
Anna Blasi of King's College London, who also worked on the study, published online on Thursday in the journal Current Biology, added: "It is probably because the human voice is such an important social cue that the brain shows an early specialisation for its processing.
"This may represent the very first step in social interactions and language learning."
The researchers used functional magnetic resonance imaging (fMRI) to scan the activity in sleeping babies' brains. MRI scanning can pick up the rate of flow of blood to different parts of the brain, an indication of activity within it.
Declan Murphy, also of KCL, said the knowledge gleaned from the study could feasibly be used "to accurately identify babies who will go on to suffer from disorders such as autism".