If a lullaby or whispered sweet nothings help get a baby to sleep, that is more than enough for most parents.
But it seems a few gentle words could deliver another benefit: a better memory for infants.
A new study revealed that babies are much more likely to remember good times than negative experiences.
The research, published in the science journal Infant Behaviour and Development, shows that babies are more likely to remember something if there is a positive emotion, or affect, that accompanies it.
Professor Ross Flom, who lead the study at Brigham Young University in Utah, said: 'People study memory in infants, they study discrimination in emotional affect, but we are the first ones to study how these emotions influence memory.'
Although babies a few months old cannot talk, there are a number of different ways that researchers can analyse how the babies respond to different stimuli.
In this study, researchers monitored infants' eye movements and how long they look at a particular image.
The babies sat in front of a flat panelled monitor in a closed off partition and then exposed to a person on screen speaking to them with either a happy, neutral or angry voice.
Immediately following the emotional exposure, they were shown a geometric shape.
To test their memory, the researchers did follow-up tests five minutes later and again one day later.
In the follow-up test, babies were shown two side-by-side geometric shapes: a brand new one, and the original one from the study.
The researchers then were able to record how many times the baby looked from one image to the next and how long they spent looking at each image.
Babies' memories did not improve if the shape was linked to a negative voice, but they performed significantly better at remembering shapes attached to positive voices.
Prof Flom said: 'We think what happens is that the positive affect heightens the babies' attentional system and arousal.
'This can raise their ability to process and perhaps remember this geometric pattern.'
The research follows on from previous studies by the same team which looked at infants' ability to understand each others' moods, the moods of dogs, monkeys, and classical music.
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