Lullabies may have evolved as a way to signal to children that their needs for attention are being met, while freeing the parents to perform other tasks, such as foraging for food or caring for another offspring, Harvard scientists suggest.
Infant-directed song might later have evolved into the more complex forms of music we hear in our modern world, researchers said.
Music is a tricky topic for evolutionary science: it turns up in many cultures around the world in many different contexts, but no one knows why humans are the only musical species.
"There has been a lot of attention paid to the question of where music came from, but none of the theories have been very successful in predicting the features of music or musical behaviour," said Max Krasnow, from Harvard.
"What we are trying to do with this paper is develop a theory of music that is grounded in evolutionary biology, human life history and the basic features of mammalian ecology," said Krasnow.
At the core of their theory, Krasnow said, is the notion that parents and infants are engaged in an "arms race" over an invaluable resource – attention.
"Particularly in an ancestral world, where there are predators and other people that pose a risk, and infants don't know which foods are poisonous and what activities are hazardous, an infant can be kept safe by an attentive parent," he said.
While there is some cooperation in the battle for that resource – parents want to satisfy infants appetite for attention because their cries might attract predators, while children need to ensure parents have time for other activities like foraging for food – that mutual interest only goes so far.
Researchers predict that children should 'want' a greater share of their parents' attention than their parents 'want' to give them.
But how does the child know it is has her parent's attention? The solution is that parents were forced to develop some method of signalling to their offspring that their desire for attention was being met, said Krasnow.
"I could simply look at my children, and they might have some assurance that I'm attending to them," Krasnow said.
"But I could be looking at them and thinking of something else, or looking at them and focusing on my cell phone, and not really attending to them at all. They should want a better signal than that," he said.
Signals in form of a song is more honest due to the cost associated with them – meaning that by sending a signal to an infant, a parent cannot be sending it to someone else, sending it but lying about it, researchers said.
Mehr notes that infant-directed song provides lots of opportunities for parents to signal their attention to infants.
The research was published in the journal Evolution and Human Behaviour.