Children who cradle dolls on the left perform better in social ability tests than their peers, a study has found.
The findings suggest the children's cradling preference could help to indicate some social developmental disorders.
The study builds on previous knowledge of a 'left-cradling bias' - the phenomenon that humans will typically cradle a baby on their left side, enabling both parent and child to keep the other in their left visual field - which is unrelated to dominance of the use of right or left hand.
Information from the left visual field is processed by the right hemisphere of the brain, which is associated with emotion and the perception of facial expression.
Further investigation could enable researchers to make important predictions about the trajectory of children's development based on their cradling responses, in association with social and communication abilities.
The research, led by Gillian Forrester of Birkbeck, University of London and Brenda Todd of City, University of London in the UK was conducted with 98 typically developing children (54 girls and 44 boys) in reception or year one at a mainstream reception school in South London, who were given a human infant doll to cradle.
They were observed to hold the doll markedly more often in a left-cradling position, and those who showed this bias had a significantly higher social ability score compared with those who held the doll on the right.
The social ability traits tested including likeliness to follow rules, willingness to share with others and wanting to please their teachers.
As part of the study the children were also given a pillow to cradle, with three dots marked on to suggest a face.
They were more likely to cradle this object on the left, which researchers say indicates the depth of the evolutionary bias, as even a hint of a face will trigger the response.
By contrast, when given a plain pillow, without a suggestion of a face, the children demonstrated neither a left nor right cradling bias.
"Even babies recognise the simple design of three dots surrounded by a circle as a face. And faces receive special attention from our left visual field (connected to the right hemisphere), which is faster and more accurate at identifying individuals and their emotional expressions than the right visual field for the majority of the population," Forrester said.
"This left-visual-field bias is a natural ability, thought to have originated from a need to identify predators in the environment. In modern humans we believe that the left visual field bias for recognising faces and expressions supports our sophisticated social and emotional abilities," he said.
"The phenomenon, known as the 'left cradling bias', is not just present in humans - it is pervasive across the animal kingdom and found in species as different as gorillas and flying foxes. Keeping a baby in the carer's left visual field allows for more efficiently monitoring of the baby's wellbeing," he said.
The left cradling bias was also seen when children held a human baby doll, indicating that this behaviour is present early in development and you do not need to have had experience of holding babies to express this preference, researchers said.
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