Mothers cradle babies on the left side because it helps them to better understand their child's emotional and physical needs, University of Sussex psychologists claim.

Research by Victoria Bourne and Dr Brenda Todd indicates that left cradling is the best way for a mother to notice and respond to a baby's behaviour, such as tears, laughter or big yawns. The position directs important infant responses to the right side of the mother's brain, the hemisphere used for emotional response. Left-sided cradling provides an advantage in the bonding process by giving the mother fast intuitive access to the baby's requirements.

Victoria explained: "The different sides of the brain do different things. For most people, the right side of the brain controls the left side of the body and helps to process emotions. Our findings suggest that, for mothers with this typical brain organisation, left cradling is the best position for interpreting a baby's well-being. The right hemisphere of the brain has also been shown to be better than the left at tasks needing external attention."

A popular myth for the occurrence of left-biased cradling focused on whether the mother was left or right-handed. Victoria said: "We wanted to look past this anecdotal assumption. Large-scale studies where handedness measures were made showed that a significant number of left-handed women also cradle on the left side."

Previous research showed that a bias of 70 to 85 per cent of women and girls cradle babies to the left. The Sussex team carried out a two-part experiment with more than 30 undergraduates to assess the idea of a link between right hemisphere emotional response and left cradling.

First, each student was observed picking up a life-like, baby-sized doll, a type used as a newborn substitute in midwifery training. Next, each volunteer was asked to look at a selection of facial images designed to reveal whether they use the right or left side of the brain for processing emotions.

Victoria said: "The results for women were very clear. The way a woman cradles a child is governed by which hemisphere of her brain is processing the stimuli she receives from the baby.

"There was no similar difference for the male volunteers. One possibility is that it is only when men become fathers they adopt this method of communication with their newborn. Brenda and I plan to further explore this line of research, particularly to look at the relevance of socialisation on how the brain processes information."

Victoria and Brenda's paper, 'When left means right: an explanation of the left cradling bias in terms of right hemisphere specializations', was published in the January issue of Developmental Science.