WHEN THEY cradle a baby, 80 per cent of women hold it against the left
side of their body, regardless of whether they are left- or right-handed.
Now researchers have discovered the female chimpanzees and gorillas show
the same bias (Animal Behaviour, vol 39, p 1224). The behaviour may be telling
us that the brains of the great apes are organised in a very similar way
to humans, with the left and right side carrying out specialised jobs.
John Manning and Andrew Chamberlain of the University of Liverpool wondered
whether cradling a baby on the left-hand side was unique to humans. In the
course of their research, they studied chimpanzees, gorillas and orang-utans
and gibbons in zoos, and looked at films and photographs of these species
in the wild. They found that most chimpanzee and gorilla mothers, and probably
also orang-utans, have a strong preference for the left-hand side.
According to Manning and Chamberlain, this indicates that the behaviour
probably originated in the common ancestor of African apes and humans, between
6 and 8 million years ago. If this is so, it predates by some time the origin
of a bias in humans towards right-handedness. This is a characteristic that
appears to be unique to our species.
The reason why left-handed cradling has evolved is controversial. The
traditional explanation is that it places the baby over the mother’s heart,
and that the sound of the heartbeat keeps the baby quiet and manageable.
Manning and Chamberlain are not happy with this explanation, however.
For a start, the two researchers are not convinced that the heartbeat
really is localised on the left-hand side of humans. The heart’s sound comes
from the closure of its valves, they say, and these are situated almost
directly under the sternum, in the centre of the chest.
Manning and Chamberlain are also not convinced that a cradled baby can
easily hear its mother’s heartbeat. They say that the preference for cradling
a baby on the left side is seen even in African mothers who carry their
infants on their backs while they carry out their work throughout the day.
Manning and Chamberlain also take issue with Lee Salk, who in the 1960s
and 1970s showed that the noise of a heartbeat will quieten a baby. According
to the researchers, many other sounds will quieten a baby just as effectively.
Manning and Chamberlain prefer to explain left-side cradling of babies
in terms of the way the brain is organised. Biologists have long known that
the left and right hemispheres of the brain do not do identical jobs, a
phenomenon known as lateralisation. In humans, one of the differences between
the hemispheres is that the right half of the brain specialises in the decoding
of information which has emotional significance. Much of the sensory input
from the left visual and auditory fields is processed by the right half
of the brain – and vice versa.
Manning and Chamberlain’s idea is that by holding the infant to the
left, a mother is able to monitor it with her left ear and visual field,
and hence the side of the brain best able to interpret emotion. Furthermore,
because there is evidence that lateralisation also applies to facial expression,
the researchers suggest that this way of cradling has its advantages for
the infant, too, allowing it to see the left-hand, most emotionally expressive
side of its mother’s face.
The two researchers are currently investigating their idea by asking
human adults and children to cradle babies or dolls, while wearing eyepatches.
If their idea is correct, blocking out the right-hand field of view should
have no effect on side preference, but blocking out the left-hand field
of view should reduce left-hand cradling. This would demonstrate in another
species right-hand lateralisation for interpreting emotion.