Why do humans and apes cradle babies on
their left side?
21 Jul 90
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.
GEORGIA MASON
From New Scientist magazine, vol 127 issue 1726, 21/07/1990, page
© Copyright New Scientist, RBI Limited 2001