The hand you favour as a 10-week-old fetus is the hand you will favour for the rest of your life, suggests a new study.
The finding comes as a surprise because it had been thought that lifelong hand preferences did not develop until a child was three or four years old.
A team led by Peter Hepper of the Fetal Behaviour Research Centre at Queen鈥檚 University, Belfast in the UK reached this conclusion after studying ultrasound scans of 1000 fetuses.
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In one study, nine out of 10 fetuses at 15 weeks鈥� gestation preferred to suck their right thumbs. Hepper鈥檚 team followed 75 of those fetuses after birth, and found that at 10 to 12 years old all 60 of the right thumb-suckers were right-handed, while 10 of the 15 left thumb-suckers were left-handed and the rest right-handed.
At 10 weeks old, even before they suck their thumbs, fetuses wave their arms about. A second study found that most prefer to wave their right arm, a preference that persisted until 24 weeks, after which the fetus is too cramped to move. Hepper reported the findings at the Forum of European Neuroscience in Lisbon, Portugal, earlier in July.
Reflex arc
Hepper is quick to point out that these observations do not show that the fetus can control its movements at such a young age. Nervous connections to the body from the brain are not thought to start developing until around 20 weeks鈥� gestation.
In addition, at the same stages of development fetuses that lack a brain cortex, a condition called anencephaly, move their limbs in a similar way, also favouring their right arm over the left.
鈥淭here is no evidence that the brain has any control over these movements at this stage,鈥� says Hepper. 鈥淚t鈥檚 most likely to be a local reflex arc involving the spinal cord.鈥� He speculates that the fetus may have a preference for one side of its body simply because that side develops slightly faster.
The findings challenge the favourite theory of how handedness in humans develops. According to that theory, it is a side effect of brain lateralisation, in which one side of the brain predominantly handles certain functions, such as language. As the fetal scans show that handedness appears long before the brain has any control over limb movement, that theory cannot be correct.
Sensory connections
Instead, Hepper speculates that the reverse may be true: the fetus鈥檚 body movements
may somehow lead to the development of an asymmetrical brain. He points out that the sensory connections from the body to the brain develop before the connections that allow the brain to control the body鈥檚 movement.
But Stephen Wilson, a developmental biologist at University College London, is sceptical. 鈥淭he movements you see in a fetus don鈥檛 have to be influencing brain asymmetries.鈥�
It is more likely, he says, that in the early fetus there is already a difference in gene activation between the right and left sides of the brain and that this leads to lateralisation.