Left handedness why

We know that left- and right-handedness has a genetic origin. However, geneticists are still trying to pinpoint which bits of DNA are involved, and there may well be up to 40 different genes at play. The right side of the brain controls the left hand, and vice versa. And so being left-handed can have knock-on effects on the way the brain is arranged. However, not everyone agrees. Dorothy Bishop is Professor of Developmental Neuropsychology at the University of Oxford and she has a personal interest.

But after looking into the data, Bishop is not convinced. Children very quickly show a preference for which hand they use Credit: iStock. The debate rages on, and there is still so much we need to discover about the left-handed brain. Part of the problem is that when neuroscientists look at various aspects of behaviour, MRI studies are only done on right-handed people, in order to try and minimise the variation between different participants.

Only specific studies on left-handedness will invite lefties to take part. He found that nine out of 10 foetuses preferred sucking their right thumb, mirroring the familiar pattern we see in the general population. And when he followed those children up many years later, the babies who were sucking their right thumb in the womb became right-handed, and the ones who preferred their left, stuck with that. If you liked this story, sign up for the weekly bbc.

The mystery of why left-handers are so much rarer. Share using Email. But a study published last year in the journal eLife found that the answer could lie in the spinal cord. The research — by Sebastian Ocklenburg, Judith Schmitz, and Onur Gunturkun from Ruhr University Bochum, along with other colleagues from the Netherlands and South Africa — found that gene activity in the spinal cord was asymmetrical in the womb and could be what causes a person to be left- or right-handed.

Arm and hand movements start in the brain, in an area called the motor cortex, which sends a signal to the spinal cord that's translated into a motion. The researchers found that while the fetus is growing in the womb, up until about 15 weeks, the motor cortex and the spinal cord are not yet connected, but right- or left-handedness has already been determined. In other words, the fetus can already start movements and chooses a favorite hand before the brain starts controlling the body.

To study this, the researchers analyzed gene expression in the spinal cord in the eighth through the 12th week of pregnancy. They found significant differences in the left and right segments of the spinal cord that control arm and leg movement. They concluded that the asymmetrical nature of the spinal cord could be down to something called epigenetics, or how organisms are affected by changes in their gene expression rather than in the genes themselves.

These changes are often brought about by environmental influences and can affect how a baby grows. These gene-expression differences could affect the right and left parts of the spinal cord differently, resulting in lefties and righties.

In , researchers at Northwestern University developed a mathematical model to show that the percentage of left-handed people was a result of human evolution — specifically, a balance of cooperation and competition.

In other words, they thought that, though the basis for right- or left-handedness may be genetic, there could be a social factor that explains why the ratio is so high. In other words, we may have, for some reason, evolved to favor right-handedness, so anyone deviating from this may have been conditioned to use that hand primarily despite their genetic predisposition.

The new study couldn't explain the majority of right-handedness, but Schmitz explained how bird research can show how genetics and environment can be the cause. Read more : Introverts are not socially anxious by default, and understanding this can help them network with people more effectively. This combination of genetic and environmental factors light induces a visual asymmetry — pigeons and chicken are better in visual discrimination, categorization, and memorization of visual patterns with their right eye than with their left eye.

If the familial genetic pool contains C genes, then hand preference becomes amenable to chance influences, including the pressures of familial training and other environmental interventions that favor the use of one hand over the other. The proposed genetic locus that determines hand preference contains an allele from each parent, and the various possible genetic combinations are DD individuals who are strongly right-handed, DC individuals who are also mostly right-handed, and CC individuals who are either right-handed or left-handed.

These genetic combinations leave us with an overwhelming majority of human right-handers and a small, but persistently occurring, minority of left-handers. Already a subscriber? Sign in. Thanks for reading Scientific American. Create your free account or Sign in to continue.

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