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Left-Handedness Does Not Originate in the Brain

What makes some people rely more on their left hands than their right hands? One answer may seem obvious: their brains are different.

And that may be a somewhat satisfying answer, because the brains of left-handed people do indeed look different than the brains of right-handed people. But is that difference the cause or the result of being left-handed?

A recent study dug into this, and came up with a surprising answer: handedness does not originate in the brain, but in the spinal cord. Conditions in the womb lead to asymmetry in the development of the spine, before the motor cortex is even connected to it. When the spine and brain do connect, the fetus is already using one hand more than the other, and the brain changes accordingly. Then it's onward to a lifetime of having to seek out left-handed scissors and can openers.

Perhaps this result suggests a deeper lesson: don't assume the brain is the root cause of every observable difference in behaviour. Sometimes it goes back further and deeper.


This post was written by Mike Battista, a staff scientist at Cambridge Brain Sciences.