Most people clearly prefer one hand for writing, throwing, brushing their teeth, and other precise tasks.
That preference is called handedness.
It is connected with the way the brain develops and divides work between its two hemispheres.
The brain and body are partly crossed
The left side of the brain controls much of the movement on the right side of the body.
The right side controls much of the left.
As the brain develops, certain functions become more specialized on one side than the other.
Hand preference appears to be one result of that specialization.
Is handedness genetic?
Genes matter, but there is no single gene that simply says, “right-handed” or “left-handed.”
Many genetic influences appear to contribute.
Development before birth and other biological factors matter too.
That is why handedness can run in families without following a simple inheritance pattern.
Why are most people right-handed?
About nine out of ten people are right-handed.
Scientists do not have one complete explanation for why the population is so strongly tilted in that direction.
The pattern is likely connected with the way language, movement, and other brain functions became organized during human evolution.
The important point is that right-handedness is common, but left-handedness is a normal variation.
Can handedness be trained away?
People can learn to do many tasks with either hand.
But that does not necessarily erase the underlying preference.
Historically, some left-handed children were forced to write with their right hand. That could change the practiced behavior without changing how naturally the left hand felt for many other tasks.
What about people who use both hands?
Some people are mixed-handed, meaning they prefer different hands for different tasks.
True ambidexterity, where both hands are equally skilled across many tasks, is much less common.
Handedness is better thought of as a spectrum than as two perfectly separate boxes.
Does being left-handed change intelligence or personality?
There is no simple rule connecting handedness with intelligence, creativity, or personality.
Plenty of popular claims go far beyond what handedness can actually tell you about a person.
So handedness is not just a habit somebody taught you when you were little.
For most people, the brain was already leaning in one direction very early on.
Pun unfortunately intended.
The same body systems connect with Why do we have fingerprints?, another everyday effect with its own underlying mechanism.
And that’s the short explanation.
Related explanations: Why do we have fingerprints?, Why do fingernails keep growing?, and Why do we get pins and needles?