We don't know, is the only real answer. In fact, scientists don't even have a scientific definition of what it means to be left-handed. (Is it the hand which can perform a task better? Or the hand you prefer to use for a task?)
There are several theories, and it's probably a combination of all of them. They include:
- Correlation with the way the brain is organised (in right-handed people, the left side of the brain deals with speech as well as with controlling the right hand, and the theory is that speech and fine motor skills are related, whereas the brains of left-handed people are often organised differently)
- The position of the foetus in the womb, especially in the last trimester of pregnancy
- A complex tie-in with genetics (two left-handed parents have a 24% chance of having a left-handed baby, compared to the general population which is 10% left-handed)
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