Some articles suggest that our educational and cultural background influences how we learn to write. But that does not explain why siblings or even twins have very different handwritings.
Articles suggest that when a person speaks, only 15-20% of brain cells are activated, whereas when a person writes by hand, nearly 85- 90% of the brain cells are activated. We know that hand writing is a highly complex bio-mechanical action, which takes years of practice to learn. However, there are important links to the brain that neuroscience is only just starting to uncover. Handwriting has been described as 'brain writing', because the brain is the ultimate controller of the writing gesture.
Rudolf Pophal, a neuropsychiatrist in the 1940s, studied the physiological basis of handwriting. He said that handwriting is a movement controlled by three parts of the brain: the cerebral cortex, the palladium and the striatum. All of these have an effect on handwriting, especially in the psychic tension produced in the written gesture. He distinguished six degrees of 'Tension' in writing, which is still used by graphologists today.
Considering how much research in the psychology field is conducted on understanding the human brain and how it affects our personality, there is relatively little understanding of why people produce different handwritings. Only graphologists seem to have an answer. It would be great if there were more debate about this fascinating issue.