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Is there a significant difference in left-handedness in RTL countries? It seems strange to me that an RTL writing system would develop in a vast majority right handed environment, for the same reasons that left handed people have issues with LTR systems

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No, RTL cultures still have the 10ish percent lefties. The difference is, RTL cultures developed writing styles and hand positions to work with RTL. Whereas LTR cultures treat lefties as an afterthought, so lefties only hear about actual good hand positions and writing styles as adults, after decades of learning what is basically a mirror to right-handed style.