What is the cause of someone being introverted or extroverted?

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I was talking to an elderly person recently and they said that social media and texting has caused kids these days to become introverted. He said that kids won’t even say hello to you in the shops or public transport. I personally thought that you were just born an introvert or extrovert as a part of your personality but now I’m rethinking? What is your opinion?

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