why thumbs bend backwords for some people, and for some they don't?

i know it's genetic, but i want an explaination about the phisical structure of whatever it is, that makes it happen (or not happen).
Answer:
it's primarily due to the laxity of the ligaments supporting the joints; the more lax it is, the easier the joints move over each otherin conditions such as Marfan's syndrome, where there is an abnormality affecting the connective tissue, excessive laxity of the joints are a feature

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The above answer is exactly correct - just wanted to echo that.

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