Question: why are some people double jointed and other people arn't

Pip Millington answered on 14 Nov 2016:

It all comes down to genetics, how that plays a role in how the bones and joints are formed, and how stretchy the soft tissues like ligaments and muscles are which support the joint.

Flexibility is a very complex trait like height where many different parts of the DNA code have an influence – there are lots of genes involved and it depends which versions of these genes you inherit from your parents’ DNA. As well as this the environment plays a role: people can become more flexible by training but some people will never be as flexible as others because of their genetics!