Someone recently shared with me this Scientific American article on the link between joint hypermobility and anxiety. Here's an excerpt:
Being double-jointed has long been linked with an increased risk for asthma and irritable bowel syndrome, among other physical disorders. “Joint
They even posit a potential mechanism for this connection: Collagen abnormalities that enable flexibility also lead to increased cardiac output in stressful situations, which means that people with this condition may experience "exaggerated cardiovascular responses," which they interpret as anxiety.
In any case, I like to see findings like this, which link anxiety disorders to seemingly unrelated or unlikely risk factors. Because it's not uncommon for people to take responsibility (read: feel guilty) about experiencing elevated anxiety. This goes to show that it's just another heritable, hardwired, physiological trait.