Joint flexibility is an oft-coveted trait that provides a special advantage to dancers and athletes, but there can be too much of this good thing. A growing body of research suggests a surprising link between high levels of flexibility and anxiety. A study published last year in the journal Frontiers in Psychology is among the most recent to confirm the association, finding that people with hypermobile joints have heightened brain activity in anxiety regions.

Joint hypermobility, which affects approximately 20 percent of the population, confers an unusually large range of motion. Hypermobile people can often, for instance, touch their thumb to their inner forearm or place their hands flat on the floor without bending their knees. The trait appears to be genetic and is a result of variation in collagen, the main structural protein of connective tissue.

Being double-jointed has long been linked with an increased risk for asthma and irritable bowel syndrome, among other physical disorders. “Joint hypermobility has an impact on the whole body and not just joints,” says Jessica Eccles, a psychiatrist and researcher at the University of Sussex in England. It was only a matter of time before scientists also looked at whether joint hypermobility was linked to mental disorders. The investigation began in 1993 and heated up in 1998 when researcher Rocío Martín-Santos, now at the Hospital Clinic of the University of Barcelona, and her colleagues discovered that patients with anxiety were 16 times more likely to have lax joints. Their findings have since been replicated numerous times in large populations.

A 2012 brain-imaging study conducted by Eccles and her colleagues found that individuals with joint hypermobility had a bigger amygdala, a part of the brain that is essential to processing emotion, especially fear. In the 2014 study, which was conducted by Eccles and her associates in collaboration with researchers from Spain, hypermobile participants displayed heightened neural reactivity to sad and angry scenes in brain regions implicated in anxiety. Researchers have also linked the condition with increased consumption of chocolate, tobacco and alcohol—items that are often used in an attempt to self-medicate anxiety.

Joint hypermobility may also be associated with an exaggerated fight-or-flight reaction. Eccles and her colleagues recently found support for this idea in a study of 400 psychiatric patients. They uncovered a simple yet powerful mechanism behind the link: the collagen abnormalities that make joints especially flexible seem to affect blood vessels, making patients prone to an accumulation of blood in the veins of the legs. This pooling may lead to exaggerated cardiovascular responses to maintain the output of blood from the heart. When the heart has to work extra hard just to circulate blood, it brings the entire body to the verge of a fight-or-flight reaction, requiring very little to set off panic.

Eccles hypothesizes that these patients might benefit in particular from beta blockers, drugs that ease anxiety by reducing symptoms of the body's fight-or-flight response. She hopes that future studies will investigate such targeted treatments for double-jointed people. In the meantime, the findings are an important reminder for clinicians to consider the possibility that a patient's mental disorder could have purely physical origins.