Hitchhiker’s thumb is a thumb that’s double-jointed and able to bend backward. Known formally as distal hyperextensibility, this condition isn’t painful and doesn’t inhibit the thumb’s function in any way.

Your thumb’s bendability is controlled by its distal interphalangeal joint, the bendy point at which the bones of your thumb are connected. People with hitchhiker’s thumb have distal joints that can bend back as far as 90 degrees. This looks similar to the classic roadside hitchhiker’s pose, thumb out in hopes of hitching a ride.

Hitchhiker’s thumb can occur in one or both thumbs.

Hitchhiker’s thumb hasn’t been studied extensively, and there is little to no data on its prevalence in the United States or worldwide. However, a 2012 study found that 32.3 percent of a random sample of 310 people had hitchhiker’s thumb. Of those subjects, 15.5 percent were male, and 16.8 percent were female.

A 1953 study, done at Johns Hopkins University, was one of the first to analyze hitchhiker’s thumb. In that study, 24.7 percent of white individuals and 35.6 percent of black individuals were found to have this condition in the United States.

Hitchhiker’s thumb may be an inherited condition with a genetic link. Some people with hitchhiker’s thumb may have acquired two recessive copies, or alleles, of the gene that determines thumb straightness. This means that the trait for hitchhiker’s thumb was present in both parents of the person born with it.

If instead one parent had the dominant gene for thumb straightness and the other had the recessive gene for hitchhiker’s thumb, their offspring wouldn’t have the condition. People with the recessive gene for this condition are called carriers. A person who carries a recessive gene would have to have a child with another carrier of the gene in order for that child to inherit the trait.

There’s some debate, however, about thumbs always being one of two kinds, straight or hitchhiker’s. An alternative theory is that thumb bendability involves a spectrum that ranges from no bendability in the joint to extreme bendability.

Hitchhiker’s thumb doesn’t result in any complications or health-related issues. It’s usually not painful, and it doesn’t make it harder to use your hands.

Hitchhiker’s thumb may be associated with several medical conditions. These include:

Diastrophic dysplasia

This is a genetic condition that affects the development of bone and cartilage. People with this condition have very short arms, and legs. They may also have curvature of the spine, club feet, and hitchhiker’s thumbs.

Hypermobility syndrome

An inherited disorder that affects connective tissue, joint hypermobility syndrome results in extremely flexible joints in multiple areas of the body, including the thumbs. People with this condition are often referred to as being double-jointed, as their joints are able to move beyond the normal range of motion.

When this condition results in painful joints, it’s referred to as joint hypermobility syndrome.

Hitchhiker’s thumb is a little-studied phenomenon that may have a genetic link. Unless it’s the result of a congenital disorder, such as diastrophic dysplasia or joint hypermobility syndrome, it isn’t painful.

Hitchhiker’s thumb doesn’t adversely affect the ability of the person with it to use their hands in any way.