When you give a thumbs-up, does it look straight or seem to bend backwards? Whether it bends a little or a lot, the ability to bend your thumb further than average could mean you have a hitchhiker's thumb. It's also likely that both your parents have it—or at least carry the gene for it. This trait tends to run in families. An AncestryDNA + Traits can shine a light on whether you have the genetic markers associated with this intriguing ability.

What Is a Hitchhiker's Thumb?

Hitchhiker's thumb, sometimes referred to as a double-jointed thumb, is the common name for distal joint hyperextensibility that affects the thumb. You have a hitchhiker's thumb if you notice that the thumb joint can extend further than 50 degrees when you give a thumbs-up.

Anatomically speaking, a hitchhiker's thumb has a hypermobile distal interphalangeal joint. This hypermobility in the joint in the middle of the thumb is typically caused by the shape of the joint and the laxity or looseness of the ligaments. When the joint and connecting ligaments are looser than average, the thumb has a wider range of motion, resulting in it extending back further than average.

This extra flexibility rarely impacts functionality. For most people, it's completely asymptomatic. If it does cause issues, however, bracing or physical therapy can help by providing additional support to the joint and strengthening the muscles to improve stability. However, hyperextensibility can be associated with genetic conditions, such as joint hypermobility syndrome.

Currently, no wide-scale research has been performed to identify exactly how common a hitchhiker's thumb is. Some studies have estimated that between 24.7% and 35.6% of the population has a hitchhiker's thumb.

The Genetic Influence on Having a Hitchhiker's Thumb

Having a hitchhiker’s thumb can often be a familial trait. Some research has suggested that the trait is recessive, and therefore can only be inherited if both biological parents have the same recessive genes involved in the trait and pass them down to their child. However, other researchers believe that this trait is more complex—that because having a hitchhiker's thumb can relate to other joint hypermobility conditions, it is therefore caused by multiple genes acting in combination.

Eager to understand how genetics impacts the hitchhiker's thumb trait, the AncestryDNA team asked over 800,000 people, "Do you have a hitchhiker's thumb?" Based on their answers and their genetic profiles, the team identified over 620 DNA markers associated with the hitchhiker's thumb.

AncestryDNA scientists further analyzed these results by calculating a polygenic risk score. This statistical tool helps determine the likelihood that someone has a trait based on their DNA. Based on the results, the team determined that 10% of the variation in whether people had the hitchhiker's thumb could be explained by differences in their genetics. However, these numbers could change as new discoveries are made about the interactions between different genes.

Non-Genetic Reasons for Having Hypermobility in the Thumb

Sometimes, people develop a hitchhiker's thumb long after birth. For example, joints, including the thumb, can become unstable and hypermobile after acute injury or repetitive strain.

During pregnancy, hormones such as relaxin can also lead to increased joint laxity. While this is necessary to prepare the body for childbirth, it can also lead to lasting joint looseness or instability across the entire body. This could present as the ability to overextend thumbs and fingers, among other joints.

Interesting Facts About Having a Hitchhiker's Thumb

Understanding the presentation of a hitchhiker's thumb gets tricky because there's no clear standard for what a hitchhiker's thumb is. In their 1953 study, Glass and Kistler established the 50-degree rule as an arbitrary dividing line for their purposes. People can present with varying degrees of flexibility. In fact, some people have been documented extending their thumbs upwards of 90 degrees. Some people have it in just one thumb, while others can bend both digits past the average limit.

Because a hitchhiker's thumb requires a degree of hypermobility, it's not uncommon for those with it to have other joints they can extend further than average. While sometimes a hitchhiker's thumb is little more than an extra-flexible joint, it can also result from the presence of other joint hypermobility conditions.

Wondering what your genetics suggest about your physical traits? Take an AncestryDNA + Traits test today and find out whether you’re more or less likely to have the genetic markers for all sorts of traits, from your hitchhiker's thumb to your finger length, earlobe type, and more. If you've already received your AncestryDNA results, check out what science has to say about your traits with an Ancestry subscription.