Bodily Oddities

Hitchhiker's thumb

Also called: Distal hyperextensibility of the thumb · Bendy thumb

A thumb whose top joint bends back past 50 degrees, once taught as a simple recessive gene. Thumb angle is really a smooth range, and the genetics are not that simple.

Bones and joints Variation Structural
Try it Debunked myth Genetic

How common

24–36% of people Common · 10–50% Depends entirely on where the cutoff angle is drawn.

Do you have this?

These are answers from visitors, not a study.

How it works

Hold your thumb up and push the top segment backward. In most people it stops close to straight. In some it folds back to a clear angle, like a hitchhiker asking for a ride. The usual cutoff is 50 degrees or more at the top joint, measured from straight1.

The angle depends on the shape of the joint surfaces and the slack in the ligaments and joint capsule. These are the same factors that decide flexibility in every other joint3. Surveys find the trait in 24% to 36% of people1.

For decades biology classes used the thumb as an example of a single-gene recessive trait. The claim rests on one study from 19532. When researchers measure thumb angles across a population, they do not find two groups. They find a smooth range, from thumbs that barely move to thumbs that bend far back2. There is no natural dividing line, and the 50-degree cutoff is arbitrary. Families do show some inheritance, but the angle is shaped by several genes and by joint looseness in general2. A hitchhiker’s thumb is one small sign of hypermobility rather than a separate trait.

Try it

  1. Hold one hand up with the thumb pointing to the ceiling.
  2. With the other hand, push the tip of the thumb backward gently. Do not force it.
  3. Look at the angle between the top segment and the rest of the thumb.
  4. If the top segment leans back by about half a right angle or more, it counts1.
  5. Compare the two hands. They often differ.

History and culture

The thumb appears in old genetics textbooks next to earlobe attachment and tongue rolling as a supposed one-gene trait. All three have since been shown to be more complex2.

Sources and further material

  1. Encyclopedia Hitchhiker's thumb
  2. Article Myths of Human Genetics: Hitchhiker's thumb John H. McDonald, University of Delaware Reviews the 1953 study behind the one-gene claim and shows that thumb angle is continuous.
  3. Encyclopedia Hypermobility (joints)
Ears Variation

Attached and free earlobes

Some earlobes hang free and others join the head directly. Textbooks taught this as one gene, but a 2017 study found at least 49 genetic regions involved, and the two shapes blend into each other.

Both shapes are common
Mouth Trick

Tongue rolling

Between 65% and 84% of people can curl the sides of the tongue up into a tube. For decades it was taught as a simple one-gene trait. It is not.

65–84% of people
Bones Variation

Joint hypermobility

About 1 in 5 people have joints that bend further than usual, measured with a nine-point test called the Beighton score. For most people it causes no trouble.

10–25% of people
Bones Variation

Clinodactyly

A finger that curves sideways toward its neighbours, most often the little finger leaning toward the ring finger. Between 1% and 20% of people have it, and on its own it is harmless.

1–20% of people