How Many People Have 6 Fingers and Why?

Roughly one in every 700 to 1,000 babies worldwide is born with at least one extra finger or toe, a condition called polydactyly.¹ That makes six-fingered hands far more common than most people assume. The reasons behind it involve a handful of genes that steer limb development in the womb, and the story gets richer the deeper you go, touching on population genetics, evolutionary biology, and even a surprising finding about what a sixth finger can do.

How Common Is Polydactyly, and Who Gets It

The one-in-700-to-1,000 figure is a global average, and averages hide wide variation. Rates differ by ancestry in ways that reflect different underlying gene frequencies rather than anything about environment or lifestyle. A study of American Indian populations found polydactyly at a rate of about 2.4 per 1,000 live births, roughly double the rate in white Americans.² Among populations of African descent, postaxial polydactyly (the type involving extra digits on the pinky side of the hand) is particularly common. A comparison of birth registries in South America and Spain found that postaxial polydactyly showed up more than twice as often in the South American registry, which the researchers attributed to the higher proportion of African ancestry in those populations.³

These disparities are not about one group being more “normal” and another being less so. They simply reflect different frequencies of the gene variants responsible. The specific types of extra digits also vary: preaxial polydactyly (extra thumb) was three to four times more common in American Indian populations than in white or Black populations in the same study.² So both the overall rate and the particular pattern of extra digits differ from one population to the next.

Where the Extra Digit Shows Up

Not all extra fingers are the same. They are classified by their position on the hand or foot:

• Postaxial: An extra digit on the pinky side. This is the most common type globally and the one most frequently seen in people of African descent.
• Preaxial: An extra digit on the thumb side. This includes duplicated or split thumbs, and it is relatively more common in Asian and American Indian populations.
• Mesoaxial: An extra digit in the middle of the hand, which is the rarest form.

These categories matter because each type tends to involve different genes and different mechanisms during embryonic development.⁴ The clinical picture also varies enormously within each category. Some extra digits are fully formed, with their own bones, joints, tendons, and even independent muscle control. Others are tiny nubs of soft tissue dangling from the side of the hand by a thin stalk. This range means that polydactyly is really a spectrum, not a single condition, and the same genetic variant can produce strikingly different results even within the same family.⁵

The Genes Behind Extra Fingers

During the first weeks of embryonic development, a cluster of cells at the back edge of the growing limb bud produces a signaling molecule called Sonic Hedgehog (yes, named after the video game character). This signal acts like an instruction manual for digit formation: it tells the cells at the front of the limb bud how many fingers to make and where to put them. When the signal is too strong, active in the wrong place, or active at the wrong time, extra digits form.

Much of the research on polydactyly genetics converges on two key players. The first is the gene that encodes the Sonic Hedgehog protein itself. Its activity in the limb is controlled by a distant stretch of DNA called the zone of polarizing activity regulatory sequence, or ZRS. Think of the ZRS as a volume knob for Sonic Hedgehog in the developing hand. Mutations in the ZRS can crank that volume up or cause the signal to leak into areas of the limb where it does not belong, leading to extra digits on the thumb side.⁶ Research in mice has shown that the ZRS is built from modular pieces: some parts control where the signal goes, others control how much is produced. Disrupting the repressor module, which normally keeps Sonic Hedgehog confined to the back of the limb, can allow ectopic expression at the front, triggering thumb-side duplication.⁷

The second major player is a gene called GLI3, which acts downstream in the same signaling pathway. GLI3 serves as a brake on Sonic Hedgehog signaling: when working normally, it limits the number of digits that form. When GLI3 is broken or weakened by a mutation, the brake fails and extra digits appear, usually on the pinky side. Researchers have identified multiple loss-of-function mutations in GLI3 linked to postaxial polydactyly across different families.^{8 9} In one study of 19 families with isolated postaxial polydactyly, about 16 percent had identifiable GLI3 mutations, suggesting that in many cases the responsible genes have not yet been found.⁹

These are not the only genes involved. Variants in GLI1, LMBR1, and other genes in the Hedgehog pathway have all been linked to extra digits, sometimes in combination with other developmental differences like heart defects or short stature.¹⁰ The genetic picture is complex, and it keeps getting more detailed as researchers sequence more families.

How It Runs in Families

The most common pattern of inheritance for isolated polydactyly (meaning extra digits without other health problems) is autosomal dominant. In plain language, that means only one copy of the variant gene, from one parent, is enough to produce the trait. If a parent has it, each child has roughly a 50-50 chance of inheriting the variant. This pattern has been confirmed in large multi-generation families: one study traced postaxial polydactyly through five generations of an Indian family, with 15 affected members out of 37.¹¹

But the dominance is not clean-cut. Variable expressivity is the norm: two people in the same family carrying the exact same gene variant can end up with very different outcomes. One might have a fully formed extra finger on both hands; another might have a barely noticeable bump on one hand only. Some carriers show no visible extra digits at all, a phenomenon geneticists call reduced penetrance. This unpredictability is one reason polydactyly can seem to “skip” a generation, even though the gene variant was faithfully passed along.

Do Environmental Factors Play a Role

Genetics dominates the picture, but the environment during pregnancy is not irrelevant. A case-control study that examined risk factors for polydactyly and syndactyly found that a family history of either condition carried the strongest association, increasing the odds roughly tenfold. Beyond genetics, exposure to hazardous substances before pregnancy tripled the risk. On the protective side, better nutrition during early pregnancy, specifically higher intake of meat and eggs, was associated with significantly lower odds.¹²

These environmental associations are modest compared to the genetic ones, and they come from a single study rather than a large body of replicated research. Still, the finding fits what we know about limb development: the window in which fingers form is early and narrow, roughly weeks four through eight of pregnancy. Disruptions to signaling during that period, whether from a gene variant or an external chemical exposure, can alter the number of digits. The overwhelming majority of polydactyly cases, though, are genetic in origin.

When Extra Fingers Are Part of Something Bigger

Most people born with an extra digit are otherwise perfectly healthy. But in a minority of cases, polydactyly shows up alongside other developmental differences as part of a recognized genetic syndrome. These syndromic forms are worth knowing about because they change the medical picture substantially.

One example is Ellis–Van Creveld syndrome, caused by weakened Hedgehog signaling. It can involve extra digits along with heart defects, abnormalities of the nails and teeth, and short stature. Variants in GLI1 have been linked to a form of postaxial polydactyly that sometimes co-occurs with atrial septal defects and minor nail changes.¹⁰ Similarly, GLI3 mutations can cause Greig cephalopolysyndactyly syndrome, which combines polydactyly with fused fingers and a broad forehead, or Pallister-Hall syndrome, which involves additional limb and organ differences.¹³

This is why prenatal detection of an extra digit often triggers a more thorough evaluation. When polydactyly is spotted on a routine prenatal ultrasound, clinicians typically perform a detailed anatomy scan and may recommend genetic testing to rule out a syndromic cause.¹⁴ In one reported case, ultrasound at 19 weeks revealed six metacarpals and a widened thumb, and whole-exome sequencing of the fetus identified a duplication in the LMBR1 gene spanning roughly 253 kilobases, confirming a diagnosis of triphalangeal thumb-polysyndactyly syndrome before birth.¹⁵ Isolated polydactyly found on ultrasound is generally reassuring, but confirming that it is truly isolated requires this kind of follow-up.

Treatment and What Parents Face

The default assumption in most medical systems is that an extra digit should be removed, usually in infancy or early childhood. How that removal happens depends on the anatomy of the extra digit.

Small, rudimentary digits that hang by a narrow stalk of soft tissue are sometimes removed with suture ligation, a quick in-office procedure where a tight suture cuts off blood flow and the nub falls off on its own. Fuller surgical excision is the alternative and tends to produce cleaner results. A study comparing the two approaches for a common form of pinky-side polydactyly found that ligation was significantly more likely to leave behind a residual bump or neuroma stump, with about 64 percent of ligation patients having a residual remnant. Parents of children who had remnants left behind reported less satisfaction with the appearance of their child’s hand and with treatment outcomes overall.¹⁶

For more complex cases, like a duplicated thumb with full skeletal elements, surgery involves not just removal but reconstruction of the remaining thumb to ensure it lines up properly and stays stable. Two common approaches, removal of the extra bone segment with joint stabilization and a combined osteotomy with fusion, have been compared in a study of 33 hands. Both produced similar outcomes in terms of joint stability, objective function, and patient-rated daily activities, suggesting either route is reliable when performed by an experienced surgeon.¹⁷

A Sixth Finger Can Actually Be Useful

Most conversations about polydactyly focus on removing the extra digit, but a 2019 study asked a different question: what if the extra finger works? Researchers at the University of Freiburg studied two people with fully developed six-fingered hands using brain imaging and detailed motor testing. They found that the extra finger had its own dedicated muscles, its own nerve supply, and its own distinct representation in the brain’s motor cortex. It could move independently of the other five fingers.¹⁸

The practical payoff was real: both subjects could perform manipulation tasks with one hand that normally require two, such as tying a knot or handling an object that demands more grip configurations than five fingers can manage. The study concluded that the human nervous system can control a body with significantly more degrees of freedom without any motor deficits or impairments.¹⁸ This finding has sparked interest in the robotics and prosthetics communities, where researchers are exploring whether augmenting the human hand with an extra controllable digit could enhance dexterity for specific tasks. The idea that a sixth finger could be an advantage rather than a defect challenges the reflexive assumption that drives most surgical decisions.

Why Five Fingers Became the Rule

If extra digits can be functional and even advantageous, why are almost all land vertebrates stuck at five per limb? The answer has less to do with five being optimal and more to do with the developmental timing of limb formation. In most land vertebrates, limbs form during a critical early window of embryonic development when signaling networks across the entire body are heavily interconnected. Changing the number of digits during this window tends to disrupt other body systems at the same time, a phenomenon called pleiotropy. Mutations that produce extra digits therefore often come with side effects elsewhere in the body, which makes them unlikely to succeed in evolutionary terms.¹⁹

Amphibians offer an interesting contrast. In frogs and salamanders, limb development happens later, after the most interconnected stage of embryonic development has passed. The limbs are more developmentally independent from the rest of the body, so changes to digit number are less likely to cause problems elsewhere. This looser constraint may explain why amphibians show more variation in hand and foot structure, with some frog species occasionally sporting six toes.¹⁹ The five-digit plan in mammals and reptiles, then, is not proof that five is the “right” number. It is an artifact of when limb development occurs relative to everything else going on in the embryo.

Polydactyly Through Human History

Extra digits have been showing up in human populations for thousands of years, and ancient peoples noticed. In the Atacama Desert of northern Chile, bioarchaeological analysis of remains from the Chinchorro culture, a group of coastal hunter-gatherers who lived between about 9,000 and 3,400 years ago, identified two individuals with polydactyly. The researchers also surveyed 75 rock art sites in the surrounding region, dating from roughly 3,000 to 550 years ago, and found depictions of hands and feet with six digits. Given the documented skeletal evidence of polydactyly in the same area, the researchers concluded it was highly probable these artworks depicted real people rather than symbolic or mythological figures.²⁰

Six-fingered figures appear in art across many cultures, from Puebloan rock art in the American Southwest to biblical references (the giant of Gath in 2 Samuel is described as having six fingers on each hand and six toes on each foot). The recurring presence of polydactyly in the human record, both in skeletal remains and in cultural depictions, reinforces how common and persistent the trait has been across millennia, long before anyone understood the genes responsible.

Extra Toes in Animals

Polydactyly is not exclusively human. Cat owners may already know about this: polydactyl cats, sometimes called “Hemingway cats” after the writer who kept a colony of them at his Florida home, carry extra toes on their front paws and sometimes their back ones too. In Maine Coon cats, the genetics have been studied in some detail. A known variant called the Hw allele accounts for polydactyly in a subset of affected cats, but researchers found that a significant portion of polydactyl Maine Coons lack this allele entirely, pointing to additional, still-unidentified genetic causes. Population analysis also showed a genetic distinction between cats that carry the Hw allele and those that are polydactyl through other mechanisms.²¹

The parallels with human polydactyly are striking: multiple genetic routes lead to the same visible outcome, the trait shows variable expression even within families, and the underlying pathway involves the same Hedgehog signaling system. Polydactyly in cats is generally harmless and rarely treated surgically, which provides an interesting contrast to the human medical approach where removal is almost automatic. Whether a sixth finger or toe is treated as a problem or a quirk depends more on the species and the culture than on the biology itself.