Can Radial Polydactyly Be a Reason for Left-Handedness?

Before we can answer this question, let us first review what this phenomenon of radial polydactyly is. I assume that we are all familiar with the terms left- versus right-handedness. Then let’s talk about the prevalence of the two phenomena in the human population. This will shed some light on the issue being raised in this question.

What is Radial Polydactyly?

Radial polydactyly, also known as preaxial polydactyly, is a condition manifested by the duplication of the thumb. "Polydactyly" [pronounced polly dack-tilly] means “more than the usual number of digits dactyls or fingers,” and "radial" refers to the "inside" or "thumb-side" of the arm. Another term for the radial side is the "axial" side, so for our purposes here, the words "radial" and "preaxial" can be thought of as being more or less synonymous.

Individual cases of it are grouped into seven grades of severity. The most common being Stage IV duplication of the proximal phalanx, which involves doubling of the thumb’s proximal and distal phalanges, which together are attached to the single metacarpal bone of the thumb—the "base" of the thumb. Stage II in which only the distal phalanx portion of the thumb "above" the knuckle forming the "tip" of the thumb and containing the thumbnail, etc., is duplicated is the second most common form. Stage IV is about three times more common than Stage II. The malformation occurs in anywhere from one in one thousand to one in ten thousand live births.

Prevalence of Left-Handedness and Radial Polydactyly

Around 10 percent of the population is left-handed. This means that left-handedness is approximately 100 to 1000 times more prevalent than radial polydactyly. Or to put it another way, there is a two to three orders of magnitude difference in the frequency of left-handedness versus radial polydactyly. In percentages, it is 10 percent of the population for left-handedness versus 0.001 to 0.0001 for radial polydactyly.

Or to put it still another way, if you collect ten thousand (10,000) or 10K people selected at random out of the whole human population, there is a good chance that there will be something like one to a hundred of them that exhibit radial polydactyly. That is, some number between 1 and 100 with the malformation. But at the same time, out of that same group of 10K people, there will be around 1000 who are left-handed.

Factors in Left-Handedness

So, if you compare 1-100 versus roughly 1000 and perhaps even a little bit more than that, you can pretty quickly see that left-handedness has absolutely nothing to do with any congenital malformations of the thumb.

Left-handedness is not a congenital defect or birth defect. In fact, it is not a defect at all. It is not caused by any genetic abnormality or birth trauma; left-handed people are perfectly normal individuals with exactly the same capabilities as their right-handed counterparts. Their hand preference is indeed inborn but it is merely a difference like hair color or eye color.

Conclusion

The evidence clearly shows that left-handedness and radial polydactyly are two entirely different phenomena. Radial polydactyly is a congenital anomaly that affects the thumb, while left-handedness is a natural variation in human handedness. Both conditions are unrelated, and neither can be the reason for the other.