Best estimates for left-handedness in the relatively permissive cultures of the developed Western world (roughly North America and Western Europe) is around 12%. In more formal cultures that stress conformity, such as Asian, Muslim and Latin American countries, the rates tend to be much lower. For example, Korea (2%), Japan (3%) and Taiwan (5%) have the lowest levels of left-handedness, possibly reflecting the systematic discouragement of left-handedness that still persists in these countries. The difficulty of teaching complex Chinese and Japanese calligraphy has been suggested as another reason why children there are more likely to adopt the same handedness as their teachers.
Overall, worldwide, a rough figure of about 10% seems likely. However, it could be argued that the statistics from more permissive and less conformist societies like North America may in fact give a “truer” and less distorted indication of the relative numbers of left-handedness, as they are less influenced by extraneous social and political factors. Indeed, it is perhaps a testament to the tenacity and persistence of left-handedness that there are still any left-handers at all in Eastern cultures like Korea and Japan.
Within the 12% of North Americans who claim left-handedness, there are some interesting demographic variations. As regards gender, men are slightly more likely than women to be left-handed, with most studies indicating that about 13% of men and just under 11% of women are left-handed (men are NOT twice as likely to be left-handed as is sometimes claimed). It has been suggested this occurs because girls are more receptive to social mores and more likely to conform than boys (although there may also be a genetic component). In the same way, the percentage of left-handers tends to be much smaller than the overall average among Chinese Americans, those who attended Catholic schools, and older generations raised in the more conformist Pre-War years.
The variations due to age demographics are particularly striking, with one major study indicating that about 15% of under-30s are left-handed writers, compared to only 6% of over-65s. Other statistics back up this trend: a century ago, only about 2-3% of North Americans were, or at least admitted to being, left-handed, compared to around 12% today. An analysis of the underlying trends shows a steady increase in left-handedness from the beginning of the 20th Century until about the middle of the century, after which it stabilizes at about 12%. This is unlikely to be due to any genetic factors, and probably reflects the changing attitudes towards left-handedness (a relaxation of the stigma and social pressure against it) and the educational reforms against systematic switching during education. By some estimates, 6-8% of right-handed Americans, particularly among older generations, are actually natural left-handers who have been forced (or at least encouraged) to switch at a young age, with an unknown number of others having switched before their memories reliably begin.
Interestingly, left-handedness is almost twice as likely among twins as among the general population. Left-handed parents are also about twice as likely to produce twin offspring. And finally, left-handers are about twice as likely to have left-handed children as right-handers. These statistics suggest intuitively that there is some sort of link between twins and handedness, but just what that link might be has been perplexingly resistant to discovery. There are, of course, also major inconsistencies between the two phenomena of left-handedness and twins, not the least of which being that monozygotic (or "identical") twins occur only 3 or 4 times out of every 1,000 births (as compared to over 100 in every 1,000 for left-handedness).
In past times (and presumably in some countries even today), it seems possible that the long-standing prejudice against left-handedness could even have interfered with the mating prospects of left-handers. A 1992 meta-study by Chris MacManus and Phil Bryden showed that, prior to 1955, two right-handed parents averaged 3.1 children, a right-handed and a left-handed parent averaged 2.69 children, and two left-handers just 2.32. Interestingly, since 1955, this situation has actually completely reversed, suggesting that left-handers are now in the ascendancy, although the implications of this in the longer term are not clear.
Most people fall towards the extremes of the left-right continuum of handedness, with over 60% strongly or completely right-handed and another 20% or more quite strongly right-handed. Very few are strongly or completely left-handed (less than 3%), and even fewer are evenly mixed-handed or ambidextrous. The resulting graph of handedness is sometimes described as “J-shaped”, with a small uptick for strong left-handedness, a big dip in the middle and a steeply-rising and much larger uptick towards strong right-handedness.
In some respects, mixed-handedness is more prevalent towards the left hand side of the scale than the right. Another way of looking at this is that left-handedness is more likely to be a weak handeness preference, whereas right-handedness is more likely to be a strong preference. For example, about half of all left-handers (who write with their left hands) prefer to throw a ball with their right hands, and actually do so more accurately than with their left. Conversely, only about 2% of right-handed writers prefer to throw left.
Other studies have attempted to estimate other aspects of laterality. Around 80% favour their right foot, 70% their right eye, and 60% their right ear. Sidedness in other parts of the body, though, is not necessarily consistent with a person's handedness: for example, around 40% of left-handers are right-eye dominant, and almost 50% are right-footed. However, we should perhaps expect handedness and “eyedness” to be independent of each other, as there is no real direct analogy between the two phenomena (both hemispheres of the brain control both eyes, with each hemisphere processing a different half of the retina of each eye). Also, foot use does not require anything like the same level of motor control as hand use.
Other (less important but nonetheless interesting aspects) of laterality include hand-clasping and arm-folding. Hand-clasping can be demonstrated by quickly clasping your hands together, interlacing the fingers in what feels like the most natural way. About 60% of people, regardless of whether they are left-handed or right-handed, clasp their hands with the left thumb on top. Arm-folding also shows evidence of a type of handedness: when you fold your arms naturally, take note of which wrist is on top. Again, about 60% fold their arms with the left wrist on top (and indeed find it quite difficult to do otherwise), and once again, it appears to be totally independent of either a person’s handedness or their hand-clasping preference.
The direction of the hair whorl on the crown of the head is another example of laterality, with about three-quarters of people having a clockwise whorl (causing a tendency for the hair to naturally part on the left), and a quarter anti-clockwise (although different studies have yielded results anywhere from 65% to 92%). Research by Amar Klar indicates that about 45% of left-handers have anti-clockwise hair whorls, compared to only about 8% of right-handers, suggesting a possible genetic link between the two phenomena, but it is by no means a simple link.