ResearchBlogging.orgJim Thomeson – who gets a brofist for being one of the most frequent commenter here – asks

I wonder about handedness. i am right handed, and my right hand can do things for which my left hand has no clue. On the other hand, both hands attended typing class at the same time, and type equally well; maybe the left is a hair better.

Ok fine, he isn’t really asking anything. There’s no question here but I felt like writing something and this is as good a starting point as any. What about handedness? Is it something we evolved because it confers an advantage or is it actually neutral and just got carried along with a beneficial gene? Or is there no genetic influence at all?

The chimp handedness data from the early study which shows that there is an almost even preference for each hand.

For the longest time these questions were made all the more fascinating by the fact it was believed that a right-handed majority was a uniquely human trait. Early studies suggested there was an even split, with half of primate populations favouring each hand. This raises the interesting question of why so many humans are right-handed given it clearly wasn’t the ancestral condition.

Except it kind of was. Later research found that there was indeed a population wide preference for a particular hand, although curiously only for certain tasks. Most primates favoured the left hand for reaching whilst the right hand was preferred for manipulative tasks.

So where does that leave us? On the one hand (if you’ll pardon the pun) there is no primate population that is as right handed dominant as humans. Even for manipulative tasks only 60-70% of the group preferred their right hand whilst 90% of humans like their right hands.

On the other hand humans do engage in more manipulation than other primate species which might explain the overall dominance of that hand. After all, are not most tests of handedness (i.e. writing) manipulative tasks? However, there is no way of knowing whether this increase in manipulation is sufficient to account for the differences between humans and primates so there is still the possibility that some recent mutation is responsible for this dominance.

Yet according to wikipdia to my knowledge only one gene associated with handedness (in particular left-handedness) has been identified yet it is not responsible for every case of left-handedness. Since it is also associated with schizophrenia (and that is associated with left-handedness) it may well be that this particular gene is just linked to schizophrenia and not involved with determining hand preference at all.

So we have no idea what’s going on with you Ned.

So if we don’t know the ultimate cause of handedness, can we still work out its evolutionary history? Pretty much. The muscles on a creature’s preferred hand are used more often and thus bigger than their other hand. Such an increase in size typically leaves a bigger muscle scar on the bone allowing handedness to be identified in the skeletal remains of our ancestors.

AL 288-1 (Lucy). Note her almost total lack of hand bones.

The results are that pretty much every specimen we can identify handedness on shows a bias towards the right hand. However, a lack of well preserved, articulated hands for early hominin species means that all of these right handed specimens come from the genus Homo. This only emerged 2.6-2 million years ago, meaning there are still several million preceding years of unknown preference.

You can also work out handedness by looking at stone tools. However, since Homo is the only genus known to manufacture stone tools this research doesn’t really add anything new to that picture.

So we don’t know how it evolved nor do we know when. Could we work out why? Err…no. Whilst some studies proclaiming an advantage to one handedness (normally left) do occasionally appear there are also many studies – including reviews of the literature – finding no such effect.

Many have suggested that it is linked to hemisphere dominance in the brain with most humans, for one reason or another, favouring their left hemisphere (linked to the right side) resulting in most humans using their right hand. So right handedness itself might not offer an advantage but is just a by-product of another beneficial trait.

However, if using the left hemisphere is so great then why do left-handed people still exist? One favoured explanation for their persistence is frequency-dependent selection. This is when the advantages of one trait are dependent upon how frequent another trait is. For example, in an environment where everyone fights if they see someone else an individual runs away when they see someone will be at a large advantage as they are less likely to be injured in a fight. Conversely, in an environment where everyone runs away someone who fights will do well as nobody fights back.

There is a ratio of fighters to runners where both are similarly advantageous. This ratio will be stable and persist through time, with any influx of runners giving an advantage to the fighters thus restoring the balance. Perhaps left handed individuals do well in a right handed world and the 90/10 ratio of right/left hands we see is the “evolutionary stable strategy” for handedness.

A table showing a hypothetical frequency-dependent scenario.

Yet worldwide studies of handedness show that this “stable” ratio can vary by as much as 10%! This isn’t what we would expect if it were an evolutionary stable strategy. This might have something to do with different cultural stigmas against a particular hand preference artificially altering the ratio. We’ve all heard tales of left-handed people not being allowed to become knights.

Whilst this explanation means left-handedness could still be the product of frequency-dependent selection it also means that we would have no way of determining if it actually is. Any divergence from predictions could just be explained away as cultural influence. Whilst a very detailed study could control for such influences, to my knowledge none have been done.

In conclusion, we know a gene causes handedness, although we don’t know which, and we know a large right-handed dominance emerged in Homo  although we don’t know when or why. Kind of a non-answer to the question really, but I think that’s allowed since it was a non-question to begin with.

Cashmore L, Uomini N, & Chapelain A (2008). The evolution of handedness in humans and great apes: a review and current issues. Journal of anthropological sciences = Rivista di antropologia : JASS / Istituto italiano di antropologia, 86, 7-35 PMID: 19934467
FINCH, G. (1941). CHIMPANZEE HANDEDNESS Science, 94 (2431), 117-118 DOI: 10.1126/science.94.2431.117
Francks C, Maegawa S, Laurén J, Abrahams BS, Velayos-Baeza A, Medland SE, Colella S, Groszer M, McAuley EZ, Caffrey TM, Timmusk T, Pruunsild P, Koppel I, Lind PA, Matsumoto-Itaba N, Nicod J, Xiong L, Joober R, Enard W, Krinsky B, Nanba E, Richardson AJ, Riley BP, Martin NG, Strittmatter SM, Möller HJ, Rujescu D, St Clair D, Muglia P, Roos JL, Fisher SE, Wade-Martins R, Rouleau GA, Stein JF, Karayiorgou M, Geschwind DH, Ragoussis J, Kendler KS, Airaksinen MS, Oshimura M, DeLisi LE, & Monaco AP (2007). LRRTM1 on chromosome 2p12 is a maternally suppressed gene that is associated paternally with handedness and schizophrenia. Molecular psychiatry, 12 (12) PMID: 17667961
Halpern DF, Haviland MG, & Killian CD (1998). Handedness and sex differences in intelligence: evidence from the medical college admission test. Brain and cognition, 38 (1), 87-101 PMID: 9735180
Hopkins, W. (1996). Chimpanzee handedness revisited: 55 years since Finch (1941) Psychonomic Bulletin & Review, 3 (4), 449-457 DOI: 10.3758/BF03214548
Keller JF, Croake JW, & Riesenman C (1973). Relationships among handedness, intelligence, sex, and reading achievement of school age children. Perceptual and motor skills, 37 (1), 159-62 PMID: 4727991
Perelle, I., & Ehrman, L. (1994). An international study of human handedness: The data Behavior Genetics, 24 (3), 217-227 DOI: 10.1007/BF01067189
Toth, N. (1985). Archaeological evidence for preferential right-handedness in the lower and middle pleistocene, and its possible implications Journal of Human Evolution, 14 (6), 607-614 DOI: 10.1016/S0047-2484(85)80087-7

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Jesse Marczyk · 13th June 2012 at 4:41 pm

There’s also been some hints at a correlation between left-handedness and homosexuality, though it’s been a while since I’ve been looking at those papers specifically.

    Adam Benton · 13th June 2012 at 6:43 pm

    Whilst that would certainly be very interesting to look at, given the way science seems to be flip-flopping on what left handedness is and isn’t correlated with it would have to be a very rigorous study to convince me of anything.

    Also, I can’t seem to find a follow button on popsych.

      Jesse Marczyk · 14th June 2012 at 5:54 pm

      That would mostly be due to my own shortcomings. Working on remedying that issue.

      Jesse Marczyk · 14th June 2012 at 6:02 pm

      And that should do it.

        Adam Benton · 14th June 2012 at 6:03 pm

        Hehe, I think I’m following now. I entered the email address and was promptly hit by a 404 error.

        As long as I’m informed of new posts I don’t really care.

Donald Miller · 14th June 2012 at 1:06 am

I was born lefthanded (although I’m not a homosexual) and my parents thought it was a good idea to turn me into a righty. I had a tough time with writing ds and bs for quite some time. Overall, I’m glad they did it.

But what I find really interesting about this is the lack of ambidexterity. I’m a bit ambi, (although not bisexual)* and I’ve always thought that ambidexterity should be encouraged. I am actually serious about that. I see advantages in it, and I think it’s probably easy to achieve, if only we used a our nondominant hand at more tasks. For instance, no one is ever graded on how far they can throw a ball with either hand. It’s always one or the other. In deed, I don’t even know what the ratio is to how far I can throw a baseball with my left hand compared to my right. If I had one, I’d give it a shot.

*Hope Jesse’s got a god sense of humour.

    Adam Benton · 14th June 2012 at 1:56 am

    The point about cerebral dominance is that by making everything occur in the same location there is less distance for signals to travel. As such it would probably be better to focus on just one hand instead of both (although I will admit that being ambidextrous is b pretty cool, even if not from a neurological point of view).

    On a related note, the Perelle & Ehrman study I cited includes a section where it categorises people on a “scale” of handedness, from very right dominant to very left dominant. Indeed, it is now the common consensus that it isn’t as binary as once thought, with people falling on this range.

    That same study also notes that the percentage of left handed people who have tried switching is significantly higher than the percentage of right (~20% vs ~6%). However, returning to the aforementioned sliding scale we find only 10% of left-handed people are ambidextrous. Thus even if every ambidextrous was the result of a switching attempt (which has not been shown to be the case) there’s still a 50% “failure” rate, with people switching but not becoming ambidextrous.

      Donald Miller · 14th June 2012 at 3:00 am

      Thanks for the interesting information.

Darby · 27th June 2012 at 11:19 pm

I learned a while ago that the alleles were dominant – right, and recessive – no clear preference. That will skew the population data (since a fraction of the righties will be genetic “lefties), and explain why a lot of identical twins show different handedness (I’m a fencer, and I’ve seen that A LOT).

    Adam Benton · 3rd July 2012 at 12:50 pm

    I did encounter a few claims like that whilst doing the research but a lot of it was based on population data rather than genetics. In other words if the trait was genetic, what would it be like based on its distribution through the population? Of course, this is rested upon the assumption that the trait is indeed genetic so whilst interesting cannot ultimately help inform us about its evolution since we need to be able to identify the actual gene to work out if has been influenced by natural selection.

    However, you may well have read a more useful study (my research was not exhaustive) and so if you do have a link for it I would most appreciate it.

    I would not have thought twins would be particularly common during fencing. I’ve only ever met one set (in general, I don’t fence).

Artem Kaznatcheev · 5th July 2012 at 1:45 am

Handedness is a pretty hard symmetry to break. For instance, in Turing’s original work on morphogenesis ( ), one of the big challenged for him was to figure out how an embryo could break handedness symmetry. He never resolved this issue, and I don’t know how (or if) it has been resolved since. Unfortunately, I know nothing about morphogenesis or developmental biology in general, and bio.SE has not yielded results, yet:

Evolutionary game theory I know a little bit more about and I am not convinced by your couple of paragraphs on it. Although you can have stable internal fixed points in some game (like the Hawk-Dove example you gave), you can just as easily have an absorbing boundary (such as in Prisoner’s dilemma), or an unstable internal bifurcation point (such as in other variants of stag-hunt games like assurance). Without a good argument for why the game involved is of a form that has an internal fixed point, there is no reason to prefer the EGT explanation.

In my reading, I have not come across a EGT analysis of handedness. I couldn’t figure out which of your references looks at handedness from the game-theoretic perspective. Could you suggest a paper for me to look at?

Hans Klamer · 21st November 2018 at 10:08 pm

Just started reading up this blog a few weeks ago, beginning at the end so as not to miss anything. That’s why my comment/question might appear to be a little late ..
Anyway, how about soccer and being rigth-footed, or left-footed, or, like me and my dad, being “ambi-footed” (although we can deliver a more secure pass with our left leg, but from a farther distance with the right leg). My son is strictly right-footed. I gather most people are right footed, but why?

    Adam Benton · 5th December 2018 at 2:11 pm

    Looking at the literature, I don’t see much published on it so can only speculate. And if I was to do so, I might talk about how there’s a lot of feedback between our hands and feet during evolution. Changes to one trigger things in the other. Maybe bias developing in one led to it in the other.

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