[Freak_NL]

So why are us southpaws a rarity? The article and the linked research paper both point to bipedalism and bigger brains as the cause, and the paper vaguely seems to hint at selective pressures leading to the right hand getting favoured by the majority of the population, but why?

The question from the headline is excellent, if only it was actually answered.

[scythe]

Here's my five minute lunchtime hypothesis: it's because the heart is on the left. As human behavior demanded increasing precision from the hands, being a little farther from the heartbeat was a slight advantage.

[yawpitch]

Here’s my multiple years of anatomy classes response: the heart isn’t on the left. The aorta is, sure, but the vena cava is on the right. Also people with situs inversus (essentially all organs flipped laterally from “normal”) aren’t obviously more prone to left-handedness.

[mortenjorck]

I’ve wondered ever since fourth grade (where an anatomical model in a corner of the classroom always made it clear that the heart is centrally located) how the vernacular conception of the heart as located on the left originated and persisted.

Your post finally made it click for me – the aorta extending to the left gives the superficial impression of that being the heart’s location because it’s easier to feel the heartbeat through the skin, versus the more deeply embedded vena cava on the right.

Presumably this means, evolutionarily, greater vulnerability on the left, predisposing the left hand to shielding duties, leaving the right to more dexterous tasks like spearing. The cardiological hypothesis of right-handedness holds!

[cortesoft]

> Also people with situs inversus (essentially all organs flipped laterally from “normal”) aren’t obviously more prone to left-handedness.

I feel like this isn’t really an argument against the theory. If right handedness did evolve because of heart position, a later genetic mutation to have the heart on the opposite side wouldn’t suddenly undo the previous evolution towards right handedness.

[yawpitch]

Why are you assuming situs inversus, which occurs in species with no handedness (or, indeed, hands) came after handedness?

The argument is that the selection bias was towards precision and the hypothesis was that precision is influenced by heart position (which is, still, in the middle in humans)… individuals with situs inversus would be more precise in the left hand, thus if the causal hypothesis is correct AND the argument holds then there should be a selection bias that would result in a correlation between situs inversus presence and left-handedness.

In the end I don’t believe either the argument or the hypothesis hold even as much water as I can in either hand.

[throwway120385]

It might be hard to eliminate confounding factors depending on when the research was done. A lot of people in my generation were still dissuaded pretty heavily from writing with their left hands. I'm not entirely convinced anymore as a lay person that "handedness" is a real, distinct phenomenon that's primarily genetically determined or a result of the organization of the brain. It's equally possible that it's a learned preference and that the way the brain organizes around it is as a result of the preference's impact on how you have to solve problems with your preferred hand in a society that preferences right-handedness.

[rybosworld]

Not disagreeing that handedness is probably unrelated to heart position.

But why would situs inversus somehow be tied to this at all? If there's a gene that favors right-handedness, it's not like it would somehow "choose" left-handedness because the individual has their internal organs flipped.

[yawpitch]

Genes don’t favor (or not favor), but if a natural selection bias for precise dexterity exists AND heart lateral orientation affects dexterity precision THEN those with flipped lateral orientation should exhibit more dexterity in the left hand, thus they should be naturally selected for because of the same bias.

Now, I’d seriously doubt there’s any evidence whatsoever for the assumed selection bias in the first place, never mind any causal relationship between fine motor control and heart asymmetry, but the selection bias should apply to both flips of the anatomical mirror.

[stackghost]

>Here’s my multiple years of anatomy classes response: the heart isn’t on the left.

Why is the left lung smaller, then?

[yawpitch]

The heart is asymmetrical, but it’s in roughly the center of the chest. The left auricle and ventricle are larger muscles because they’re pumping through the descending aorta to the extremities, that’s the systemic circulatory branch, the plumbing for which is also largely to the right, while the right are pumping into the lungs alone as part of the pulmonary circulatory branch. The left lung (right on those with situs inversus) has two lobes and basically accommodates the extra muscle mass on its side of the heart, but if you really want to kill someone you stab them through the sternum, kind of dead center, not where they hold their hand when performing patriotism.

[stackghost]

>if you really want to kill someone you stab them through the sternum, kind of dead center, not where they hold their hand when performing patriotism.

Noted, thanks.

[rolph]

even this is wrong, a penetrating weapon aimed for the heart is applied below the sternum at roughly the positionof the 3rd shirt button, and thrust upward at shallow angle topass behind the manubrium, and is then levered into a pommel upward position so as to lacerate the heart

[rolph]

not only smaller but having 2 lobes rather than 3, the left lung is possessed of a featureknown as the cardiac "notch" an involution of the lobe that corresponds to the larger left ventricle of the heart.

[altruios]

More piping to and from the heart exists on the left instead of the right?

[rolph]

the Aorta and Vena Cava are muchmore central than sinistral.

the aortic arch begins decent left of the coronary corpus, but becomes centralized, tandem with the Vena Cava.

[gherkinnn]

Wikipedia on Situs Inversus (visceral organs are mirrored, heart on the right, liver on left) [0], mentions mixed results regarding handedness. There would be a load of other confounding factors here and I know nothing about medicine.

0 - https://en.wikipedia.org/wiki/Situs_inversus

[scythe]

Situs inversus ("dextrocardia") is a rare disorder. What I postulated is a (very) small selective advantage leading to a neurological mechanism evolving over generations, not a direct line from the heart to handedness during development. Anyway, the effect would be very slight, and even if it did exist, it could have gone away later, but dexterity would have been baked in at that point (see also the ocular blind spot).

[xattt]

Childhood handedness development within the brain became independent of organ positioning, after positioning had become established.

[emporas]

That's a long time hypothesis of mine as well, but I think it stems from being stung or bitten by venom. If venom is injected into the bloodstream, it is desirable to be injected as far away from the heart as possible.

Some centimeters might not sound much, but over millions of years, the cumulative effect might be that 1% of human population every 10.000 years gets genetically optimized to hold their heart at a more protective spot.

[busyant]

Interesting!

Handedness is probably not (often) captured in healthcare records, but I'm wondering if epidemiologists could mine insurance claims (or some other data rich resource) to see if there's a correlation with serious outcomes (death, hospitalization, etc.) from venom and handedness.

[emporas]

That's a good idea, a very good idea actually, but I wonder about it's effectiveness due to a very small total number of snake bites nowadays, compared to the past.

Hundreds of thousands years in the past, hominids lived into much more tropical areas than today and there are a lot more spiders, scorpions, lizards and snakes in these warm places. It makes sense that insects and especially reptiles pushed the evolution of mammals in certain directions and the positioning of the heart in the human body might be one of them.

Today people live a much different lifestyle than having to deal with insects and reptiles all day long. I don't know if it is possible to decipher the past from today's data.

[gpm]

If this was the case wouldn't it be easier to measure the pulse in peoples left wrists? Which doesn't seem to be a thing?

[Tor3]

But it is a thing. That's how it was always explained in school, among other places, when I was a kid.

[abernard1]

There is also a bias for how babies are held [1]. It holds even with left-handers. Holding a baby's head near the mother's heart helps the baby get to sleep. Which means the baby doesn't cry (and attract predators) and also gives the parents more time to sleep at night.

It also allows right-handed mothers to do something with their dominant hand while cradling the baby in that position.

[1]: https://sites.psu.edu/clarep/2024/04/12/the-left-cradling-bi...

[thaumasiotes]

> Here's my five minute lunchtime hypothesis: it's because the heart is on the left.

Your hypothesis can't possibly be correct, because the only premise is false.

[booleandilemma]

I wonder why you're getting downvoted? Even if it turns out you're completely wrong it's still an interesting point and something I never even considered before.

[scythe]

Sometimes I think people downvote me because they're frustrated that I didn't engage further. After twenty years of Internet discussions, I'm a little burned out and I tend to fire and forget.

[booleandilemma]

You might enjoy this: https://serjaimelannister.github.io/hn-words/

[dwd]

I would think right-handedness is largely reinforced through learning gross motor skills as an infant. If you always use your right, your brain optimises for that.

I wonder whether something simple like being allowed to select and use an object with either hand rather than having it offered to your right hand retains ambidextrous by the time handedness became fixed in the brain around age 4-6.

[nephihaha]

I remember reading that there is evidence that Neanderthals tended to be left handed. Someone else might be able to confirm/debunk this.