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Comment by theoreticalmal | Hacker News Reader

“ when you look at rare conditions, like Down Syndrome, epilepsy and cerebral palsy, the ratio of left- to right-handers is more like 50:50 rather than 1:10.”

Is this a typo? Or are left-handed people way, way more likely to suffer from these rare diseases?

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Let's work it out: 1 in 10 are lefty, 1 in 1000 have Downs. Supposedly 50% of people with Downs are lefty, therefore:

P(A) = 0.001

P(B|A) = 0.5

P(B) = 0.1

P(A|B) = P(B|A)xP(A)/P(B) = 0.5x0.001/0.1 = 0.005 = 0.50%

So the chances of having Downs given being a lefty is 1 in 200, or which is 5 times higher than the base rate, but still quite rare. It's probably more a case of "having developmental abnormalities interferes with handedness" rather than "being a lefty predisposes you to having sundry developmental abnormalities".

People who have them are more likely to have other disturbances. One of the few things I recall clearly from embryology in med school is that any disturbance of the midline and sided structures of the body often reflects some abnormal development: there's rarely only one (e.g., if their brain is abnormal, you had better look for cardiac abnormalities).

Left-handedness is unusual, but not per se pathologic. There is a running joke among neurologists that the average neurologist is a left-handed person with migraines. They aren't all, of course, but if you had chronic headaches and were left-handed, wouldn't that predispose you to pursuing a specialty where you try to figure out what's going on in brains? I majored in chemistry; it's not a huge stretch to imagine why I became an anesthesiologist. It's interesting, and it makes some intuitive sense. We tend to be more of the "analyst" personality; former engineers and accountants are not rare in anesthesia.

Or there are simply a lot of people who might be left handed by default but then adapt and learn to be right handed. People with rare conditions like the above might either have more trouble adapting or perhaps socially we put them more in special ed programs where they are observed more closely and stick with their default.

It might just mean that people with certain disorders have much bigger immediate problems in hand-movement to overcome, so that there's no extra capacity/opportunity to engage in mimicking the right-handedness of people around them.

Down syndrome (trisomy 21) and cerebral palsy are something you're born with no? Epilepsy too maybe? (even if it only declares later on in life?)

So how is this not inversing cause and effect:

> Or are left-handed people way, way more likely to suffer from these rare diseases?

There are a lot of myths and anecdata about lefties having shorter life expectancy. That might indicate health consequences in some way.

IIRC the major factor in that was found to be that left-handed people within the age ranges where significant numbers of them are dying of natural causes had usually been brought up to suppress left-handedness, so are reporting as right-handed and skewing the data.

But why would that mean that the people who didn't suppress died so much younger?

> [A]ccording Chris McManus, the researchers made a "very subtle error"...

> Halpern and Coren took a list of the people who had recently died and contacted their families, asking whether or not their relative had been right- or left-handed.

> Looking at 2,000 cases, they saw that the average age at death of the left-handers was about nine years younger than of the right-handers.

> On that basis, they concluded that left-handers died earlier.

> At first glance, that seems persuasive. What did the researchers do wrong?

> "Their mistake was that they only looked at the dead," Chris McManus explains.

> The point is that left-handers are more common now than they used to be, so - at least at the time the research was published - left-handers were on average younger than right-handers.

https://www.bbc.com/news/magazine-23988352

Could also be due to right-handed tools. Chainsaws...

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