Miscalibrated Minds: Why Don’t We Apply What We Know About Twins to Everybody? | Hacker News Reader

Miscalibrated Minds: Why Don’t We Apply What We Know About Twins to Everybody?

michaelgr.com

12 pointsMikeCapone23 comments

I'd contest the statement that "nobody’s surprised when identical twins turn out to have very similar bodies (weight, muscle mass, etc), even into adulthood." From the research I remember reading about, there does seem to be some genuine level of surprise as to how much influence genetics has over even twins separated at or near birth - and that the level of genetic determinism that twins exhibit is certainly rather controversial even amongst those that study them.

Twins also shares a common pre-birth environment, with the same environmental influences. I've read the argument that probably helps a lot to make them similar.

Actually, those identical who grow up apart (in the same country) are more similar to each other than those who grow up together.

Also, there's quite a bit of research comparing monozygotic (i.e. "identical") and dizygotic twins. The differences in correlation are stark. Twins who share the same genes are far, far more alike than those who don't: http://en.wikipedia.org/wiki/File:Heritability-from-twin-cor...

>>Actually, those identical who grow up apart (in the same country) are more similar to each other than those who grow up together.

Not relevant. I noted that the environment in the womb was very similar for twins, which certainly influences a lot.

(It was a nitpicking note, I am aware of the strength of the twin arguments in general and not one of those idealists which dislikes arguments which goes against Marx -- or whatever it now is that the twin arguments contradict...)

Only yesterday there was another submission about a twin who weighs 15 pounds (or kg?) more than his brother.

Really, what DO we know about twins? Most twins also grow up in the same environment and with the same education.

In fact when I looked up some IQ studies of twins, it turns out that there really aren't many cases of twins that have not grown up together. Usually some war happenings were the background. As a result, those studies don't typically look like what you would a scientific experiment to look like. Some separated twins might only be discovered in their 50ies, others in their teens.

You can not just go ahead and say "let's look at 50 pairs of twins, of which one grew up in a poor family and one in a rich family", because you will not find 50 such twins from the same era.

Maybe you find some poor twin who grew up in the 50ies, and another poor twin who grew up in the 80ies, or one grew up in Africa and the other in Europe, so comparison already becomes questionable.

From the other story:

"For a start I was 15kg heavier than my brother."

but...

"Of the thousands of twins enrolled in the research program only 10 were more different in weight than us."

So it seems that in general twins weigh roughly the same.

15kg is quite a lot. Most men in my street (by sight) would be within 15kg of me, I'm about 83kg ~ 13 stone (slightly over WolframAlpha's reported median USA male weight in 2006).

I think the chance of being more than 15kg outside the weight of someone who has grown up in the same lifestyle and with the same food opportunity is probably pretty low.

When I went back to the states, I saw people that weighed 50kg more than others near them pretty much every time I went out.

"So it seems that in general twins weigh roughly the same."

It should be easy to study the eating habits of these twins. In most cases, why should they differ much, if they have been raised in the same household?

Or, maybe, twins share similar thought patterns and therefore make similar lifestyle choices (how much to eat, how often to exercise, and so on) and that's why they share similar weights and muscle mass.

That said, the nature/nurture pendulum has been stuck on the "nurture" side for a while, and it's now starting swinging back to the middle where it belongs. It's got a ways to go though. I'm reading Steven Pinker's The Blank Slate now, and I highly recommend it to people who are interested in this topic.

It's all nature. Nurture is what you do with it.

Twins may be identical physically, but they present a challenge to the assumption that being is merely physical because they are often completely different when it comes to who they are (and what is that, really?)

I have lived with twins for 3 years (married to one of them) and you couldn't find two people more different. Physically they look the same, but as far as personality, you couldn't find two people more different.

They were both "nurtured" from the same woman, same environment, yet they are opposites. I think there is an ontological question raised. Is there a non-physical component to our existence? Something to think about.

Why does a non-physical component need to be invoked? Why could there not simply be genetic mutation that affects personality but not physical stature? It seems to me that there's no reason to believe some non-physical component is at work until the physical components have been ruled out.

(Identical) twins are genetically identical.

(edited--thanks patio11)

Monozygotic (identical) twins are. Dizygotic (fraternal) twins are not.

Actually, clarification to the clarification: fraternal twins, like any two siblings from the same parents, are merely highly unlikely to be (close to) genetically identical. For any given chromosome, you either get one of Mom's or one of Dad's. That gives you a finite number of possible chromosomal choices. Since they're essentially chosen with replacement between fertilizations (in the discrete sense of "chosen with replacement", its not like the egg says "OK, I'm done with this X chromosome mom, you can have it back now"), you could randomly end up with two siblings (and, by extension, two fraternal twins) with the same chromosome choices (i.e. pretty close to genetically identical, allowing for some minor and likely consequence-less mutation).

My naive calculation of the probability of any two siblings being genetically identical without being identical twins is 1 / (2 ^ 48), though. :)

Much, much smaller than 1/(2^48). You don't get whole chromosomes from each parent.

Suppose one of your father's chromosome pairs is (ABCDEF,abcdef) and your mother's corresponding pair is (PQRSTU,pqrstu). Then you might be, e.g., (ABcdef,pQrSTu). Except that actually there are hugely many points at which crossover (i.e., a switch between ABCDEF and abcdef) can occur, and it can happen a variable (though usually not very large) number of times on each chromosome.

Oh, and after that you'll have maybe 100 mutations (which could be anywhere) relative to your parents.

Crude back of envelope calculation: you have on the order of 10^8 base pairs on each chromosome; there might typically be two crossovers per chromosome pair from each parent, giving you on the order of 10^16 possibilities for the chromosome you get from your father, the same for your mother, and hence ~ 10^32 possibilities for each of your chromosome pairs, so ~ 10^700 possibilities in all. Then you have ~100 extra mutations in your ~10^9 base pairs, giving another 10^900 possibilities; so there are ~ 10^1600 ways to make a typical human child's genome from a given pair of parents.

That's a super-crude calculation, of course, but it's much nearer the right order of magnitude 2^48.

Oh, twaddle. What differencse between "identical" twins challenge is the idea that being is purely genetic. Fortunately, no one actually believes in that idea.

All apples of a given variety are (I think) genetically identical, because of the way they're grown. (You start with a rootstock of some kind of apple tree that doesn't produce nice fruit, and then graft on the thing you actually want. This has to be done because if you grow apples from seed you tend to get trees that are very unlike their parents.) But -- gasp! -- not all Granny Smith (or whatever) applies look identical. Is this evidence for a non-physical component to applehood?

In the back of my mind I recall a study that said identical twins raised separately will often act differently to try to differentiate themselves, whereas identical twins raised separately have no such motivation and thus often end up more similar.

If anyone could verify my hazy recollection I'd be most interested.

1) even if their environment was very similar, it wasn't identical, if only because they interacted with each other. As you probably know, living system are roughly chaotic (except that, formally, chaotic systems are strictly deterministic, whereas determinism in the real world is debatable), and a tiny difference at one time point can have drastic consequences later on.

2) identical genes can lead to different phenotypes. Take the example of handedness. There is a dominant allele for right handedness and a recessive allele for random handedness. The people with two random genes have 0.5 chances of being right handed, and 0.5 chances to be left handed or ambidextrous.

The same gene also determines if the hairwhorl turns clockwise or counter-clockwise, with the same behaviour. The dominant allele mean clockwise, and the recessive one is random. AFAIK, in the random case, handedness and whorl direction are independant.

I don't think that your example is a good case for dualism.

Edit: Were you trolling?

Looking at twins that grew up in the same environment isn't all that appropriate for making statements about the dominance of genes. Of course, if the data doesn't fit our assumptions we could always cite Cyril Burt or do it like he did.

The jury seems to still be out on Burt. Hamilton as a defender is not bad...

http://en.wikipedia.org/wiki/Cyril_Burt#.22The_Burt_Affair.2...

As I'm about half done with Good Calories, Bad Calories, this post rather resonates. My mother is an identical twin and she is about as equal in stature to her sister as you can be. It's curious to me that I've always accepted that since they're twins, they must have similar body types. It is very quickly apparent to me that, because they live apart and don't eat the same things, weight and/or body type must have some genetic or metabolic component unrelated to consumption or exercise.

The disconnect between my observation and my previous thinking of obesity being related to will power or exercise habits is sort of mind blowing.

Are you sure they don't eat the same things? They grew up together? This is most likely to mean that they eat a similar diet - if they co-habit with people outside the others social setting then that's probably going to lead away from similar diets.

My cooking is very much influenced by the things my mother cooked for me; I also borrow recipes from my sister occasionally.

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