

Miscalibrated Minds: Why Don’t We Apply What We Know About Twins to Everybody? - MikeCapone
http://michaelgr.com/2009/09/30/miscalibrated-minds-why-dont-we-apply-what-we-know-about-twins-to-everybody-else/

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JCThoughtscream
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.

~~~
berntb
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.

~~~
xiaoma
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...](http://en.wikipedia.org/wiki/File:Heritability-from-twin-
correlations1.jpg)

~~~
berntb
>>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...)

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Tichy
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.

~~~
ZeroGravitas
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.

~~~
pbhjpbhj
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.

~~~
xiaoma
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.

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a-priori
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.

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

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donniefitz2
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.

~~~
latortuga
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.

~~~
philwelch
(Identical) twins are genetically identical.

(edited--thanks patio11)

~~~
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. :)

~~~
gjm11
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.

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xtho
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.

~~~
berntb
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...](http://en.wikipedia.org/wiki/Cyril_Burt#.22The_Burt_Affair.22)

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latortuga
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.

~~~
pbhjpbhj
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.

