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Why Don’t We Apply What We Know About Twins to Everybody Else?

13 Post author: MichaelGR 01 October 2009 04:23PM

I think our intuition might be miscalibrated when it comes to evaluating how much a person’s genes impact how they turn out physically (which isn’t surprising). What’s a bit strange is that we seem to be closer to the truth when it comes to twins.

Nobody’s surprised when identical twins turn out to have very similar bodies (weight, muscle mass, etc), even into adulthood.

But when it comes to non-twins, people seem to think that “making the right choices” and “willpower” are primary factors in how human bodies turn out, and that we can assign a good amount of personal credit or blame to individuals for good and bad outcomes.

There is a disconnect between these two visions, and I think that it’s the latter that needs to be updated.

After all, even if we put aside the direct ways in which our genes build our bodies (encoding how our tissues grow) and instead look at our abilities to “make the right choices” and exert “willpower”, we find that those are also greatly determined by genetic factors. Identical twins probably turn out very similar in good part because they have almost identical amounts of those qualities of mind.

This doesn’t mean that all is pre-determined and that if we all stop trying we’ll turn out the same we would have otherwise, but rather that we are playing within certain parameters, and that the part we control is probably smaller than most people think (not non-existent — we still deserve some credit — just more modest).

To be clear, I’m not saying the situation was white and we thought it was black, or even that it’s a black & white thing, but rather that most people’s intuition might be the wrong shade of gray. Otherwise, I would think there would be a bigger variation between identical twins, but they spend their lives making different choices yet most stay very similar to each other (as far as I know — if you know of a study on this, please send it my way).

Comments (26)

Comment author: PhilGoetz 01 October 2009 09:57:02PM *  6 points [-]

A study of fat/skinny twins was only able to recruit 14 such twins from a registry of 2,453 twins. It doesn't say what fraction of those 2453 were fat/skinny ("obesity discordant"). The BMI ratio between those 14 was only 6/5.

They suggested that the obesity difference might be accounted for by mitochondrial DNA copy number. mtDNA copy number is regulated, not simply "genetically innate"; so I don't see that as a "genetic" explanation.

(I put "genetic" in quotes because the things we say have genetic causes are only a small subset of the things which have genes as causes.)

The researchers recruited 14 pairs of genetically identical Finnish twins born between 1975 and 1979 who were “obesity discordant”—that is, one twin of each pair had a BMI of about 25 (not obese); the other had a BMI of about 30 (obese). The researchers took fat and blood samples from each twin, determined the insulin sensitivity of each, and measured the body composition and various fat stores of each. They found that the obese twins had more subcutaneous, intra-abdominal, and liver fat and were less insulin sensitive than the non-obese twins. Insulin sensitivity correlated with the amount of liver fat. Analysis of gene expression in the fat samples showed that 19 gene pathways (mainly inflammatory pathways) were expressed more strongly (up-regulated) in the obese twins than the non-obese twins, whereas seven pathways were down-regulated. The most highly down-regulated pathway was a mitochondrial pathway involved in amino acid breakdown, but mitochondrial energy metabolism pathways were also down-regulated. Finally, mitochondrial DNA copy number in fat was reduced in the obese twins by nearly half, a novel observation that could partly account for the obesity-induced metabolic defects of these individuals.

Comment author: Psychohistorian 01 October 2009 08:22:07PM *  8 points [-]

the part we control is probably smaller than most people think

Think it would be more accurate to say

the part we control is probably different from what most people think

The idea that body type is basically determined by genetics simply doesn't work, because current body types diverge rather dramatically from body types of a hundred or even fifty years ago. Genes don't change that fast.

It's more likely that people are responding to culture and incentives that are largely beyond their control. Culture and incentives can change dramatically in the necessary time frame. This doesn't mean that people can't make the right decisions, so much as it suggests the right decisions may be hard to figure out. Choosing to associate with people who will look down on you for super-sizing your bacon double cheeseburger may do a lot more than deciding to get a diet coke with it, but good luck selling a diet book centered on that idea.

Comment author: AndrewKemendo 03 October 2009 08:00:43AM 2 points [-]

Current body types diverge rather dramatically from body types of a hundred or even fifty years ago

Dramatically? I have to disagree with that. I wasn't around 100 years ago but I've seen pictures and they didn't look much different than us, really at all. From the medical texts I am familiar with, our insides work pretty much the same as well.

I agree with Michael below that nutrition has made differences in how, say bodybuilders change their bodies, but the average person from what I can tell is about the same in type - if perhaps marginally taller on average.

Comment author: Psychohistorian 03 October 2009 07:38:01PM 2 points [-]

people seem to think that “making the right choices” and “willpower” are primary factors in how human bodies turn out,

The post is clearly referring to obesity and physical fitness. Average bodyweight, BMI, and obesity rates have risen rather dramatically since 1900.

You're completely ignoring the context of the statement. With specific respect to fatness, demographics have changed very dramatically very fast, which suggests a non-genetic cause. Average number of lungs had not changed all that much, which suggests a genetic cause, but no one even pretended to challenge that.

Comment author: MichaelGR 01 October 2009 08:27:46PM 2 points [-]

The idea that body type is basically determined by genetics simply doesn't work, because current body types diverge rather dramatically from body types of a hundred or even fifty years ago. Genes don't change that fast.

A change in available nutrition in the past few hundred years is probably the biggest factor to explain this.

Comment author: loqi 01 October 2009 07:50:01PM 3 points [-]

This doesn’t mean that all is pre-determined and that if we all stop trying we’ll turn out the same we would have otherwise, but rather that we are playing within certain parameters, and that the part we control is probably smaller than most people think (not non-existent — we still deserve some credit — just more modest).

A lot of these concepts (willpower, choice, responsibility, blame, credit) really start to break down if you look at them too closely, and this post is using them a little too formally for my comfort. What do you actually mean by "the part we control", or "if we stop trying"? This only makes sense if we all agree upon some set of counterfactuals, some set of events that "could have gone differently".

People do what they do. If you can predict what that is (genetically or otherwise), then you can predict it. If you can't, then you can't. To reify this difference into some intrinsic property of minds like "free will" is to commit the mind projection fallacy.

You've singled out genetics and seem to be implying that people shouldn't be blamed for a "lack of willpower" if that lack has a genetic origin, but any such lack must have some cause, so why are genes so special? If parental behavior has a significant effect on apparent willpower, is the individual suddenly "blameworthy", or do we start down the infinite regress of blaming their parents?

What if the person in question takes a prescription medication and suffers a side effect that diminishes their willpower, are they to blame for that? ...If the side effect was unknown? ...If the side effect was known to be extremely rare? ...If the side effect was known to be fairly common, but the person's perceived need was great enough to take the risk?

To be clear, I’m not saying the situation was white and we thought it was black, or even that it’s a black & white thing, but rather that most people’s intuition might be the wrong shade of gray.

If "most people's intuition" is founded on an incoherent notion of free will, then I don't think minor tweaks to the shade of their delusions will help them get any closer to the truth. Our intuition of blame has more to do with what we can imagine intentionally changing (parental behavior, environments) than any fundamentally meaningful distinction. As gene therapy becomes more effective and accessible, this line is only going to get blurrier.

Comment author: [deleted] 07 October 2009 03:40:01AM *  4 points [-]

It is not willpower that prevents heterosexuals from having homosexual sex (or vice versa), it is not willpower that prevents classical music lovers from indulging in pop music (or vice versa), it is not willpower that prevents young athletes from lying around on the couch all day, and similarly in the typical case it may not be willpower that prevents thin and normal people from becoming fat. If that is so, then what distinguishes a fat person is not necessary a lack of willpower, but something else - such as a damaged sense of hunger which continues to send "I am hungry" messages when they should not be sent.

Even the overweight stop eating at some point. Suppose someone is 240 pounds. It is willpower that prevents him from being 250 pounds? Or is it that he stops being hungry at an earlier point than someone who is 250 pounds?

I think willpower in eating is best defined relative to a person's sense of hunger. If a person stops eating despite still feeling hungry, and he does this in order to keep his weight down, then he is exerting willpower. The sense of hunger is not easily observable, but in principle it could be observable, once we have worked out the mechanism of hunger.

It's been recently in the news that there is a finite amount of willpower. Now imagine that many of the obese actually are exerting some willpower - someone who would feel content at 250 pounds may be constantly struggling against his hunger in order to maintain 240 pounds. Indeed many of the obese do struggle. So they are exerting willopower in that area - and therefore presumably have less willpower left for other areas of their lives. We should therefore observe that many of the obese have observably less willpower in various areas of their lives.

Some (though not all) people who are thin or normal do exert some willpower. Someone may, for example, notice that he is 10 pounds (of fat) above the normal weight for his age group, and deliberately keep himself hungry in order to lose that 10 pounds. But he is not necessarily exerting any more willpower than someone who is 220 pounds but who, if he stopped keeping himself hungry, would rise to 230 pounds.

None of this should be interpreted as making any statement about blame. As mentioned in other comments and in the main post, if our amount of willpower itself is something we are given and beyond our control, then we can hardly be blamed for not having enough of it.

Or maybe we can:

Blame is a tool to manipulate and ostracize (and not, I am suggesting here, a factual statement with truth conditions). If we blame someone for something, one of two things is likely to happen: they will change in a way that pleases us, or they will be socially distanced from us. It doesn't really matter to us which they do - there are other fish in the sea. If people really can't control their weight, then blaming the fat for their fatness will exclude the fat from our social circles, and so will keep the short lifespan and health problems associated with obesity away from us. It's better to have longer-lived friends than to have shorter-lived friends, better to have healthier friends than to have sick friends, so it is a wise policy to blame the obese for their obesity, regardless of whether that is "fair".

If blame can be said to have truth conditions, then those are something like, "enough people refrain from this behavior I am calling blameworthy that I can ostracize all who engage in this behavior and still have many friends", or more precisely, "blaming people for behavior X will yield an optimal balance between quality and quantity of friends". To really make this non-subjective, we can define "optimal" in terms of evolutionary fitness.

Comment author: pdf23ds 07 October 2009 03:59:47AM 1 point [-]

it is not willpower that prevents classical music lovers from indulging in pop music (or vice versa)

Since the two are definitely and emphatically not mutually exclusive, that's a pretty bad example. Otherwise, nice comment.

Comment author: [deleted] 07 October 2009 04:47:29AM 1 point [-]

You're right, the example was not a good one. My bad.

Comment author: Aurini 01 October 2009 06:30:16PM *  2 points [-]

Freakonomics looked at this, coming up with some interesting conclusions. For one thing, reading to your children didn't seem to have any effect on education scores, but owning books did (did the babies inherit book-reading genes?). Another example: adopted children tended to conform to the lifestyle predicted by their biological parents, not their adoptive parents, in regards to crime and income (which implies some uncomfortable ideas about poverty and class). I don't remember the exact details of their analysis, but the general approach was to take commonly accepted statistical measurements and check for correlations. The most specious claim in the book (AFAIK) was that legalized abortion leads to lowered crime-rate twenty years down the road; while this is probably true, I don't think the evidence demonstrates anything beyond correlation. His chapter on babies, though, was pretty good.

I've only met one pair of twins who were distinctively divergent (though both were in the military), but I believe they were fraternal.

Comment author: MichaelGR 01 October 2009 08:30:51PM 6 points [-]

adopted children tended to conform to the lifestyle predicted by their biological parents, not their adoptive parents, in regards to crime and income (which implies some uncomfortable ideas about poverty and class).

The Blank Slate by Steven Pinker touches on this a lot. Pinker basically throws out most studies that don't control for genes (ie. You read to your kid and he develops good verbal skills.. But was that because you read to him/her, or because he inherited the genes that give good verbal skills), and instead he focuses on studies with adopted children.

Turns out that they are a lot more like their biological parents than their adoptive parents, and that twins that have been separated and adopted are still very similar to one another despite having grown in a different environment.

But Pinker mostly looked at psychological attributes, not at physical bodies.

Comment author: Psychohistorian 01 October 2009 08:24:58PM 5 points [-]

The abortion claim was pretty well backed up by numbers; crime declines began earlier in states that legalized earlier, and crime fell by less in states where fewer providers existed. Moreover, the theory behind it makes perfect sense: higher capital investment (be it time or money) in children tends to lead to better life-outcomes; children who would otherwise have been aborted receive lower capital investment, either because their parents aren't ready for them financially, or because their parents don't like them as much.

Comment author: taw 02 October 2009 03:19:21AM 1 point [-]

I find their claim extremely dubious - abortion availability correlates with too many political and demographic variables for tests like that to work, and worldwide there seems to be no relation between abortion availability and crime at all.

Comment author: Stuart_Armstrong 02 October 2009 06:11:52PM 2 points [-]

I have a nit-pick with these studies, which I haven't seen addressed: they show that a much larger part of the variance of results results from the birth parents than from the adopted parents.

However, the variance in birth parents' IQ seems to be much larger than the variance in adopting parents IQ; has this been corrected for?

Comment author: John_D 12 May 2013 02:37:03PM *  0 points [-]

This post sort of reminds me of this study: http://www.apa.org/research/action/smarter.aspx

" As compared to the control group, students who learned about intelligence's malleability had higher academic motivation, better academic behavior, and better grades in mathematics "

I suspect that belief that one can self-improve one's intelligence is partially explain by genetics as well. Another example of a trait that correlates with both behavior and intelligence is openness. Openness correlates with personal growth, need for cognition and crystallized intelligence (and certain facets correlated with fluid intelligence), so a predisposition to want to improve oneself may cause one to search for ways to do it. I also noticed that many people who eat well and exercise regularly are very knowledgable of its effects on cognition and positive mood, so there could be a predisposition for self-improvement and desiring exercise.

If intelligence can be partially explain by behavioral factors that enhance one's intelligence, then it also leaves room for improving it as well. (can these behaviors be taught, or does the average person just give up?)

Comment author: MichaelVassar 09 October 2009 12:05:51PM 0 points [-]

"the part we control is probably smaller than most people think" who is this "we" that control part of it if not the abstract ideal dynamic which is us, and which is largely our genes? Have you read the stuff here about free will?

Comment author: dial911 07 October 2009 05:38:22PM *  0 points [-]

This sounds like a nature vs. nurture discussion.

Nature + Nurture = Who the person grows up to be.

Genes (nature) are the functions but life experiences, environment, etc. (nurture) are the parameters.

GeneX(Twin1, ExperienceA) { return x; }

GeneX(Twin2, ExperienceB) { return y; }

The genes are like restricted scales and within each hold all possible outcomes for an individual. Some of these genes have a greater influence on the end result of an individual than others. Different life experiences, environments, etc. result in shaping each twin differently within each of their genes - since even twins cannot live identical lives and experience the same exact experiences.

Comment deleted 01 October 2009 07:13:02PM [-]
Comment author: orthonormal 01 October 2009 09:09:48PM 2 points [-]

True, but irrelevant to the present discussion; we're talking about studies that show explicitly how much similarity we can expect with identical genes at the start, but different environments after birth.

Comment author: smoofra 01 October 2009 05:31:16PM 0 points [-]

it's perfectly possible for one twin to get fat while the other doesn't. If it doesn't happen often, it's because features like willpower are more controlled by genes than we think, not because staying thin doesn't depend on willpower.

Comment author: MichaelGR 01 October 2009 05:38:46PM 2 points [-]

Indeed. This is why I wrote:

After all, even if we put aside the direct ways in which our genes build our bodies (encoding how our tissues grow) and instead look at our abilities to “make the right choices” and exert “willpower”, we find that those are also greatly determined by genetic factors. Identical twins probably turn out very similar in good part because they have almost identical amounts of those qualities of mind.

Comment author: pwno 07 October 2009 03:52:31AM 0 points [-]

I don't think thin people are thin because of willpower.

Comment author: MichaelGR 07 October 2009 08:02:37PM 1 point [-]

Willpower was listed as just one factor. It wasn't mean to be an exhaustive list of all factors.

Comment author: NancyLebovitz 03 October 2009 02:06:09PM 0 points [-]

I've also read that gene expression diverges in twins over time-- so if a lot of the difference in body composition is about gene expression, there might be a few pairs of twins where, just by chance, either the willpower or the fat storage changes kick in earlier or more strongly.

"Willpower" is not just one thing-- there are people who can't stick to diets who show a lot of will power in other parts of their lives, and vice versa.

Comment author: pdf23ds 04 October 2009 05:33:27AM 0 points [-]

As a young teen I played basketball with a pair of identical twins. One was fairly slight, the other rather stocky and an inch or two taller. Not sure why.

Comment author: [deleted] 05 October 2009 12:55:32PM 1 point [-]

maybe they weren't identical twins. Unless they did a genetic test they wouldn't know for sure. I read something once that said that a significant number of same-sex fraternal twins are misidentified as identical.

Comment author: MichaelGR 07 October 2009 08:03:33PM 0 points [-]

That is very interesting. I hadn't thought yet that the way we identify twins right now is just by looking at them ("they look pretty much the same, they must be identical").