Thursday, November 4, 2010

Nature/nurture and causality

We intuitively understand that things can have many causes.  For example, what caused the election of Obama in 2008?  Obviously, it was a combination of factors, including public disapproval of the war, the disaster of hurricane Katrina, Obama's popular election campaign, and so forth.  Now I ask, what percentage of the victory was caused by McCain's choice of Palin as running mate?

To answer this question, we could perform a thought experiment where McCain chooses someone else, and then we predict how well Obama would have done.  Divide the change in votes by the total number of votes and we have a percentage.  But even supposing we could perfectly predict the outcome of this thought experiment, this answer is problematic.  Wouldn't it depend on who McCain chose instead of Palin?

Furthermore, if we did the same thought experiment for all sorts of different causes, the total percentage would be much greater than 100%, which just doesn't make any sense.

I think this is because the question simply isn't well-defined enough that it can have a definite percentage as an answer.

And yet, when it comes to human biological traits, we ask the same question and expect a definite answer.

What percentage of a given human trait is caused by genetics rather than environment?

This question is problematic, because it's unclear how exactly we would do the thought experiment.  How much do we vary genetics in our thought experiment?  How much do we vary the environment?  And you can't just say that we vary genetics and environment by the same amount.  Given a certain variety of genes, how much is an equivalent variation in environment?  There simply isn't an answer to these questions, because they are bad questions.

But even though it's a bad question, scientists still deliver!  Scientists talk about measuring the "heritability" of a human trait, which is just a number between 0 and 1 that can be experimentally observed.  How do scientists answer the impossible?

They do it by being tricky.  In fact, heritability is the answer to a different question.  If you're a non-scientist, now you are put in the odd situation of having a number for an answer, but not knowing what the original question was.

Google knows all the answers

But enough suspense.  Heritability answers the following question:

How much of the variation in a given human trait is due to genetic variation between individuals of a population?

The question sounds almost the same, but perhaps that's just because I couldn't word it more precisely in a single sentence.*  This question calls for a thought experiment where we take a large population, and hold all environmental conditions constant.  We vary just one factor, genes, and we vary it by the amount we see in the real world.  What percentage of the trait's variation in the population remains?  Alternatively, we can try a thought experiment where we take a large population, and leave their environment as is.  But we change the population's genes, making them all genetically identical.  By what percentage does this decrease the trait's variation in the population?

*The more precise definition has to do with variance, which is a statistical concept that I won't attempt to explain.

 This is a convenient definition because we no longer have to rely on thought experiments to vary the genes and environment.  Now we can look at the real world, and use the real world variation in genes and environment.  The primary difficulty is isolating the two factors.  This is usually done using studies of identical and fraternal twins.  It's by no means a straightforward experiment, and there are all sorts of methodological difficulties, but it's possible.

But even if we can measure heritability perfectly, there are some rather strange consequences of the definition:
  1. Heritability increases as the genetic diversity of a population increases.
  2. Heritability decreases if a population is exposed to a wider variety of environments.
  3. Thus, heritability depends on which population we consider, and what point in history.
  4. Heritability does not tell you how hard it is to change a trait by changing environment.
I should re-emphasize that last point, because it undercuts a lot of political abuse of heritability.  For example, let's consider a people that doesn't get any schooling.  How much variation in literacy is due to variation in schooling?  None, because there is no variation in schooling.  But that's not the same as saying that increased schooling will fail to increase literacy.  What it really means is that being a privileged member of this population gets you nowhere because not even the privileged go to school.

Even if heritability is close to 1, it might still be possible to change the trait by changing the environment.  You just have to change the environment by more than is typical for the population.  This could be easy or hard, depending on specifics.

Further difficulties are caused by the fact that genetics and environment are not exhaustive categories, unless you define environment to be everything that isn't genetic.  For example, what about random developmental factors in the womb?  Technically, this is an environmental factor.  But just like genetics, it's just a bunch of chemicals bouncing around in a way that we have no control over.

And we haven't even gotten into gene-environment interaction.  For instance, what if there's a gene that simply increases human receptiveness to environment?  Is that variation due to environment or genetics?  Nature/nurture is a false dichotomy.

Other posts in this mini-series:
Colds and Causality
Women and Causality
Responsibility and Causality
Nature/nurture and Causality
Physics and Causality


Red. x said...

I had a nice lecture off a biopsychologist today, who left us with a nice analogy; "asking whether something is down to nature or nurture is like asking if a football field's area is due to its length or its width"

Larry, a.k.a. The Barefoot Bum said...

Not apropos of this post... what do you think of Quantum Darwinism?

miller said...

Based on the articles I read... It's just a way of thinking about how classical physics emerges from quantum physics. It's not a new interpretation. I think it relies on the Copenhagen interpretation, but the article never says. I am not too impressed by the analogy with natural selection, because it seems misleading.

Larry, a.k.a. The Barefoot Bum said...

Thanks, M. I mention it because an reasonably intelligent non-physicist (economist) I read brought it up as something interesting that he lacked the capacity to evaluate.

What interested me is that it seemed to give a slightly more detailed explanation of the measurement problem than the sources I've read.

Zack said...

Excellently argued!

David T. said...

And we haven't even gotten into gene-environment interaction. For instance, what if there's a gene that simply increases human receptiveness to environment? Is that variation due to environment or genetics? Nature/nurture is a false dichotomy.

is there something independent from the environment other than fantasy and even that too be considered a part of the environment? So the statement makes really no logical sense, it's sort of like saying self organizing.

miller said...

You'll have to phrase that question more intelligibly, David.

Zack said...

The comment about the football field's area is insightful. As a function of the dimensions of a field, you could take the derivative of the area of the field with each dimension. For long fields adding a bit more length will not matter as much as adding some width. Likewise, you can ask, given the same environment, what is the effect of a particular gene? Or given the same genes, what is the effect of changes in the environment? This is one of the reasons why twin studies (comparing fraternal and identical twins, or comparing identical twins raised apart with those raised together) are so interesting.

miller said...

Yeah, causal relationships are basically like partial derivatives. If only multivariable calculus were common knowledge, then this would be easy to explain!

And then I would go further to explain that phenotype is not a function of genes or environment, but a function of their tensor product. It's just so much simpler to say in mathematical language!

drransom (aka toillE) said...

So the basic problem here is that asking what "percent" of X is caused by Y is ill-formed, no? Is there something unique about evolution, genetics, or the nature/nurture debate?

miller said...

It's not unique, though it's an interesting example. I think the one thing that's unique is that scientists manage to produce an answer to a bad question, thus illuminating what parts of the question make sense and what parts don't.