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It may be useful for the discussion here to define what "heritable" means. I like what Eric Turkheimer, a psychologist who investigates the heritability of huma

It may be useful for the discussion here to define what "heritable" means. I like what Eric Turkheimer, a psychologist who investigates the heritability of human intelligence, says about that. "But back to the question: What does heritability mean? Almost everyone who has ever thought about heritability has reached a commonsense intuition about it: One way or another, heritability has to be some kind of index of how genetic a trait is. That intuition explains why so many thousands of heritability coefficients have been calculated over the years. Once the twin registries have been assembled, it's easy and fun, like having a genoscope you can point at one trait after another to take a reading of how genetic things are. Height? Very genetic. Intelligence? Pretty genetic. Schizophrenia? That looks pretty genetic too. Personality? Yep, that too. . . . "Unfortunately, that fundamental intuition is wrong. Heritability isn't an index of how genetic a trait is. A great deal of time has been wasted in the effort of measuring the heritability of traits in the false expectation that somehow the genetic nature of psychological phenomena would be revealed. There are many reasons for making this strong statement, but the most important of them harkens back to the description of heritability as an effect size. An effect size of the R2 family is a standardized estimate of the proportion of the variance in one variable that is reduced when another variable is held constant statistically."[1] [1] Turkheimer, E. (2008, Spring). A better way to use twins for developmental research. LIFE Newsletter, 2, 1-5 http://people.virginia.edu/~ent3c/papers2/Articles%20for%20Online%20CV/Turkheimer%20(2008).pdf http://people.virginia.edu/~ent3c/papers2/Articles%20for%20O...

jcampbell1 13y ago
> Heritability isn't an index of how genetic a trait is. I am totally confused. My understanding is that heritability is a measure of how much genetics accounts for the observed variability in a given environment. I understand both that we don't understand the genetic recipe for height, but also that identical twins grow to be the same height. What is the author saying that I am missing?
- Symmetry 13y ago
  I think the author is saying that the heritability of a trait depends on the range of environments being considered, but saying it in a needlessly grandstanding and obfuscatory manner? Having now read the paper, yes, that was what he was saying, and the non-quoted sections of the paper are much clearer. Example For the modeling underlying twin studies to make sense, one has to assume that the environments experienced by identical twins raised in the same family are no more similar than those experienced by frater- nal twins, that genes and environments are independent and additive (again, in the usual ANOVA sense) in the determination of the trait, and that parents mate randomly with respect to the trait. That's totally true. If you compared an organism with a randomly generated genome to a human and exposed both to what would be a normal range of environments for a human, you would find that all traits are entirely genetic. And if you selected people from the normal range of human genetics but only gave half of them access to important things like a uterus then you would find that all traits are almost entirely environmentally determined. So you can't say that "Trait Foo is 50% heritable" unless you also specify the populations and range of environments you're talking about. EDIT: Or you could say that heritability is ordinal but not cardinal, given some reasonable assumptions.

jcampbell1 13y ago

> Heritability isn't an index of how genetic a trait is. I am totally confused. My understanding is that heritability is a measure of how much genetics accounts for the observed variability in a given environment. I understand both that we don't understand the genetic recipe for height, but also that identical twins grow to be the same height. What is the author saying that I am missing?

Symmetry 13y ago
I think the author is saying that the heritability of a trait depends on the range of environments being considered, but saying it in a needlessly grandstanding and obfuscatory manner? Having now read the paper, yes, that was what he was saying, and the non-quoted sections of the paper are much clearer. Example For the modeling underlying twin studies to make sense, one has to assume that the environments experienced by identical twins raised in the same family are no more similar than those experienced by frater- nal twins, that genes and environments are independent and additive (again, in the usual ANOVA sense) in the determination of the trait, and that parents mate randomly with respect to the trait. That's totally true. If you compared an organism with a randomly generated genome to a human and exposed both to what would be a normal range of environments for a human, you would find that all traits are entirely genetic. And if you selected people from the normal range of human genetics but only gave half of them access to important things like a uterus then you would find that all traits are almost entirely environmentally determined. So you can't say that "Trait Foo is 50% heritable" unless you also specify the populations and range of environments you're talking about. EDIT: Or you could say that heritability is ordinal but not cardinal, given some reasonable assumptions.