…it’s nature and…we dunno…miscellaneous/unknown/noise?

from steven pinker’s response to this year’s edge questionWhat scientific idea is ready for retirement? (pinker’s reply: behavior = genes + environment):

Even the technical sense of ‘environment’ used in quantitative behavioral genetics is perversely confusing. Now, there is nothing wrong with partitioning phenotypic variance into components that correlate with genetic variation (heritability) and with variation among families (‘shared environment’). The problem comes from the so-called ‘nonshared’ or ‘unique environmental influences.’ This consists of all the variance that is attributable neither to genetic nor familiar variation. In most studies, it’s calculated as 1 – (heritability + shared environment). Practically, you can think of it as the differences between identical twins who grow up in the same home. They share their genes, parents, older and younger siblings, home, school, peers, and neighborhood. So what could make them different? Under the assumption that behavior is a product of genes plus environment, it must be something in the environment of one that is not in the environment of the other.

But this category really should be called ‘miscellaneous/unknown,’ because it has nothing necessarily to do with any measurable aspect of the environment, such as one sibling getting the top bunk bed and the other the bottom, or a parent unpredictably favoring one child, or one sibling getting chased by a dog, coming down with a virus, or being favored by a teacher. These influences are purely conjectural, and studies looking for them have failed to find them. The alternative is that this component actually consists of the effects of chance – new mutations, quirky prenatal effects, noise in brain development, and events in life with unpredictable effects.

“Stochastic effects in development are increasingly being recognized by epidemiologists, frustrated by such recalcitrant phenomena such as nonagenarian pack-a-day smokers and identical twins discordant for schizophrenia, homosexuality, and disease outcomes. They are increasingly forced to acknowledge that God plays dice with our traits. Developmental biologists have come to similar conclusions. The bad habit of assuming that anything not classically genetic must be ‘environmental’ has blinkered behavioral geneticists (and those who interpret their findings) into the fool’s errand of looking for environmental effects for what may be randomness in developmental processes.

did you get that?: “The bad habit of assuming that anything not classically genetic must be ‘environmental’ has blinkered behavioral geneticists (and those who interpret their findings) into the fool’s errand of looking for environmental effects for what may be randomness in developmental processes.”

jayman has been trying to tell us this for a while now:

“The heritability of behavioral traits is typically on order of 50%. However, what’s left (after you subtract the ‘shared environment’, which is generally 0, but more on that soon) is just the ‘unexplained variance.’ We don’t know what that is. Much of it, perhaps a good deal, is measurement error. Evidence suggest that that is actually missed heritable influence.

“However, what’s left over, after you’ve accounted for ‘attenuated heredity’ may be what’s known developmental noise. This is ‘environmental’ in the sense that it’s not inherited, but is essentially random and not subject to controlled manipulation.

“Or we think it’s random. See Kevin Mitchell on it:”


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“Even developmental noise appears to heritable, to a degree. Whether or not this is ‘on purpose’ or an evolutionary accident is unclear.

“And finally, and this is an ‘advanced’ topic, impact of the ‘unique environment’ – what makes identical twins raised together different from one another – could itself significantly genetic in nature, because identical twins aren’t actually genetically identical, but have different de novo mutations.

“You see why I’m a little hard on the ‘nurturists’ out there. Broadly, the evidence has not been kind to ‘environmental’ influences. Note that this is not to say that they don’t exist.”

there’ll be more on this — much more i’m sure — coming down the pipeline. in particular, watch this space (jayman’s blog!) for a more detailed explanation of all this — coming soon! (^_^)

see also Pinker on interpreting twin studies from steve sailer.

(note: comments do not require an email. twins.)

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