Nature, nurture and noise


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  1. I’ve nearly asked about non-genetic biological effects on traits several times here, but never did. I’m glad to see it addressed.

    It always struck me as a bit premature to attribute differences to environment when there are so many biological steps involved in the actual expression of a gene. Random small changes early on can have massive consequences for the entire system, especially in development.

    Genes are the blueprint for a pattern, that does not mean the pattern will always look like its blueprint.

  2. Great post. This is an especially poignant question: “Non-shared experiences can make me differ from my twin but shared ones cannot make us more similar?”

    I would point out that shared environmental effects may be significantly underestimated by twin (and adoption) studies. Take the example of IQ, a psychological trait with a high heritability:
    1. Twins reared together are significantly more similar than twins rared apart, even at middle-age. This comparison is more valid than the usual MZ-DZ comparison because it doesn’t make so many assumptions. (see my post here

    2. The existence of a generational Flynn Effect proves that there are shared environmental influences that change significantly with time. At the very least, you must admit that the shared environment matters between generations.

  3. Clearly there can be environmental effects on behavioural or psychological traits – sometimes quite large (such as from extreme neglect). All the twin studies show is that within a given population at a given time, shared environment effects (usually taken to mean shared family effects) do not contribute much to the overall variance of the trait. (They may contribute something – my reading of the effects on IQ is that any shared environment effects decrease sharply with age).

    Because of the way they are designed, these studies say nothing about differences between populations or between generations, which may be (and commonly are) entirely due to environmental differences. So, for example, a person’s IQ may be affected significantly by both their genes and their nutrition and general health status – however, the differences between people in a population where general nutrition levels are fairly equal will be more due to differences in their genes.

    At the same time, genetic and environmental factors are insufficient to explain all the phenotypic variance – there must be something else adding to the variance and I think there is likely a major role for intrinsic stochastic developmental variation in contributing to phenotypic variance.

  4. I think there is likely a major role for intrinsic stochastic developmental variation in contributing to phenotypic variance.

    This IMO is a very important post and point being made, and of course it bodes poorly for would-be genetic engineers, who would like to re-engineer more intelligent phenotypes out of the available genotypes – in that it may never be possible with any certainty to do this.


    I don’t see the problem here. If you have a monozygotic twin then your genes would be identical. If you also had all the same experiences (including pre-natal) then you would be completely identical people. No problem so far?

    Now suppose your genes are still identical, but some of your experiences are not identical. The result is that you are no longer identical people. And it is your non-shared experiences that have made you different! Your remaining shared experiences are still there in the mix, but the answer to the question “why are we different?” is the non-shared experiences. To me this looks like a perfectly good answer to the question.

  6. jb,

    The studies suggest that shared environmental influences do not have any effect on making twins more similar to each other. The key part of the question is why this is the case.. why should shared environmental influences exert *no* influence according to these studies?


    Thank you Chris!

    I believe that I’ve read somewhere that monozygotic twins have many genetic differences, e.g., SNPs and Copy Number Variations?

    If one were to make a chocolate cake and put all the ingredients into a bowl such as the flour, sugar, baking powder, cocoa powder, egg, etc., mix it together thoroughly and evenly pour the batter into two identical baking pans and bake them together in a single oven, one would not end up with two identical cakes.

  8. Thanks for your comment jb. The point from the twin and especially adoption studies is that monozygotic twins who are reared apart are almost as similar to each other as those reared together – a shared family environment is thus NOT making the ones reared together more similar to each other (and the implication is that shared experiences have little effect on the phenotype). It is thus not clear to me how one can argue that the non-shared experiences are making them different from each other – this seems simply like the flip side of the same equation. It is effectively arguing that an experience can have an effect on me only if it is not shared by my twin.

  9. Fraternal twins are more similar in IQ to each other than ordinary siblings. This suggests that twins share an environment, either pre-natal or post-natal (or both), that tends to make them more similar to each other. I’m not sure if this effect persists to adulthood though. Some of it might be due to the Flynn effect.

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