Progress in science isn't always as straightforward as it seems long after the event. For example, when Mendel's discovery of the laws of inheritance was first widely recognized at the beginning of the 20th century, it was wrongly interpreted as contradicting Darwin's theory of evolution. It took a generation to resolve the confusion and establish modern Mendelian population genetics as the mathematical foundation of twentieth-century Darwinism and the tool which allowed W. D. Hamilton and others to establish our modern, "selfish gene" view of evolution by natural selection.
Now, it seems to me, something very similar is happening in evolution and genetics, and it could take another generation--or more--to resolve. What has happened is that we now know that there is what you might see as a third dimension to the nature/nurture dichotomy: so-called epigenetics. This describes molecular factors which affect gene expression, rather than simply inheritance.
Mendel was right about inheritance. But we now know that other, non-Mendelian factors can affect gene expression: notably genomic imprinting (which silences genes according to their parent-of-origin); X-inactivation (or Lyonization, which silences genes on one copy of the two X chromosomes present in mammalian females); and copy number variation (which means that sometimes genes are lost or multiplied in any particular person's genome).
Before we knew about epigenetics, nature/nurture was simply two-dimensional: something was attributable to one or the other. The paradigmatic methodology was to compare identical twins raised apart. It seemed obvious that where they were the same, it had to be because of their shared, seemingly identical genes, and where they differed, it had to be because of their obviously different environments. But as I have pointed out in previous posts, we now know that this is not the case. On the contrary, as a recent study of the DNA of identical twins showed clearly, "molecular mechanisms of heritability may not be limited to DNA sequence differences." Indeed, the authors speculate that, because identical twins reared together or apart are generally quite similar on measures such as brain-imaging, IQ, and other psychometrics, epigenetic differences between identical twins "are much more important than environment."
As the authors of this study point out, this has profound implications for twin studies in particular and for the nature/nurture debate in general. Put most simply, it means that where in the past any differences between identical twins raised apart seemed self-evidently environmental, now we must consider the possibility that they are instead genetic--or at least, epigenetic. Indeed, according to the imprinted brain theory, this is the key to unlocking the mystery of how mental illnesses like autism and schizophrenia can demonstrably run in families yet defy Mendelian laws of inheritance. According to the theory, such illnesses are the result of epigenetic factors: with all three mechanisms listed above implicated, and perhaps with more to be discovered.
But as I also pointed out in a recent post, epigenetics can be used to promote pre-scientific Lamarckian or anti-scientific Lysenkoist notions of inheritance of acquired characteristics and evolution-by-will. If epigenetics is indeed a new, third dimension to the nature/nurture dichotomy, then it will be up for grabs by both sides of the debate. The nature side has its own, selfish-gene view of epigenetics as a conflict between the sexes and individual genes as I have frequently explained in these posts and argue at length in my book.
But fashionable PC dogma makes recognition of sex differences taboo and any concessions to genetic determinism anathema. The result, it seems to me, is inevitable: We face another generation or more of confusion, obfuscation, and denial of the truth about epigenetics, which may produce the illusion that Lamarck and Lysenko were right after all. Indeed, as my review of one particular piece of explicit neo-Lamarckianism shows, writers who take this view specifically target the selfish gene paradigm in doing so.
One thing though, seems certain. If the imprinted brain theory succeeds in its attempt to explain the mind and mental illness in terms of selfish-gene epigenetics, a decisive victory would have been won for the scientific view. The nonsense of Lamarckian/Lysenkoist interpretations of epigenetics would then be seen as the 21st century equivalent of the early 20th century belief that Mendel had disproved Darwin. The truth was that Mendel added something that Darwin sorely needed but could not provide himself: a secure foundation in genetics. Ultimately, epigenetics will do something similar by adding a third, real dimension of depth to our understanding of individual development. And that will be no illusion!