The bane of agricultural statistics is what biologists call Genotype by Environment interaction, or GxE for short. Broadly speaking it is the old problem of nature vs. nurture. But for some reason when we’re talking about a stalk of corn there’s really no debate about which dominates. Environmental effects can easily swamp genetic expression. This is why it’s so difficult to predict which varieties are superior. So why is it that so many people insist that when we’re talking about humans, gene’s dominate? Humans are organisms that are not only affected by environmental factors like nutrition and sleep, but we can also be influenced by ideas, by imagination and by a sense of wonder. At our best we can even be persuaded by reason.
At least with corn you can produce genetically identical varieties in hundreds of locations to estimate how robust the genes are. It would be impossible, or at least unethical, to clone a few dozen individuals and raise them in diverse, controlled environments. We can talk about twin studies till we’re blue in the face, but they are not well designed experimental trials and are prone to all the problems of observational studies. And that’s the bane of the social scientists.