1) All human behavioral traits are heritable.Harris mentions that, in fact, the shared environment (families) accounts for a miniscule portion of the variation. Turkheimer's pdf assigns ~50% of the variation to the nonshared (or unique) environment, which comprises the things which happen to one sibling but not the other, and which are not an effect of genetic differences -- so, this would exclude differentially treating two children due to one being genetically predisposed to crybaby-ness and the other to being well behaved. As such, this can include things like the open slot left in a child's peer group, pathogenic contagion, and other chance affairs. I assume most reading this have already heard this and refer any readers who haven't to the accessible papers at Turkheimer's site & the references therein, or to the wonderfully written chapter on "Children" in Steven Pinker's The Blank Slate.
2) The effect of being raised in the same family is smaller than the effect of the genes.
3) A substantial portion of the variation in complex human behavioral traits is not accounted for by the effects of genes or families.
So what does this have to do with art? Harris' claim that normal parenting has little influence on how children turn out, and that chance events are key, sounds a lot like Gornik's view that "the exact effect of art can't be controlled or fully anticipated." I thought of how BG could relate to artistic creation in 2004 when I was reading a lot of interviews of David Lynch, who consistently drives home how his works come to fruition -- the ideas come to or call him, and he is but a faithful attendant to them, like a gardener who waters seeds that will grow into different creatures, depending in large part on what their genes specify. See here, here, and here. For example, in describing how a work gets its features like sound, style, characters, etc., he says (first link above): "...everything comes out of ideas. Never go against the ideas, stay true to them. And it will always tell you the way you go." Now, Lynch is hardly to first artist to liken their ideas to children, but what is unusual is his insistence that they play a larger role in how the final work turns out than any deliberate nurturing on his part -- again, separate from the effect of how the idea's nature evokes a certain treatment. Well, there's that whole history of artists who believe they're possessed by foreign thoughts and only give birth to their art rather than calculatingly shaping a lump of undifferentiated thought-clay. But it's the first I recall hearing it from a major modern or avant-garde figure.
So let's explore this traditional view of artistic creation and what BG would predict. Again, I don't commit to fine-grained quantities, as this is art crit, but here goes. The original idea is like a child, born of inspiration in the unconscious mind -- say, two bees in the bonnet of the artist's uncosciousness connect, one fertilizes the other, and eureka! an inspirational idea is born. The idea-child, like all children, needs a healthy environment to nourish it into maturity, and this is the role of the artist's conscious action during the development stage. However, just as BG suggests, the effects of parental nurture are limited and mostly consist of helping the idea-child grow into what it wants to be, rather than trying to manipulate it against its predispositions. As all idea-children are reared by the same conscious mind, the deliberate part of the artist's thinking is like the shared environment; the unique environment includes things like the peer group of fellow idea-children with whom an idea socializes, chance occurrences in the artist's brain-home, as well as chance occurrences outside -- say, it's introduced to new idea-peers by a collaborator. The genetic endowment consists of the union of various unconscious sparks that gel together, forming a single idea-fetus. Ever felt your unconscious sparking but not resulting in anything solid? Not every mingling results in conception. Once fully developed, the idea instructs the artist how to behave (i.e., use this hue, weave in unrequited love, etc.).
What about the adoption & twin studies which BG researchers use? In brief, the findings are that unrelated siblings reared in the same adoptive home are no more similar than strangers despite their shared environment; identical twins separated at birth & reared apart are pretty similar but not terribly so; and identical twins reared together are no more similar than if they were raised apart. So again, genes & unique environments explain most of the variation, parental nurture very little. The adoption analogy would be ideas conceived within two separate unconscious minds with little genetic relatedness but reared in the same brain-home. Say, Mozart gives early-era Stravinsky an undeveloped idea for a composition, which is reared alongside one of Stravinsky's own. Assuming normal development -- i.e., assuming he isn't a wicked stepparent who maliciously bends the idea-child's nature -- wouldn't you think the compositions would be quite different? I'm sure they'd be somewhat similar as well, but I'd attribute it more to how Mozart's idea-child interacts with its newfound idea-peers that inhabit Stravinsky's brain (i.e., unique environment), since to get Mozart to sound strongly like early Stravinsky would require abusive / pathological upbringing. BG also shows that parental influence can have a noticeable effect during childhood, but once children grow into full adults and leave their homes, their genetic nature starts asserting itself and the effects of upbringing wane as parents no longer wield the whip over their children's heads. So part of the similarity could also be due to the fact that Mozart's idea can't escape Stravinsky's brain-home -- though if it did and found a more understanding parent (like a humble songwriter), it would end up more like a Mozart composition.
As for the identical twins separated at birth, let's assume that Mozart kept and raised his original idea but also made a clone (i.e. identical twin) which he then let Stravinsky adopt and raise -- a classical cover song, not unlike those of the latter's Neo-Classical period. Continuing to assume normal parental upbringing, wouldn't you expect them to sound pretty similar, even if not completely so? And wouldn't you account for the differences in the Stravinsky version by its interaction with its fellow idea-peers that inhabit Stavinsky's brain-home but not Mozart's, such as those still left over from the former's Primitive period, or those for orchestral techniques developed only after Mozart's death during the Romantic movement?
And what about the identical twins reared together, who in the BG literature are no more similar than twins reared apart? It's hard to think of how this would play out, since typically Stravinsky is contemplating how to develop just one copy of his idea-child, not an identical copy of it as well. But perhaps a close analogy would be that of the same parental conscious rearing the idea at one point in time, and then returning to it far later on to redevelop an identical copy of it in some new direction. It's as if a human parent raised a child a particular way, had a clone made of it 20 years later, and tried to raise it in the same encouraging fashion. But after 20 years of comings and goings from the peer group, the younger twin of Stravinsky's original idea may have a substantially different peer group from its older twin. We still expect them to turn out similarly given their common origin, but given differences in the unique environments, they wouldn't be indistinguishable. For example, say the idea is telling Stravinsky to compose a work on a pagan theme -- developed during his early Primitive period, it might sound like The Rite of Spring, whereas developed later during his Neo-Classical period after his brain-home's begun to house a somewhat different population of idea-peers, it might sound more like Orpheus. As for the non-peer aspect of the unique environment, say a whim strikes him which directs his conscious thought down a different path than that which originally occurred to him. This idea-peer / whim interaction often strikes many people when pondering how they'd re-do their work if given a second chance -- the "Oh! You know what I should've done there...?!" moment.
Well, I could go on, but you get the idea. The take-home message is that the BG picture of development, as well as the usually concomitant prescription to parents that they should encourage their children to find themselves, corresponds pretty closely to the traditional idea of artistic inspiration and creation -- that the original endowment and unique environment matter a lot, while conscious nurture (separate from genetic effects) seems to matter little. So, there really is something special about Mozart's brain that makes it capable of producing such singularly amazing works, not that he's simply a harder working parent to his idea-children -- in fact, if we believe the popular portrayal of him and his rival Salieri in Amadeus, Mozart's offspring are like the children of smarties who on average grow up smart, more or less no matter what assuming basic nutrition is met; whereas the offspring of duller folks on average don't reach the same level despite a heavily orchestrated campaign of nurture to force them in that direction. I've left out two things in this discussion. First, the nature of scientific output -- I think that, aside from the exceptional "Aha!" insight of a genius, a lot of scientific output comes from being a wicked stepparent who through trial-and-error strives to mold one's idea-children this way or that until something works, and if not, try again with a different child until one of them ends up explaining something interesting. Artists do this too but are more likely to "listen to their ideas" no matter what since no empirical issues are at stake. Second, I haven't discussed memetics because the ideas I'm discussing here look more like children that a parent raises into maturity, and less like genes (or how I see them, pathogens -- something distinct).