A DNA Blueprint for the Future

A new book explains how DNA determines psychological traits.

Posted Dec 31, 2018

MIT Press
Source: MIT Press

Robert Plomin’s Blueprint (left) is subtitled How DNA makes us who we are. But a more accurate subtitle might be How DNA makes us what we are rather than anything else, such as childhood (below, left).  

Amazon
Source: Amazon

As Plomin says, “Most people accept that DNA matters for psychological traits, even though they underestimate its influence.” What he calls the first law of behavioural genetics states that “All psychological traits show significant and substantial genetic influence.”

Indeed, as Plomin points out, even physical parameters such as weight bear this out, with DNA accounting for 70 percent of this accurately measured but often stigmatized personal trait (at least if it is deemed excessive and a reflection of a heavier person’s lack of willpower to stay fashionably thin). 

Furthermore, as he also shows, "Heritability increases during development.” For example, as I have pointed out in previous posts, the psychometrics of identical twins converge as they age, rather than diverge as official psychology with its dogmatic nurture-not-nature ideology might suggest.

In behavioural genetics, environmental influence—the determining factor for just about everything psychological according to official dogma—“is defined very broadly to mean all influences that are not due to inherited DNA differences,” such as “family, neighbourhood, school, peer and work environments,” so that “in this sense, a better word for what geneticists mean when they refer to environment is ‘non-genetic.’”

In summary, parents matter, schools matter, and life experiences matter, but they don’t make a difference in shaping who we are. DNA is the only thing that makes a substantial systemic difference, accounting for 50 percent of the variance in psychological traits. The rest comes down to chance environmental experiences that do not have long-term effects.

How could it be otherwise? The only thing that determines whether you are a woman or a worm, a man or a mouse, is DNA—and only about half of it in the case of the worm and the woman and an astonishingly little of it in the case of mice and men (almost all human genes have murine equivalents).

Furthermore, what determines how the environment affects an organism is itself determined by its DNA, even in the case of higher cognitive functions, such as learning, which is the most nurtured of all traits. But the case of autistic savants, who show prodigious learning ability far beyond the normal range, proves that what they learn is almost exclusively mechanistic rather than mentalistic, and to that extent determined by their DNA—certainly not by their parents or teachers (the most common skills being calendar calculation, rote memorization, and maths and musical ability).

Plomin says that the main reason he wrote this book “was to foster this discussion and provide the DNA literacy that we need to address these complex issues in an informed way.” As the title of his concluding chapter proclaims, “our future is DNA.” Indeed, as I pointed out in a previous post, genes matter more than ever in the modern world and the advent of personalised genomics will make DNA even more important, as Plomin readily recognizes.

But of course, if DNA is our future and likely to become even more so as time goes on, so too was it our past, and it is in this respect that evolutionary genetics can build on the solid statistical foundations of behavioural genetics to suggest that, far from over-egging DNA as most of his critics claim he has, Plomin has in fact been remarkably reticent. 

To take one example which goes to the heart of the central paradigm of behavioural genetics—its definition of heritability—we now know that female identical twins are not always as identical genetically as male ones, thanks to possible differential Lyonization/X-inactivation of one of their two parental X chromosomes in every cell of their bodies. As I pointed out in an earlier post, identical female twins can be discordant for classic X-linked genetic diseases, notably muscular dystrophy. And even if you rightly call this an epigenetic effect, outside the narrow definition of heritability, it is nevertheless genetic in its own way and indeed under the control of a single gene, XIST, as explained in a previous post. To this extent, estimates of heritability like those cited by Plomin are almost certainly underestimates of the role of genetics, not exaggerations as his critics might claim.

Again, Plomin’s comments about nurture being in part genetically-determined are only the start of the story. A theorem proved by Robert Trivers reveals that nurture as objectively and quantifiably defined as parental investment is not merely an aspect of nature, but is one heavily biased by the parent’s genetic self interest, predicting that parents will usually demand twice as much co-operation/self-sacrifice or half as much selfishness (which comes to the same thing) as offspring will demand of themselves. The conflict between the civilizing, educating, and nurturing role of the parent and the recalcitrance of the child is rooted in genetics and the very nature of nurture, whatever parents and their surrogates similarly motivated to mould minds such as priests, prophets, pundits, professors, politicians or psychotherapists may say.

The standard slander against any mention of genetics in connection with behaviour is that DNA is deterministic/pre-formationistic. But the first principle of evolutionary behavioural genetics is that DNA builds brains to make difficult decisions in real-time for which genes could not legislate in advance. Indeed, according to the imprinted brain theory, the grotesque inequality of the sexes in human development, with everything being contributed by the mother during gestation and lactation and nothing save his DNA by the father, means that the parents need not agree on exactly how the brain of their offspring should be built to best suit their conflicting evolutionary self-interests.

And in any event, if DNA directly determined behaviour, who would ever be in doubt about what to do, feel ambivalent, or experience inner turmoil? The truth is that our warring genes dictate that we as their agents are riven by conflict and often disabled by it—sometimes to the point of becoming mentally ill. 

Plomin has a worry about his polygenic score indicating an enhanced risk of schizophrenia (85%). But I can reassure him: according to the diametric model of mental illness, it also indicates the enhanced mentalistic skills which he so tellingly reveals in this book.

And of course, if the book really did explain how DNA makes us what we are, it would have had to give the lie to the official with-thousands-of-genes-involved-in-making-us-no-single-genes-are-significant mantra by providing an account of epigenesis and pointing out as I did in an earlier post that complex traits such as sex or whole-body colouring can be, and indeed are, controlled by single genes. The key to understanding mental illnesses such as schizophrenia lies here, and in the developmental details of gene expression, not just in polygenic scores. As Plomin himself concludes, genetics is much too important to leave to geneticists.

References

Robert Plomin, Blueprint: How DNA makes us who we are. Allen Lane, 2018.