Rethinking Nature Versus Nurture

Photo by Marlon Alves on Unsplash

Did you happen to catch the 2018 movie Three Identical Strangers? In it, identical triplets are separated at birth and adopted by three different families. Reunited at age 19 they are found to each smoke the same brand of cigarettes, drive sports convertibles, and prefer to date older women. There is also a darker side to the similarities which we discover later but I don’t want to spoil it for you.

The Nature versus Nurture debate has been raging for centuries but it’s only in the last 50 years or so that the debate has moved from the realm of philosophy to that of science. And science has brought along its own language, so we now refer to the influence of genetic factors (“nature”) compared to that of environment factors (“nurture”) in shaping our personalities. In this post I’ll be putting forward some relatively recent scientific claims and the evidence supporting them.

We start by observing that research studies done in the past would invariably find strong correlations between the behaviour patterns of children and that of their parents and then go on to attribute the entire effect to some aspect of parenting. These studies are wrong. Wrong because they do not account for the fact that parents not only create the home environment, they also pass on genes to their children. For example, it has been observed that children whose parents read to them at bedtime grow up to be book-lovers as adults. Is this on account of genes or parenting? How can we tell?

How It Works

More generally, for any given behavioural trait, given that parents determine the genetic makeup of children and provide the environment in which children are raised, how is it possible to separate the effect of genes from the effect of environment?

Well?

The answer lies in Twin and Adoption Studies

Image Credit: author

Twin and Adoption Studies are a research design which allows us to tease out the effect of genes by controlling for environment or the effect of environment by controlling for genes. For example: identical twins (or monozygotic twins) share 100% of their genes and experience very similar environments. Fraternal twins (or dizygotic twins) share only 50% of their genes on average but experience very similar environments. So if a given trait shows a significantly higher correlation among identical twins than among fraternal twins, it must have a heritable basis.

This gives us the concept of Heritability, denoted and defined as the proportion of variation in a trait that can be explained by variation in genes.

Similarly, you could ask whether a given behavioural trait shows any correlation between adopted siblings. Adopted siblings are, genetically speaking, no more similar than random strangers, but they share the same home environment. So if a given trait shows a higher correlation among biological siblings than among adopted siblings, it again must have a high heritability. These are the ways in which heritability is quantified as a number between 0 and 1.

What kind of behavioural traits have been tested for heritability? Everything from cognitive abilities to personality to school achievement to mental health.

What We Have Learnt

Note that we are trying to explain differences in personality and behaviour in terms of differences in genes and environments. So, for example, you cannot say that 70% of a person’s height is due to their genes and 30% is due to the environment — that’s just absurd! What you can say is that 70% of the variation in height between individuals in a population is explained by variation in their genes. Note also that twin and adoption studies are not about comparing a single pair of twins or even 5 or 10. Most studies have sample sizes of thousands or tens of thousands of twins/adoptees.

Against this background, here are some of the latest findings from Behavioural Genetics studies done over the past 40 years:

  1. All behavioural traits have a heritability greater than 0 and less than 1. This means that while every trait has some genetic influence, no aspect of our behaviour is “genetically determined”.
  2. Intelligence (as measured by IQ tests) is heritable and its heritability increases with age. Puzzling but true.
  3. Psychological disorders like autism spectrum and schizophrenia have high heritability. So does addiction and substance abuse.
  4. The effects of ‘environment’ factors on behaviour are often mediated via genes. This happens because children actively select and modify their environment based on their genetic propensities rather than passively having an environment imposed upon them.

In general, the genes that we pass on to our children have a greater influence on their behaviour than parenting practices though this is true within the “normal” range of home environments and does not apply to cases of abuse or neglect. We know this because identical twins raised in different families show far greater similarities than adoptive siblings raised in the same home. In fact, adoptive siblings are no more similar than random strangers.

Eugenics And Other Horrors

Unfortunately, despite the careful design of these studies, outside the scientific community the results have been treated with skepticism and often just outright disbelief.

Part of the reason for people’s discomfort could be the long shadow of the past. During the first half of the 20th Century the science of genetics, still in its infancy, was being used to justify the despicable ideology of Eugenics, culminating in the egregious practice of selective mass sterilizations. Any talk of genetic influence on behaviour today tends to be misconstrued as being a claim of “biological determinism” — the idea that we are just puppets programmed by our DNA.

It is obvious that every individual is a product of both genes and environment. By using genetically sensitive designs such as twin studies, behavioral genetics has revealed almost as much about the environment as about genetics. Remember, past studies which did not control for genetic influence were wrongly attributing all differences in behavioural traits to real or imagined differences in parenting and early childhood experiences.

Parents Matter

Most parents would like to believe that the sacrifices they have made, the time, effort, and resources they have invested in their children are not in vain. And indeed they are not. In the words of Robert Plomin, a pioneer of behavioural genetics, “parents matter but they don’t make a difference”. What this cryptic-sounding statement means (and this is my interpretation) is that the home environment parents create matters a great deal to their children. But differences in home environments explain very little of the differences we observe in the behaviour and personality of people around us. In that sense, but only in that sense, DNA is Destiny.

Reading List

  1. Blueprint, Robert Plomin (2018)
  2. Top 10 Replicated Findings from Behavioral Genetics https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4739500/
  3. The Blank Slate, Steven Pinker (2002)

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