It is one of the crowning findings of behavioural genetics: the older we get, the more our specific genetic differences are associated with the degree to which we differ on measured phenotypic traits; like weight, disease prevalence and cognitive diversity. This is interesting, because genetic variance of a population doesn’t change over time (each person still has the same genetic material), so how can it be that heritability estimates account for more of the variance in a population as a function of brain maturation?
One set of traits: memory, spatial reasoning, verbal fluency, abstract rule finding, commonly referred to as general intelligence, have been well known to demonstrate this linear heritability trend over time. During childhood, intelligence is around 20 per cent heritable; get to adolescence though and that number increases from 20-to-40 per cent. Compare this to a 60 per cent heritability estimate during adulthood.
Geneticists obtain these heritability measures through a couple of methods, but the most reliable estimates come from adoption studies of monozygotic twins (MZ) raised apart from one another. The method utilises the rear occurrence of twins being separated at birth; one goes to one adoptee family and the other goes to a completely different family. These twins don’t differ in genes, they only differ in the environments they’ve being raised in, which is important – as any measured trait similarity (co-variance) between the two twins will be due to the thing they share i.e. genes; whereas, any dissimilarity of traits can be attributed to the things they don’t share i.e. environments. So, their correlation of a measured trait will directly estimate heritability. If intelligence test score correlations for MZ twins raised apart is 0, heritability is 0, however a correlation of 1 infers heritability of 100 per cent. To ascertain a sense of whether heritability is changing, researchers assess twin correlations at different points during their life spans (known as a longitudinal study).
Why, then, does the heritability of intelligence increase linearly from childhood to old age, as seen in longitudinal twin studies? One possible explanation is that further genes begin to contribute during development, summating their effects on intelligence with the genes already active during childhood. But, recent analyses of genetic correlations from age-to-age reveals that genes affecting intelligence at childhood are the same as those influencing intelligence during adolescence.
The heritability of intelligence, then, appears to increase as development progresses in spite of the trait’s contributing genes remaining stable throughout life. So, same genes, but with more influence over an expressed trait as time goes by. How? The answer could be that those genes acquire some sort of amplification system as the brain develops.
This seems to be the answer. A person’s genes shape their phenotype (all measurable mental and physical traits), which is the biological organism that acts on the world. Through the phenotype, genes can exhibit some influence over the wider environment that they’re involved in. Initially, small genetic differences can multiply their significance on heritability estimates as a person becomes more mature and further able to shape their own environment through their phenotype. For instance, a child that enjoys reading will probably ask their parents for more books; it is up to the parent to oblige. That child is now an adult, with their own money, and can now determine for themselves how many books they’d like. As can been seen with this example, we appear to seek, mould and manipulate environments to fully utilise our genetic predispositions.
A study that gives credence to this gene-environment interplay hypothesis was done by Robert Plomin, a prominent behavioural geneticist and Professor at Kings College, London. In the 1980’s it was widely assumed that children’s television time had effects on child development that were entirely environmental in essence. It was assumed that TV time completely dictated by parents, with the remote control never falling into the hands of the child. Plomin, however, discovered that TV time was never really that regulated by parents, with children usually deciding how much TV they’d like to watch. With this insight, Plomin decided to research the heritability estimates for time spent watching TV. He found that adopted and non-adopted children, in the same household, with the same access to TV time, did not correlate in their indulgence with the box, giving a heritability estimate 50 per cent. This means that children with different genes but with exactly the same environment were choosing to utilise that environment in a different way.
Our unique genetic profiles and the phenotypic brain they aid the environment in producing — once fully able to break free from environments imposed upon then during early childhood — are able to amplify their individuality for all to see (or — if you’re a behavioural geneticist – measure).