Post No.: 0362
In the field of behavioural genetics (the study of how genes influence our behaviours), and regarding quantitative or polygenic traits (traits that are influenced by more than one gene, which is most of them) – ‘additive genetic effects’ explain the hereditary portion of the influences on one’s life; and ‘non-additive genetic effects’ seemingly only explain a negligible portion of the influences on one’s life for most behavioural conditions.
On average, additive genetic influences account for ~50% of the total influences on the behavioural traits of an individual, with the other ~50% being environmental influences. So although some traits are more genetically, and other traits are more environmentally, influenced – it’s on average ~50:50 genes:environment. It’s also virtually always down to a combination of genetic and environmental factors, and all behaviours in the animal kingdom can be completely explained by genetic and environmental factors alone.
‘Non-shared environmental influences’ explain the environmental portion of the influences on one’s life that make two people different from each other (e.g. differential parental treatments, different peer groups, stressful events that affected one person but not another). These account for most of the total environmental influences that affect the behavioural traits of an individual when it comes to traits such as neurodevelopmental conditions like autism, ADHD, schizophrenia, personality disorders, and personality itself (i.e. most behavioural and personality traits). The magnitude of the effects of non-shared environmental influences remains stable whatever one’s age.
‘Shared environmental influences’ explain the environmental portion of the influences on one’s life that make two people similar to each other (e.g. a shared upbringing style and environment). These account for most of the total environmental influences that affect the behavioural traits of an individual when it comes to traits such as general cognitive ability, social attitudes, religiosity, rule-breaking and moral values (i.e. things that parents generally care to shape).
However, some research suggests that heritability actually increases with age, so the older one gets, it’s not the freer one becomes to be whatever one wants to be, and it’s not even the more that one’s accumulated shared life experiences will shape one’s choices and destiny – but research actually shows that the older one gets, the more that one’s genetics and non-shared environmental influences shape one’s choices and the less that shared environmental influences such as upbringing matter. A possible explanation for this is that, when young, one’s choices are constrained, but when people leave their parents, have their own money and can choose their own place to live and lifestyle to lead, they can start to express their own genetic predispositions more, and so they do?
A limitation of the above model, though, is that it doesn’t take into account ‘gene-environment interactions’ (GxE effects). For example, how a heritable disease or personality trait may or may not express depending on the presence of environmental stressors. These GxE interactions can be highly complex and their products can be drastically life changing too. And not only are identifying specific risk alleles, accounting for polygeneity (multiple genes affecting one trait), pleiotropy (one gene influencing multiple traits), the effects of ageing on our DNA and epistatic (gene-gene combination) interactions, all highly complex – but non-shared environmental influences are idiosyncratic, unsystematic and highly individual, which makes them hard to firmly identify and study too.
All we know so far are very broad generalisations about the balance of genetic and environmental influences on our behaviours. There’s still currently a lot of research required in the field of behavioural genetics. It’s complex because one needs to work out the gene-environment combination interactions, gene-gene combination interactions and environment-environment combination interactions, and the past and present combination interactions of all of these together! Whatever the case though, we are a function of our genes and environment, along with the law of causality (effects have causes), at least above the atomic level where our living experience resides, thus these two areas are where we need to look for understanding all of our behaviours.
Genetic variation plays at least some role in every single trait or behaviour scientists have studied so far. But very few traits in humans are actually controlled by a single gene in ‘dominant/recessive allele’ relationships (despite the impression one might have gotten from (old) Secondary/High school biology classes if one didn’t continue one’s education in biology). Most genotype-to-phenotype relationships are highly complex because of complex GxE interactions.
As aforementioned, it’s seemingly overall ~50:50 genes:environment, nature:nurture, when aggregating everything including personality, IQ, psychiatric disorder risk, religiosity, risks of divorce, crime or alcoholism. Yet of that environmental portion, only seemingly ~5-10% is down to shared environmental influences. So parenting methods and the home environment only apparently play a minor role in shaping a child’s personality or behaviours, with non-shared environmental influences playing the major role regarding environmental influences. However, environmental influences might be severely underestimated – we must note that heritability estimates can change over time, across cultures and over a person’s own lifespan, as the environment changes.
The influence of genetics can vary depending on the environment we’re in. The environment can change and this’ll ultimately affect the relative importance of genetics. This is like saying that the influence or importance of a car’s aerodynamics can vary depending on the circuit that’s being raced. For example, if people received different levels of access to income, education, health services and social services then we’d expect the environment to play a greater role in explaining the differences between people, and vice-versa if there’s less inequality in society, where genetics will then play a greater role in explaining the differences between people.
Also, because the field of behavioural genetics research only tends to focus on (thought to be) stable traits (e.g. OCEAN personality traits) and traits that are (thought to be) mediated by genetic variability (rather than whether learning particular skills, such as playing the piano (which also depends heavily on having access to a piano and lessons) i.e. specific real-world traits we really care about, are heritable), and because adoptive families aren’t that variable for they tend to be wealthy families, which introduces sampling biases (the data for these studies come from twin studies, where some twins are adopted into different families, some are adopted into the same family, and some aren’t adopted at all, since this is the next best thing to unethical or impractical randomised-controlled trials (RCTs)) – this might possibly mean that the effects of genetics are (quite) over-estimated, and therefore the effects of the environment are (quite) under-estimated?
Furthermore, behavioural genetics currently doesn’t tell us what specific genes or environmental factors (e.g. which parenting practices) are the reasons for these differences between individuals, or how strongly each specific gene or environmental factor contributes to these differences.
So it appears that parenting methods, as inferred from the shared environmental influences, have much less of an impact on children than expected – yet we must be cautious about the limitations of current behavioural genetics research. With current limitations in fully understanding genetic heritability (partly due to a lack of an ethical way to conduct many RCT experiments on the lives of parents and children), we can only say something ‘influences’ rather than ‘determines’ a trait. No two humans ever have the exact same life experiences at exactly the same times in their lives – even monozygotic (identical) twins living under the same roof – so that the only independent variable is the experimental variable being tested. There are umpteen examples of monozygotic twins where only one of them is a sports star, movie star or otherwise considered ‘successful’ – indicating how genes alone hardly determine fate. Woof.
But even though genes overall appear to only account for ~50% of all behaviour, there are 3 ways that genes possibly affect the environment we live in too when we examine ‘gene-environment correlations’. Parents of a certain genetic disposition not only tend to mate with each other (e.g. academic people tend to mate with other academic people) but create an environment that promotes their, and relatedly their offspring’s, dispositions (e.g. academic parents value academia thus tend to create an environment that promotes achieving high academic grades for their children, whom share related genes too). This is called a passive correlation because the child/person in question never chose it, and this influence has a greater effect on a person when they’re young because that’s when they’re with their parents.
How a person behaves will influence how others behave towards them too, and how a person in turn reacts back towards that behaviour; and how we behave and react in the first place may be a function of our own genetics. This is called a reactive correlation, and this influence is assumed to be stable throughout one’s life.
Finally, our genetic disposition will shape the choices of experiences and types of environments we want to select and create (as long as we have the opportunities and means to do so that is e.g. money and laws), which in turn will reinforce our behaviours and the expression of our genetic propensities (e.g. if we’re good at physical activities then we might choose to do more sports and live in an open space away from the city). This is called an active correlation because the child/person in question directly chooses it, and this influence has a greater effect on a person when they’re older because that’s when they’re more autonomous.
All this suggests that even the environmental portion of the influences on our lives seems to be heavily based on the genetics we didn’t choose, and therefore ultimately all of our choices are based on things we didn’t ultimately ever choose. But a big note of caution in this research is that these correlations between a parent and a child, or a gene and the environment, don’t necessarily mean there’s a genetically causal relationship. Because the gold standard in working out whether there’s a causal relationship is a double-blind, randomised, large sample size experiment with a control group, we may never ethically know whether they’re causal relationships or if they’re merely effects of some other cause(s). There are far too many variables and complexity in real life and therefore in observation studies to know what confounds may be at play. We cannot ethically lock people up for a lifetime to study them – the types of people who’d volunteer may present a biased sampling and it wouldn’t be ecologically valid (representative of real life) anyway!
And although any scientific results are still scientific results (albeit lots more further research is still presently needed in this field) – what we choose to do with that information i.e. the social and political decisions, is highly debateable (e.g. should we help the disadvantaged, handicap the advantaged, promote the advantaged (positive eugenics) or constrain the disadvantaged (negative eugenics), and if so then how should we decide who falls into which category, and so on?) This field is controversial because of what it may seem to justify to some people.
We never chose our genetic inheritances or de novo (from new) mutations, and even if we could in the future, what we’d wish to change our genes to will ultimately be a function of whatever genes we were personally initially born with in the first place(!) Plus the environment we’re born into is down to either someone else’s choice such as the parents we never chose, our own genetic propensity’s choice, how others react to our phenotypes, or chance… and all that breeds and follows from all the above i.e. there’s apparently, from an ultimate level analysis, no true free will/choice at all.
But I personally feel that if our personal genetic propensity is to trust in education and act upon any solid scientific findings (e.g. like regular exercise benefits everyone), as well as to pro-socially help each other to reach better outcomes, then there may be hope for reshaping our environments to help us all reach better outcomes regardless of our arbitrary starting points.