The Psychological sciences (and many other disciplines as well) are suffering a so called “replication” crisis. The problem is that many reported scientific findings do not replicate when other research groups try to redo the study. For example, an attempt to test psychological studies for replication showed that out of 100 studies only 36% showed significant replication (Open Science Collaboration, 2015). We can’t trust the findings which do not replicate, because the final say on whether findings are robust depends on reproducing the previous results.
The percent of replication varies inside the psychological science, e.g. the replication for behavioural genetics is much higher than for neuroscience. In his paper, Professor Plomin presents 10 findings from behavioural genetics, which have showen consistency in replication and thus can be considered trustworthy. These findings are crucial and have a huge impact on psychological science.
The idea of Heritability comes up a lot in these studies and so we need to establish a good understanding of this term. Heritability refers to individual differences across a particular population at a particular time with its particular mix of genetic and environmental effects. It does not relate to the genetic make-up of an individual (that’s heredity), but rather to the variation seen across a population in the trait of interest. So if a trait is 80% heritable, it means that 80% of the trait differences in people in a population can be accounted for by genetic differences, the remaining 20% would be accounted for by environmental differences.
So, here we are: The top 10 replicated findings of Behavioural Genetics:
1: All psychological Traits Show Significant and Substantial Genetic Influence
You probably will not manage to find even one trait which doesn’t show a genetic influence. Almost everything in our life is genetic in some way, all our psychological traits and individual differences have their roots in our DNA. Traits such as political beliefs, religiosity, altruism, and food preferences have all shown significant genetic influence, as have factors such as intelligence and weight. Even our sudden death chances show heritability, as ALL our traits, including risk-taking and liability to heart attacks, are under the influence of genetics to some extent. Even something which may seem entirely environmental, such as being involved in a car accident can be a result of genetic influence. For example, through a tendency for aggressive driving and/or a lack of attention. I’ve said “almost” because there are always some unpredictable events such as famines and floods, but these events hardly constitute day-to-day living for most of us.
2: No Traits are 100% Heritable
We’ve already said that our life is linked strongly with our genes, but are there any traits which are entirely genetic? Can I blame the fact that I am tone deaf entirely on my parents? Well, no, there are no complex traits which are 100% heritable. There is an environment around us, there are different situations, nurture issues, friends, work, even accidents and illnesses which shape our lives. So there is always some environmental influences effecting each of our traits. Heritability estimates are substantial and typically lay between 30% and 70%, but this range of estimates is a long way from 100%. Even height, with a heritability estimate of about 90%, leaves room for environmental influences. So, environment matters, ALWAYS. .
3: Heritability is Caused by Many Genes of Small Effect
The two previous findings come mostly from family-based genetic designs, primarily twin and adoption studies. Though good, these methods can’t estimate the specific genes which are involved in heritability and the proportion of their influence (effect size). To do this one of our options is to turn to animal research, where genetic and environmental influences can be more precisely controlled and measured. From this and other research we know that almost every trait is polygenic, i.e. there are many genetic variations that relate to each complex trait. It is because of this that the selective breeding of animals is so time consuming and complex, taking many generations to achieve the desired characteristic (drought resistant cattle for example); effects are cumulative and they tend to sum up across generations. It is the same with people; we don’t have purely categorical traits, every trait of ours is a result of the sum of huge variety of genetic influences of small effect. Even in traits such as autism, that we often think of as categorical (you do or don’t have it) there are actually many genetic variations at work. In truth, autism exists on a spectrum, to which we ascribe a threshold for diagnosis. If I, for example, have one (or ten) of these genes of small effect, I won’t show much difference from the average of a population, but if I have a hundred of those, then I may pass the threshold for diagnosis.
4: Phenotypic Correlations Between Psychological Traits Show Significant and Substantial Genetic Mediation
Much psychological research is about the relationship between traits (e.g. do people who perform will in maths also do well in reading?). Behavioural genetics also asks why does this relationship exist (or not). Scientific journals have reported associations between creativity and mental health, stress reactivity and neuroticism, empathy and moral behaviour, etc. However, genetically informed designs shed light on the fact that many correlations between psychological traits are mediated by genes and are not just environmentally driven.
It is obvious that intelligence, reading, mathematics, and language are connected somehow, in fact they correlate significantly and substantially, but these correlations are mostly (more than 53%) correlated because of the underlying genetic mechanisms, so they are mediated by genetics. Interestingly, scientific studies have also shown that anxiety and depression are correlated entirely for genetic reasons. This makes them genetically identical conditions, with variations begin down to environmental influences. Furthermore, recent molecular genetics studies have shown that “there is evidence for shared genetic risk between schizophrenia, bipolar disorder, autism spectrum disorders, intellectual disability and attention-deficit and hyperactivity disorder”, indicating that the genetic structure of psychopathology does not map neatly onto current diagnostic classifications.
5: The Heritability of Intelligence Increases Throughout Development
This one finding is limited to a specific domain (general cognitive ability), but it is one of the most surprising and counterintuitive findings from behavioural genetics. Psychological studies show that the heritability of intelligence increases across the lifespan. As we learn more throughout our lives it would seem logical that genetic influences on intelligence would decrease, but this is not the case. More than three decades of research has shown that intelligence actually becomes more genetically influenced as we age. A possible explanation for an increasing heritability estimate could be the fact that as we age, additional genetic influences come into effect, a process called innovation. However, our next finding can also shed some light on this unexpected phenomenon.
6: Age-to-Age Stability is Mainly Due to Genetics
Longitudinal studies consistently show that trait correlations (for example the relationship between reading and spelling) from age to age are largely due to genetic stability. I.e., genetic effects contribute to continuity (the same genes affect trait relationships as we age), whereas age-to-age change is primarily related to environmental factors. Therefore, if a child does well in an IQ test in his early years, he will be likely to perform in the same range as he ages. Given what was said above, this example may seem counter-intuitive. How can the heritability of intelligence increase throughout development if genetic effects are stable? How can the same genes largely affect intelligence across the life course and yet account for more variance as time goes by? This phenomenon can be partly explained by genetic amplification, the process where genetic nudges early in development are magnified as time goes by, increasing heritability. In other words, during childhood we are all equally limited in our life choices (almost all children go to nursery, and then school, with very little or no say in which nursery or school). Because the environment is more imposed in childhood it has a larger influence. As we grow up we have more say in our environments and so the genetic influence for intelligence increases because now we can choose what is best for us, for our well-being, skills and character. Our genetic influences drive us to seek out environments where those influences can thrive. Our environments become more ‘in tune’ with our genetic propensities, and so those propensities come more to the fore. The result of this is that heritability estimates for intelligence increase as we age.
7: Most Measures of the “Environment” Show Significant Genetic Influence
We have talked a lot about factors which can seem to be purely environmental: nurture, parent behaviour, car-accidents etc. If they are truly measures of the environment, they should not show genetic influence. But as soon as we start speaking about genetically informative studies of these factors it turns out they are also under the influence of genetics. Significant genetic influence was found for parenting, social support, life events and many others. But how so?
The story so far is that such measures do not assess the environment independent of the person. As we’ve noted already, humans select, modify, and create environments correlated with their genetic behavioural propensities. For example, studies have shown the significant genetic influence on parenting both from child and parent characteristics. Also, some exceptions have emerged. Not surprisingly, when life events are separated into uncontrollable events (e.g., death of a spouse) and controllable life events (e.g., financial problems), the former show no genetic influence.
8: Most Associations Between Environmental Measures and Psychological Traits are Significantly Mediated Genetically
If genetic factors affect environmental measures as well as behavioural measures, it is reasonable to ask to what extent relationships between environmental and behavioural measures are mediated genetically. For example, rather than assuming that correlations between parenting and children’s behaviour are caused just by environmental effects of parenting, one should consider the possibility that the correlation is in part due to genetic factors that influence both parenting and children’s behaviour. For instance, two thirds of the correlation between maternal negativity and adolescent children’s antisocial behaviour has been attributed to genetic factors. The possible underlying mechanism could be that children’s antisocial behaviour is not only the result of their mother’s negative attitude as an environment factor (driven by genetics obviously), but also by the childs genetic liability to negative attitude which he received from his mother. Also there could be a situation when the children’s antisocial behaviour (again driven by genetics) could evoke negativity in their mother, to which she is liable genetically.
9: Most Environmental Effects are not Shared by Children Growing up in the Same Family
It is reasonable to think that growing up in the same family makes brothers and sisters similar psychologically, which is what developmental theorists from Freud onwards have assumed. However, for most behavioral traits and disorders, it is genetics which accounts for similarity among siblings, with environmental influences leading to differences. Although environmental effects have a major impact (see Finding 2), the salient environmental influences do not make siblings growing up in the same family similar. As we’ve mentioned above, people tend to look for and create and environment which suits them and also they have their own attitude to it. For example, a child may suffer extremely depressive feelings due to parental divorce where their sibling feels relief because of the decrease of the tension in the family. Although children growing up in the same family might appear to share their environments, the thousands of different ways each responds to their environment (their “experiences of small effects”) means that environments tend to account much more for differences than similarities.
10: Abnormal is normal
This last, and possibly most important finding is that the abnormal is normal. As we’ve already seen in the example of autism (point 3), we tend to think about such disorders as being categorical; you either have it, or you don’t. However, and with the exception of some rare genetic disorders such as Phenylketonuria (PKU) and Huntingtons, this is not actually the case. For example, achievement at school varies (grades A, B, C etc.) and the genetic influences at the top end of the scale (A+ students), are the same as for students at the other extreme. As a society, we may ascribe a point at which we ‘diagnose’ someone as having a learning disability, but this does not make them part of a special and separate group, rather they are at one extreme of the normal variance seen in a population. When it comes to complex traits, there is no “them and us”, we all fall somewhere in measures of intelligence, psychopathy, autism, height and weight etc. and the extremes of each trait are influenced by variations in the same genetic influences.
In short, behavioural genetics research has consistently and reliably shown us that our genes influence who we are, informing our appetites as well as our abilities. Everything that makes us who we are has also been shaped by our environments and those environments are moderated by our genes. Far from determining who we are and what we become, our genetic influences interplay with our environments, and the more we are able to choose environments that suit us, the more scope we have for development and growth. Behavioural genetics research also shows that the notion of “them and us” can rarely be justified on biological grounds. The news is good, and we need to pay more attention to it if we are to make real changes in how we live and grow.