How Genetics and the Environment Shape Early Life Cognitive Development
What causes differences in children’s early cognitive development?
In the first years of life, children develop important cognitive skills like language, attention, and memory. These cognitive skills can influence how well children do later in school: those with better language, attention, and memory typically achieve higher school grades. Doing well in school, in turn, predicts many important life outcomes, for example how much someone earns, how healthy they are, and how happy they feel.
Differences in children’s cognitive skills are apparent from as young as 2 years of age, and often increase over time from infancy and throughout childhood. Our new study, funded by the Nuffield Foundation, aims to understand the differences in children’s cognitive development in the early years. By looking at the environment that children grow up in, and the genes that they inherit from their parents, we will try to predict children’s differences in cognitive skills in early life. Our research will contribute to developing interventions that improve children’s cognitive skills and in turn reduce the gaps in school performance and academic achievement.
Differences in our physical and psychological traits, for example our height, weight, and cognitive skills, are partly influenced by the genes that we inherit from our parents. To study the influence of genetic inheritance, classic twin studies compare the resemblance of identical twins, who share 100% of their genes, with non-identical twins, who only share 50% of their segregating genes. These comparisons can help us to estimate the heritability of traits – that is, what proportion of the differences in an observable trait is due to genetic differences. Results from the classic twin design have shown that, on average, genetics can explain 50% of the differences that we observe between people.
Recently, a new approach has emerged to identify the actual genetic variants that drive the heritability of traits. This approach involves calculating a genome-wide polygenic score, an aggregate of a person’s DNA variants that make up their genetic likelihood for a certain trait or behaviour. Let’s look at these scores in more detail.
To understand genome-wide polygenic scores, we first need to talk about genome-wide association studies, which search people’s genomes for tiny differences in DNA variants. They look for DNA variants that occur more often in people with a particular trait than in those who don’t show the trait. One or two of these DNA variants alone won’t explain people’s differences in traits because their individual effect sizes are too small. In fact, genome-wide association studies have shown that people’s differences in traits related to cognitive skills are due to many thousands of DNA variants. If we know which DNA variants are associated with a trait, we can test individual people to see how many of those DNA variants they carry. We can then add the DNA variants together, and this aggregate is known as a genome-wide polygenic score.
Genome-wide polygenic scores capture a person’s genetic propensity for traits, such as cognitive skills. Because people’s differences in cognitive skills are at least partly heritable, genome-wide polygenic scores are good predictors of a person’s cognitive skills.
An increasing number of research studies use genome-wide polygenic scores to predict psychological traits, medical disorders, and psychiatric diagnoses. In psychological research on cognitive skills, genome-wide polygenic scores have been used to predict children’s differences in school performance, reading ability, and intelligence.
Studying genetics and the environment
Children’s differences in cognitive skills are driven by both genetic and environmental factors. Environments can facilitate or block opportunities for genes to be expressed. Likewise, genetic propensities can make accessing certain environments easier or harder. Although many researchers suspect that genetic factors interact with environments, we do not know much about when these interactions happen, and how they cause differences in the development of cognitive skills.
Gene-environment interactions describe how the effects of our genes on our development depend on the environment that surrounds us. We’ll use an example to explain how gene-environment interactions work, based on research into how genes and the environment affect the development of musical ability.
Imagine a child with a high genome-wide polygenic score for musical ability, who grows up in an environment where they hear and play music often. This child is likely to develop better musical ability than a child who also has a high genome-wide polygenic score, but grows up in an environment that promotes other skills, such as sports. Next, think of a child with a low genome-wide polygenic score for musical ability who grows up in a musical environment. Their musical ability will be more advanced than a child with a similar genome-wide polygenic score who grows up in a non-musical environment. However, a child with a low genome-wide polygenic score for musical ability is unlikely to achieve the same level of musical ability as a child with a high genome-wide polygenic score, even if they both grow up in a musical environment.
The musical ability of children with low and high genome-wide polygenic scores is different depending on whether they grew up in a musical or non-musical environment. This pattern of association is known as an interaction, where the relationship between two variables, in this case genetic propensity and musical skill, varies according to a third variable, the type of environment.
In this project, we will try to identify environments that interact with genome-wide polygenic scores in the prediction of cognitive skills, and we will study how important these interactions are for children’s early cognitive development.
The data for this project will be derived from The Twins Early Development Study, in short TEDS. TEDS is the largest genetically sensitive study of development in the world, for which all twins born between 1994-1996 in England and Wales were invited to take part. The twins were assessed for the first time at age 2, and then followed up regularly until today, when they are in their early twenties.
Our analyses will use data from over 10,000 TEDS participants, for which lots of information is available, including data on the environments that they grew up in, their family home and neighbourhood, and their cognitive skills at age 2, 3, and 4 years. We will calculate genome-wide polygenic scores to capture TEDS children’s genetic propensity for cognitive skill development and analyse these alongside a variety of environmental measures. Here are some examples:
- Children’s thoughts about their family home, parenting styles, parent disciplining strategies, and nursery environment.
- How many siblings the children have.
- Childcare arrangements and whether or not children go to nursery.
- Behaviour of the children at home and at nursery.
- Details about the parents, such as their job, education, and marital status.
- Mothers’ pregnancy and birth experiences.
- Characteristics of the home, such as location, type of housing, and internet, energy and water usage.
- Characteristics of the external environment, such as air quality, access to green spaces, and life expectancy in that area.
What do we know so far about gene-environment interactions and cognitive development?
The effects of gene-environment interaction are thought to impact early life cognitive development, but to date no specific environments have been identified that interact with our genome to affect cognitive development. Studying gene-environment interactions will greatly improve our understanding of cognitive development in the early years.
Previous research has identified some key challenges in studying gene-environment interactions. For example, gene-environment interactions are likely to have extremely small effect sizes, so we need very large samples to be able to detect these effects.
Another challenge is that the environment around us is constantly changing, unlike our genes, which don’t change over time. As we grow, we are exposed to different physical, sensory and emotional environments which may impact our development in different ways.
Our project will address these challenges and help to improve our understanding of the role of gene-environment interactions for early life cognitive development. We will communicate our research widely and in accessible ways, to help other researchers work in this area, to inform the public about gene-environment interactions, and to provide a foundation for discussions about how gene-environment interactions are relevant for best practice and policy in early years education.