What causes differences in early behaviour between young children?
Picture a party of young children. Some are charging around, moving everywhere. Others are hanging back at the entrance, clinging to their parent's leg. Some children play alone. Another group are already meeting each other and making friends. And a few are tearful and looking for some adult reassurance.
These behaviours reflect differences in activity level, shyness, sociability and emotionality between young children. Parents notice these differences. Nursery workers notice them. But what causes them? Why do children differ so much in these early behaviours, even at one and two years old?
Theories about early temperament
The scientific study of children's early behaviour, often referred to as temperament, has a long history. Theories and models date back at least 50 years to the 1970s. In terms of the causes, the focus has been on early environment and parenting. The role of genetics was acknowledged in past theories, but not tested.
Our lab is interested in understanding how genes and environment both influence children’s development. We were curious: does genetics play a role in early temperament? And if so, do the genetic factors influencing infant and toddler behaviour have any relationship with traits and conditions that emerge later in life?
Past twin studies gave us a clue
We knew that a big gene discovery study of toddler behaviour was needed. We had learnt from our own reading and reviewing of the literature that infant and toddler behaviour is heritable (Austerberry et al., 2022). Twin studies compare identical and non-identical twins to estimate how much of the variation in a trait is explained by genetics. Twin studies consistently show that early temperament is partly heritable.
But twin studies have an important limitation: they do not examine DNA itself. They tell us that genes matter, but they do not discover which genes, or how they work. To answer those questions, we needed to conduct a gene discovery study known as a genome-wide association study.
The genomics revolution that is ignoring early childhood
There is a genomics revolution underway and it is extraordinary in its scope. Thousands of gene discovery studies have been conducted, using data from participants in many large cohorts and studying many traits and disorders. But we noticed something disappointing. Infancy and toddlerhood were being almost completely ignored in this revolution! In our review of the field, we found only 3 studies on psychologically-relevant traits in this age group (Ronald & Gui, 2024). We wanted to discover the genetic influences on this important and formative stage of life.
Setting up our study
We set out to conduct genome-wide association studies of infant temperament, specifically the four traits of Emotionality, Activity, Shyness and Sociability. To do this, we needed large cohorts of children for whom both DNA data and measures of early temperament had been collected. Bringing together cohorts requires careful harmonisation of measures and collaboration across many teams.
Children vary in their behaviour
We know that children differ in their behaviour. But the true degree of this variation became striking when we looked at data from tens of thousands of children. When we plotted histograms of temperament across our large samples, the spread was considerable. There is no single typical toddler. Understanding what drives this variation is one of the central goals of developmental science.
Our large samples meant that we were in a strong position to find results. In the biggest analysis, we studied over 78,000 toddlers.
Figure 1. Histograms show variation in levels of temperament in young children
What we found
Our results reveal that infant temperament is influenced by common genetic variation, the small differences in DNA sequence that exist across the population. In science, we call these genetic differences “Single Nucleotide Polymorphisms” or SNPs (pronounced “snips”). We identified ten independent SNPs in the genome associated with one or more of the four temperament traits (Hollowell et al., 2026). This is the first time specific genetic variants for infant temperament have been identified.
We were also able to estimate the heritabilities of toddler temperament, this time estimated directly from DNA rather than inferred from twin pair comparisons. These “SNP heritability” estimates indicate how much of the variation in a trait is explained by common genetic variants. The SNP heritability for toddler activity levels was 10%. This means that 10% of the variation observed between children in their activity level is explained by the common genetic variation we identified in our study. The other SNP heritabilities were 7% for Emotionality, 15% for Shyness and 3% for Sociability.
Genes that act in the brain
Among our findings were results that point to specific genes. One genetic variant is associated with activity level, and this genetic variant is known to affect the expression of the gene RHEBL1. Another genetic variant is associated with Emotionality and evidence suggests this variant affects expression of the gene MR1. Both of these genes are active in the cortex. This means that genetic variants influencing toddler activity level and emotionality do so by affecting the expression of these genes in the brain. These are new targets for future investigation into the biological mechanisms underlying early behaviour.
Is the biology of early childhood linked to the biology of later life?
Interestingly, we found genetic links between toddler temperament and outcomes measured much later in life. The “genetic correlations” we found mean that some of the genetic influences on toddler temperament overlap with genetic influences on later outcomes. Toddler emotionality was genetically correlated with adult neuroticism. Toddler activity level was genetically correlated with ADHD. Toddler sociability showed a genetic correlation with lower likelihood of autism. And toddler shyness was genetically correlated with lower adult extraversion.
At the same time, we note that the genetic correlations were not at their maximum value. This means that our results indicate some overlap in biology across development but not complete overlap. Our results also do not mean that all active toddlers will develop ADHD, or that all shy children will grow up introverted and so on. What these findings do tell us is that some of the same biological factors influencing behaviour in toddlers are also associated with traits and outcomes later in life.
A message for parents
If your toddler is particularly active, or particularly cautious, or particularly emotional, this may be, in part, their inherited predisposition. The wide differences we see between children in these early behaviours are expected and are due to a combination of both genetics and the environment. Our results do not negate the need for a supportive environment for young children, which is essential for healthy development. But our results uncover a more accurate picture of the range of causes of toddler behaviour.
What's next
This study opens up many new questions. We want to understand how early genetic influences work together with early environments. Also, while we were able to include some multi-ancestry analyses, much more can be done to study all populations. Finally, we would like to understand whether our genetic discoveries can be useful for the development of early interventions for some children who may benefit from support early in life.
References
Austerberry, C., Mateen, M., Fearon, P. & Ronald, A. (2022). Heritability of Psychological Traits and Developmental Milestones in Infancy: A Systematic Review and Meta-Analysis of Twin Studies. JAMA Network Open, 5(8):e2227887.
Hollowell A., Gui A., Wigdor E.M., Morgan M.J., Hannigan L.J., Corfield E.C., Pool, R., Bruins, S., Ask, H., Middeldorp C.M., St Pourcain B., Bartels M., Boomsma D.I., Hartman, C.A., Noda, A., Takahashi, I., Ishikuro, M., Obara, T., Kuriyama, S., Mufford, M.S., Lake, M.T., Stein, D.J., Zar, H.J., Hoffman, N., Robinson E.B., Børglum, A., Zhang, X, Warrier V., ASD PGC working group, Arichi T., Johnson M.H., Dudbridge F., Sanders S.J., Havdahl A. & Ronald A. (2026) Genome-wide association studies of infant and toddler temperament in European and multi-ancestry populations. Nature Human Behaviour. doi: 10.1038/s41562-026-02486-5
Ronald, A. & Gui, A. (2024). The potential and translational application of infant genetic research. Nature Genetics, 56, 1346–1354.