Toddler Temperament Is Genetic—And Linked to ADHD and Autism

Activity levels and behavior vary widely between children—and this variation is partly due to genetics.

Researchers from the University of Surrey have identified genetic differences that affect toddler temperament, some of which are also linked to attention-deficit/hyperactivity disorder (ADHD) and autism. 

The importance of toddler temperament 

During toddlerhood, children are learning a wide variety of skills, from developing their motor skills and exploring their surroundings to expressing emotions, building emotional intelligence, socializing, and building relationships. 

They also begin to develop their personality. Key behavioral domains, including emotionality, activity, sociability, and shyness, form toddler temperament, which represents emerging personality traits. These traits are likely to impact the parenting a child receives, their learning experiences, and their resilience in life. 

Toddler temperament is also connected to neurodevelopmental conditions and mental health later in life—higher emotionality in toddlerhood is associated with ADHD, autism, and psychiatric disorders in childhood and with emotional disorders in adolescence. In addition, activity levels are associated with ADHD symptoms and diagnosis in childhood, and shyness has been linked to anxiety and depression symptoms in adolescence, as well as autistic traits in childhood. 

For these complex behaviors, environmental and genetic factors combine to explain individual differences.  

In infancy and toddlerhood, the brain is growing and developing rapidly, making this a critical window for understanding how genetic variation shapes behavioral differences. 

“From 12 to 36 months is a very exciting time for parents and children, with a lot of new skills being learnt. During this time, we know that toddlers vary widely in how much they move around,” said Dr. Angelica Ronald, professor of psychology and genetics at the University of Surrey, and senior author of the study. “Activity levels are important and likely influence a wide variety of experiences including learning, exploring the wider environment, development of motor skills and the type of parenting that children receive.” 

“We were interested to learn about the underlying biology of early activity levels and whether these behaviors share some of the same genetic influences that affect things late in life, such as diagnoses of ADHD and educational outcomes,” she continued. 

Genome-wide association revealed loci linked to toddler temperament 

Ronald and her team conducted a genome-wide association study (GWAS) of emotionality, activity, shyness, and sociability traits in toddlers from five established cohorts. Age-specific analyses were performed for the second and third years after birth, and a cross-age average was taken across the two years.  

The researchers found that common genetic variations can explain differences in shyness, activity, emotionality, and sociability. Genetics accounted for 15% of the variation in toddler shyness, 10% of the differences in activity, 7% of the variation in emotionality, and 3% of the variation in sociability. 

They also identified ten significant genomic regions associated with emotionality, activity, and shyness traits. Within these loci, the researchers further identified cross-age and age-specific genes mapped to each trait. 

Most of the genes significantly associated with temperament traits were age-specific, but two genes in particular—TRIM26 and RBL1—were significantly correlated with cross-age activity, and IMMP2L was strongly associated with cross-age shyness. 

“For the first time, we have identified the genetic variation associated with these early behavioral traits in toddlers,” said Ronald. “Our work highlights the significance of ‘nature’ in a young child’s behavior. This complements the existing research on environmental influences on early childhood.” 

Genetic differences linked to temperament are also associated with neurodevelopment and adult personalities 

The researchers discovered an overlap between the common genetic differences associated with toddler activity and those associated with ADHD. 

“Our findings show that very early activity levels are influenced by many of the same genetic markers that influence later ADHD,” said Dr. Anja Hollowell, research fellow at University College London, and first author of the study. 

“We did not know until now that the genetic basis of ADHD was shared with these early behavioral traits.” — Dr. Anja Hollowell 

A similar trend was also observed for toddler sociability and autism, with genetic variation linked to increased sociability associated with a lower likelihood of the child being autistic. 

Toddler temperament was also genetically associated with adult personality, the researchers found. Toddler emotionality was positively correlated with neuroticism in adulthood, and toddler shyness was negatively correlated with adult extraversion. 

Underlying biological mechanisms 

Two of the ten genome-wide significant loci were found to colocalize with expression quantitative trait loci (eQTLs)—areas of the genome that directly impact gene expression—in the cortex of the brain. 

Lower activity levels in toddlers were associated with variations in the RHEBL1 gene in the GWAS, and further analysis showed that these variations increased the expression of RHEBL1 in the cortex. 

RhebL1, the product of the RHEBL1 gene, is a GTPase that regulates NF-κB-mediated transcription and is involved in the mTOR signaling pathway, offering a potential biological mechanism behind how genetic variation influences behavior. 

The second eQTL discovered by the researchers was related to emotionality. The locus for cross-age toddler emotionality colocalized with the eQTL for the MR1 gene. Variations at the locus were associated with decreased emotionality and increased MR1 expression in the brain. 

These findings may help scientists understand the relationships between temperament, brain development, and learning. 

The authors of the study outlined that further functional validation studies will be needed to identify the mechanisms by which RHEBL1 affects activity in toddlers and the pathways underlying MR1’s association with emotionality. 

“These new results advance our understanding of the early pathways influencing individual differences in child development,” said Ronald.

Reference: Hollowell A, Gui A, Wigdor E, et al. Genome-wide association studies of infant and toddler temperament in European and multi-ancestry populations. Nat Hum Behav. 2026. doi: 10.1038/s41562-026-02486-5

This article is a rework of a press release issued by The University of Surrey. Material has been edited for length and content.