Why does one toddler walk boldly into a room full of strangers while another clings to a parent? An international study published in Nature Human Behaviour showed that common DNA variations are linked to behavioral traits in the first three years of life. These same variants overlap with adult personality traits, cognitive abilities, attention deficit hyperactivity disorder (ADHD) and autism.
Temperament refers to early personality traits that determine how a child reacts to the world. Psychologists distinguish four parameters: emotionality (how easily and intensely a toddler experiences negative emotions), activity (physical energy and pace of movement), shyness (tension around strangers) and sociability (the drive to interact). Twin studies estimated the contribution of genetics to these differences at 30-60%, but did not identify specific DNA regions.
The work, led by Ani Hollowell of Birkbeck, University of London, and Angelica Ronald of the University of Surrey, combined five European cohorts. Parents rated their children's temperament at ages 18-24 months and 36-38 months. The scientists used a genome-wide association study, a method that tests millions of small DNA differences for association with a trait.
They identified 10 independent genomic regions associated with emotionality, activity or shyness; no such regions were found for sociability. Common genetic variations explained about 15% of the differences in shyness, 10% in activity, 7% in emotionality and 3% in sociability. The pattern was stable between the second and third years of life.
Variants associated with high activity in toddlers correlated moderately positively with ADHD and more strongly with earlier onset of walking, but negatively with educational attainment and cognitive test results in adults. Sociability variants were associated with a lower likelihood of autism and greater extraversion. Shyness showed no significant genetic link to anxiety disorders, although observations had previously pointed to such a link.
An analysis of gene expression in postmortem adult brain tissue identified two regions with a high probability of a shared genetic basis. One is linked to the RHEBL1 gene, which regulates a cellular signaling pathway important for the growth and connections of nerve cells. The other is linked to the MR1 gene, which helps immune cells recognize microbial products; it had not previously been linked to behavior.
Important: these are statistical correlations that do not prove causation. Genetic variants increase the likelihood of tendencies but do not predetermine a child's character or a future diagnosis. Polygenic scores - a combined measure across many DNA variants - explained less than 2% of the differences in temperament.
The authors note limitations: most participants were of European ancestry, the expression analysis used adult rather than child brain tissue, and the 10 identified regions have not been replicated in an independent sample. The scientists call for larger studies of diverse populations and for laboratory work on the mechanisms by which genes such as RHEBL1 and MR1 influence early development.