Scientists find genetic connection between early vocab size and later ADHD

There are genetic associations between children’s early vocabulary size and later-life attention deficit hyperactivity disorder (ADHD), literacy, and general cognition, an international research team led by the Max Planck Institute for Psycholinguistics in Nijmegen, Netherlands has found. 

 

These associations change dynamically across the first three years of life, researchers said, finding that both producing more words in infancy and understanding fewer words in toddlerhood were associated with a higher risk of ADHD.

 

Early language development is an important predictor of children’s later language, reading and learning skills, and language learning difficulties are often related to neurodevelopmental conditions such as ADHD and autism spectrum disorder (ASD).

 

Children typically start to utter their first words between 10 and 15 months of age. At around two years of age, they may produce between 100 – 600 words, and understand many more. Each child embarks on its own developmental path of language learning, resulting in large individual differences. 

 

Some of these variations in language development, senior researcher Beate St Pourcain said, can be related to variation in the genetic code stored in our cells.

 

To understand how genetics plays a role in the development of children’s word production and understanding, the team carried out a genome-wide meta-analysis study (GWAS) of infant (15-18 months) and toddler (24-38 months) vocabulary size. 

 

In early measures of vocabulary size, parents reported which words their children said and/or understood from a given word list.

 

The team used vocabulary and genetic data from 17,298 English-, Danish- or Dutch-speaking children. The number of spoken words was available for both infants and toddlers. The number of understood words was only available for toddlers. 

 

Later-life outcomes were mostly studied with genetic summary information from large independent consortia. These included literacy (spelling, reading, and phoneme awareness), cognition (general intelligence and number of years of education) and neurodevelopmental conditions (genetic risk of ADHD and ASD, as well as directly observed ADHD-related symptoms in some of the studied children).

 

“Learning to speak” and “Speaking to learn”

 

The researchers identified multiple genetic factors underlying vocabulary size in infancy and toddlerhood. Consistently, genetic associations with later-life literacy, cognition and ADHD-related measures varied during development. 

 

Both infant and toddler word production were related to literacy abilities such as spelling, but associations with general cognition were only found for toddler vocabulary scores. Toddlers have mastered some language fluency and may “speak to learn”, involving higher-level cognitive processing, while the development of verbal abilities may start earlier.

 

The team also found that in infancy, a larger number of spoken words was genetically associated with both an increased risk for ADHD and more ADHD symptoms. However, this genetic relationship was reversed in toddlerhood: there, a smaller number of understood words was associated with more ADHD symptoms. 

 

It is possible that in infancy, when children are “learning to speak”, the number of spoken words captures speech-related processes. Also, children with a higher genetic risk of ADHD may be inclined to express themselves more. In contrast, during the phase of “speaking to learn” when vocabulary size is linked to cognition, higher genetic ADHD risk may be associated with lower verbal and cognitive abilities.

 

To access the findings in full please see here.