A landmark global study has, for the first time, homed in on 12 specific genomic regions associated with attention deficit hyperactivity disorder (ADHD). The research reveals the genetic underpinnings of this highly heritable condition and suggests this discovery is only a small part of the wider genetic puzzle that makes up ADHD.

A large number of family, twin and adoption studies have revealed that ADHD is incredibly heritable. Between 70 and 80 percent of cases display some kind of family history yet no clear genes have so far been found to be significantly associated with the condition. It has been suggested that ADHD is a polygenic condition, meaning no single genetic variant is the cause but rather it arises through a large coalescence of many different variants.

This new research is the biggest genetic study into ADHD ever conducted, comprising 20,000 people with ADHD compared to a control of 35,000 healthy subjects.

"This is a landmark study because it involves patients from all over the world," explains Anita Thapar, one of the researchers on the project from Cardiff University. "This large number of patient samples has been lacking for ADHD, meaning our understanding of ADHD genetics has lagged behind physical disorders and other psychiatric disorders like schizophrenia and depression."

The research uncovered 12 common genomic regions that for the first time could be clearly associated with ADHD. While many of these regions share associations with other diseases, several of them play a key role in healthy brain development and learning.

One of the more interesting outcomes from the research seems to lend weight to the hypothesis that clinically diagnosed ADHD is the extreme end of a spectrum of genetically influenced behaviour. A strong genetic similarity was found between people who are clinically diagnosed with ADHD and those who just display some of the same behavioural traits, such as fidgetiness and inattention.

"The correlation between these rather different definitions of ADHD suggests that clinically diagnosed ADHD may be the severe end of a continuous distribution of symptoms in the general population," says Joanna Martin, another Cardiff researcher working on the project.

The study also suggests that these 12 genomic regions are just the tip of a potentially massive genetic iceberg that is associated with ADHD. It's hypothesised that common genetic variants may only account for around 22 percent of a person's ADHD risk, with yet-to-be-discovered rare genetic variants and environmental factors accounting for the rest.

Unfortunately, those looking for a single, easy genetic cause behind ADHD will be disappointed, however this study is a valuable addition to a body of growing science tracking the genes that help us understand the heritability of ADHD.

"The genetic risk variants related to this condition play a significant role in brain-related and other core biological processes," says Thapar. "The next step is to determine the exact role of these genes in ADHD to help us inform better treatments to support those affected by the condition."

The new study was published in the journal Nature Genetics.