Shared influences, many small effects

Shared influences, many small effects

The opening introduces many small ingredients that shape inattention and hyperactivity. Very early birth, lead exposure, and extreme lack of early care are introduced as risk factors. Thirteen numbered DNA symbols have distinct highlighted positions and colors. Most common DNA variants linked to ADHD each have only a tiny effect. Three fictional people, Alex, Robin, and Sam, stand on stacks of different combinations of influences. DNA appears first, followed by other known risk factors. Unknown influences are added later. Alex has a lower stack, Robin a middle stack extending past an illustrative line for small issues with focus or hyperactivity, and Sam a higher stack extending above an illustrative diagnostic threshold line. Sam seeks help. The stacks and lines are illustrations, not measured effects or diagnostic scores.

There is no single cause that explains ADHD.

Instead, many small ingredients shape inattention and hyperactivity in everyone.

Here are some risk factors scientists have found.

Very early birth

Lead exposure in childhood

Extreme lack of care early in life

Many DNA variants can influence inattention and hyperactivity.

Many different variants can contribute.

Let’s illustrate how different influences can combine in three people.

Most common DNA variants linked to ADHD each have only a tiny effect.

Several influences can combine with an ordinary level of inattention and hyperactivity.

For some people, the combined effects contribute to difficulties that seriously disrupt daily life.

Some influences are still unknown.

These difficulties can lead someone to seek help and an ADHD assessment.

ADHD has many contributing causes. Many of the same influences also shape inattention and hyperactivity across the whole population.

What the examples mean

A risk factor is linked to a higher chance of an outcome. It does not tell us what will happen to one person. People with similar ADHD traits can have different histories.

Shared influences across the continuum

Research supports substantial overlap in the genetic influences on ADHD diagnoses and ADHD traits in the wider population. This does not mean that every influence is identical for everyone.

Many small genetic effects

Most common DNA differences associated with ADHD have very small individual effects. Many can contribute together. Some rare genetic variants have larger effects.

The DNA numbers identify illustrative variants, not named genes or a complete list. People differ in which variants they carry. Having a risk-associated variant does not mean that someone will have ADHD.

The deprivation example refers to extreme lack of early care studied in institutions. It does not mean ordinary differences in parenting cause ADHD.

How the illustration was drawn

The three people and their combinations of influences are fictional. The icons, stack heights, and dashed lines are illustrative, not measured effects or a way to calculate a diagnosis. The drawing highlights influences associated with greater difficulties; other influences can reduce difficulties, and effects depend on the person and their circumstances. The diagnostic threshold line is not a universal boundary: clinical assessment considers symptoms, their history, and impairment across settings.

Evidence behind the examples