Previous studies have demonstrated significant associations of attention-deficit hyperactivity disorder (ADHD) with monoamine transporter genes, including dopamine transporter gene (DAT1), norepinephrine transporter gene (SLC6A2), and serotonin transporter gene (SLC6A4) as well as the important role of environmental factors in the pathogenesis of ADHD. Hence, investigating how genes and environments interact with each other may contribute to the understanding of the pathophysiological mechanisms of ADHD. In this 3-year project, investigators will explore the complex gene-environment interplay in ADHD with multimodal neurobiological approaches, including neuropsychology, neuroimaging, and metabolites, in order to identify the crucial pathophysiological pathways from genes to the brain.

In the present project, investigators aim to explore the complex gene-environment interplay in children with ADHD to identify the crucial pathophysiological pathways from genes to the brain. To achieve our objective, investigators will use multimodal neurobiological approaches to effectively resolve the four major challenges in exploring gene-environment interactions on ADHD.

1. investigators will use multiple levels of neurobiological approaches to identify the interactions between monoamine transporter genes and environment on ADHD, including neuropsychology, neuroimaging, and neuroactive metabolites in plasma, which will provide larger effects than clinical diagnosis.
2. investigators will employ correlation analyses to test for the presence of correlations between the monoamine transporter genotypes and the environmental factors.
3. investigators will use both categorical (diagnosis of ADHD) and dimensional (inattention and hyperactivity-impulsivity symptoms of ADHD) approaches to assess the interaction effects between monoamine transporter genes and environment.
4. investigators will conduct interactions with ageitems to model more accurately the effects of age, which may be non-linearly related to the neurobiological basis of ADHD.

Inclusion Criteria:

• Children or adolescents, between 7 and 16 years of age, must have clinical diagnosis of ADHD according to the DSM-5 diagnostic criteria.
• They have to be medication-naïve. They never receive any medication for the treatment of ADHD.
• They and their parents must understand sufficiently to communicate properly with the investigators.
• They must have a Full-Scale Intelligence Quotient(FIQ) score greater than 80.

Exclusion Criteria:

• They have a major psychiatric comorbid disorder, including schizophrenia, schizoaffective disorder, affective disorders, or autism spectrum disorder.
• They have a past history of seizure, or they are taking antiepileptic drugs.
• They have a past history of substance dependence or abuse, including nicotine, alcohol, amphetamine, or any over-the-counter medication.
• They have a past history of major systemic disease.
• They are using Chinese herbs or health supplements.

120 drug-naïve patients with ADHD, aged 7-16, and 120 age- and sex-matched typically developing controls will be recruited in this project.