The National Institute of Mental Health notes that scientists aren’t entirely sure what causes attention deficit hyperactivity disorder. Anything from genes and brain injuries to low birth weight and exposure to environmental toxins at a young age may play a role. However, of these variables, experts tend to agree that genetics is the most likely link. But this isn’t to say the other factors go entirely by the wayside. For example, one study, published in Human Genetics, mentions that although ADHD is “highly heritable,” it’s a “multifactorial disorder, in which many genes, all with a small effect, are thought to cause the disorder in the presence of unfavorable environmental conditions.”