However, this article argues that ADHD is harder to identify and diagnose in girls compared to boys because the symptoms manifest differently. Sadly, girls are underdiagnosed while boys are overdiagnosed.
Girls have more internalizing symptoms. For example, they tend to suffer from greater bouts of major depression, anxiety and eating disorders which can turn into low self-esteem. Boys on the other hand, have more externalizing symptoms that manifest as physical aggression, hitting and impulsivity. This brief article does a great job of delineating these differences.