The prevalence, age of onset, and clinical symptoms for virtually all neuropsychiatric disorders differ between men and women. Among the disorders with pronounced sex bias are Attention-Deficit/Hyperactivity Disorder (ADHD) and Autism Spectrum Disorder (ASD), where the ratio of males/females diagnosed is approximately 4 to 1. Whether this skewed ratio arises from roles played in brain development by sex-specific DNA sequences or hormones or reflects the way that biological mechanisms and environmental influences elicit behavioral patterns differently in males and females, remains an area of open investigation.