Why are there many similarities between adhd and autism?

Both brain diseases: The similarities r minimal. Ad kids can be impulsive & hyperactive, but this is from their inability to connect with a language based world. They self stim to alleviate anxiety or panic. They live in a material tactile world. Adhd kids are quite verbal but don't modulate correctly. They usually don't self stim, they connect emotionally well. Both are compromised socially but in different ways.

There really aren't: Not sure what similarities you are describing. Clinically, they are quite distinct, with very different long-term courses and key symptom presentations. There are some superficial similarities, e.g., people with either may have few if any friends; but, the causes for that are very different. Also, adhd responds nicely to a variety of treatments; autism is much less responsive generally.

To the trained eye, : they are not similar; however, 40-70% of people with Autistic Spectrum Disorder also have ADHD. Some kids with ADHD-Primarily Combined Type, have hyperactivity/impulsivity that impacts their peer interaction & prosocial problem-solving skills. A child with ADHD-Primarily Inattentive Type can seem " spacey.", However, the symptoms of ADHD do not include any atypical behaviors