Answer: Complexity
Explanation: Attention deficit hyperactivity disorder(ADHD) is a type of mental disorder .As this activity is related with the brain , the person having this disorder can behave in showing excessive activity, , not paying attention etc.
These are brain related acts which are not bound with the particular reason and symptoms as every person have different thoughts,response etc toward a certain situation . It could impact any person of any age due to several reason and can also affect others with such behavior .Thus ADHD is a case of complexity which is difficult to be analyzed and studied by human being.