According to the DSM-5, a mental disorder is defined as "a syndrome characterized by clinically significant disturbance in an individual's cognition, emotion regulation, or behavior that reflects a dysfunction in the psychological, biological, or developmental processes underlying mental functioning."

Those who identify as otaku exhibit some worrying symptoms, making it seem as if being an otaku is itself a mental disorder. For instance:

Otaku exhibit signs of hypersexuality: Otaku are known to exhibit bizarre and/or racy sexual desires. The "hentai" anime they enjoy often depicts extremely grotesque sexual imagery that mentally "normal" individuals would not find appealing, and perhaps even find disgusting. Otaku also engage far more frequently than the control population in "sexual roleplay" over the internet, and often have larger pornography collections than the general population. Additionally, as otaku media site GoBoiano admits, anime fans develop strange fetishes relating to anime that the general population does not exhibit (http://media.goboiano.com/list/2751-23-anime-fetishes-that-are-running-wild-in-the-community), including fetishes that border on pedophilia and bestiality.

Otaku are socially inept: Far more than the normal population, otaku exhibit adverse social symptoms. They are often extremely avoidant of meaningful interaction with others, and by their own admission have difficulty forming relationships with people, especially romantic relationships. They act very strangely when they are in a social situation, doing things like "glomping" (forcing themselves on an innocent passerby in an oftentimes violent "hug"), being obnoxiously loud, and other strange behaviors. A decent part of otaku culture, especially in Japan, is being a NEET (not in education employment or training), or hikikomori. These people hide in their rooms all day, rarely ever going out to interact with the world. You do not find this in communities other than otaku, especially in such worrisome numbers. In many cases the social behavior of the otaku mirrors autism.

Otaku have poor hygiene and self-preservation habits: Many otaku infrequently engage in hygienic activity such as bathing, causing many to exhibit horrid body odor and look unappealing. Additionally, many are overweight, at a higher rate than the general population. Especially for NEET/hikikomori otaku, they often live a slovenly lifestyle, inhabiting messy homes/apartments with no real desire to have a clean dwelling or lifestyle. Many will spend a whole day and well into the night in front of a computer screen, watching anime or engaging in fora where they can discuss with other otaku. Obviously none of these habits are healthy, and are in fact self-destructive, which can result in disability-adjusted life years.

Many more things are rampant in the otaku community than have been expressed above. However, this small glimpse shows just what kinds of symptoms being an otaku can result in, making it necessary that the otaku lifestyle at least warrant psychological study so that it may be better understood how and why these people are the way they are. And hopefully, with a better understanding of the otaku, we can better know how to treat them so that they have a higher standard of life and can blend into the population better.