Pedophilia (US spelling; UK paedophilia) is a mental disorder made by clinical diagnosis. It is characterized by frequent and intense sexual urges directed towards prepubescent children. The term pedophile or paedophile refers to any adult or adolescent individual who has such urges. Despite the colloquial usage of the term to refer to a person who sexually abuses minors, one does not need to actually sexually abuse a child in order to be considered a pedophile by a mental health professional. The desire to do so, along with the presence of a series of intrusive urges to do so, alone, are enough, even if these urges and desires are always resisted. Furthermore, a significantly older adult who is attracted to post - pubescent individuals (i.e. 13-17 year olds) would not technically be considered a pedophile, under the health science usage of the term. Instead, that attraction, also considered abnormal, would fall under the category of "ephebophilia", which refers to the sexual attraction towards adolescents.
With rare exception, sexual contact with prepubescent children is socially discouraged, socially unacceptable, or a criminal offense in all modern societies. It is reasonable to suppose, therefore, that at least some people who have compulsive erotic thoughts towards young children deny them even on a personal basis (to themselves) , and that at least some of those who have such thoughts and are able to acknowledge these thoughts to themselves will deny having pedophilic fantasies if asked by others. Accordingly, accurate statistics on the prevalence of pedophilia in society are hard to gather. Most of the published data on pedophilia rely solely on those who have acted on those fantasies and subsequently become involved in the legal system. Thus, the major studies of pedophiles are likely biased in reporting the personality traits and shared characteristics of offenders that have been caught, rather than the entire set of individuals legitimately classed as pedophiles on the basis of their attractions. 
According to the American Psychiatric Association, pedophilia is defined based on the following criteria:
- Over a period of at least 6 months, recurrent, intense sexually arousing fantasies, sexual urges, or behaviors involving sexual activity with a prepubescent child or children (generally age 13 years or younger);
- The person has acted on these sexual urges, or the sexual urges or fantasies cause marked distress or interpersonal difficulty;
- The person is at least age 16 years and at least 5 years older than the child or children in Criterion A. 
Furthermore, in cases where the individual is very young, or is still an adolescent, emotional and sexual maturity may be considered before a diagnosis is made. 
Types of pedophiles
Exclusive vs. nonexclusive
Based on the pedophile's specific sexual attraction, he or she can usually be categorized as exclusive or nonexclusive. Exclusive pedophiles are attracted to children, and children only. They show little erotic interest in adults their own age and in some cases, can only become aroused while fantasizing or being in the presence of prepubescent children. On the other hand, nonexclusive pedophiles are attracted to both children and adults, and can be sexually aroused by both. According to a U.S. study on 2429 adult male pedophiles, only 7% identified themselves as exclusive; indicating that many or most pedophiles fall into the nonexclusive category. 
Homosexual vs. heterosexual
Most pedophiles do not only prefer children of a specific age, but also of a specific sex. Most pedophiles identify themselves as homosexual, heterosexual, or bisexual. However as not to create confusion, the use of these terms by pedophiles does not imply their sexual orientation towards adults, but exclusively refers to the gender of the preferred child. According to U.S. studies, pedophiles attracted to children of the same sex can range from 9% to 40%. One possible reason for this phenomenon lies in opportunity. For example, if a pedophile finds it easier to relate to males and/or create opportunities to be alone with them, then he or she may exclusively seek out male children even if the perpetrator is also aroused by female children.
Occurrence in child sex offenders
- Hall, Ryan C. W. MD, et al. "A Profile of Pedophilia: Definition, Characteristics of Offenders, Recidivism, Treatment Outcomes, and Forensic Issues." Volume 82(4), April 2007, pp 457-471. Mayo Foundation for Medical Education and Research. 2007.
- "American Psychiatric Association Statement Diagnosis Criteria for Pedophilia." American Psychiatric Association. 17 Jun 2003.