A sex offender (sexual offender, sex abuser, or sexual abuser) is a person who has committed a sex crime. What constitutes a sex crime differs by culture and legal jurisdiction. The majority of convicted sex offenders have convictions for crimes of a sexual nature; however, some sex offenders have simply violated a law contained in a sexual category. Some of the crimes which usually result in a mandatory sex-offender classification are: a second prostitution conviction, sending or receiving obscene content in the form of SMS text messages (sexting), and relationship between young adults and teenagers resulting in corruption of a minor (if the age between them is greater than 1,060 days). If any sexual contact was made by the adult to the minor, then child molestation has occurred. Other serious offenses are sexual assault, statutory rape, bestiality, child sexual abuse, female genital mutilation, incest, rape, and sexual imposition.

Sex offender registration laws in the United States may also classify less serious offenses as sexual offenses requiring sex offender registration. In some states public urination, having sex on a beach,^[1] or unlawful imprisonment of a minor also constitute sexual offenses requiring registration.^[2][3]

In looking at various types of offenses, an example of a digital obscenity offense is child pornography. In the modern world of technology, many jurisdictions are reforming their laws to prevent the over-prosecution of sex offenders and focusing on crimes involving a victim. The term sexual predator is often used to describe a sex offender or any of the "tier offenders"; however, only the category just below sexually-violent sexual predator is reserved for a severe or repeated sex offender: sexual predator.

In the United States, the Adam Walsh Act (AWA) proposed to provide funding to each jurisdiction which would agree to incorporate its Act into their law. In the few jurisdictions accepting the agreement, there are Tier I, Tier II, or Tier III sex offenders. Individuals convicted of petty crimes not covered by the AWA are still liable to abide by the previous regulations denoting them as a sex offender (or habitual sex offender, sexual predator, sexually violent sexual predator, or child-victim offender).

In the United States, the United Kingdom, and other countries, a convicted sex offender is often required to register with the respective jurisdiction's sex offender registry. In the U.S., registry databases are often open to the public. Sexual offenders are sometimes classified by level.^[4] The highest-level offenders generally must register for their entire lives; low-level offenders may only need to register for a period of time.

The level of recidivism in sexual offenders is lower than is commonly believed.^[5][6] A 2002 study by the Office of Justice Programs (OJP) of the United States Department of Justice following 9,691 male sex offenders released from prisons in 15 US states in 1994 indicated that within the first 3 years following their release, rearrest and reconviction rates for new sex offenses were 5.3 and 3.5 percent, respectively; that is, about 1 in 19 of released sex offenders were arrested within three years for another sex crime. The same study found that during the same 3 years from release, 68 percent of released non-sex offenders were re-arrested for any crime (and 47.8 percent reconvicted), while 43 percent of the released sex offenders were rearrested for any crime (and 24 percent reconvicted).^[7]

According to the Office of Justice Programs (OJP) of the United States Department of Justice,^[8] in New York state, the recidivism rate for sex offenders has been shown to be lower than any other crime except murder. Another report from the OJP which studied the recidivism of prisoners released in 1994 in 15 states (accounting for two-thirds of all prisoners released in the United States that year) reached the same conclusion.^[9]

Of released sex offenders who allegedly committed another sex crime, 40 percent perpetrated the new offense within a year or less from their prison discharge. Within three years of release, 2.5 percent of released rapists were rearrested for another rape, and 1.2 percent of those who had served time for homicide were arrested for a new homicide. Sex offenders were about four times more likely than non-sex offenders to be arrested for another sex crime after their discharge from prison (5.3 percent of sex offenders, versus 1.3 percent of non-sex offenders). In 1991, an estimated 24 percent of those serving time for rape and 19 percent of those serving time for sexual assault had been on probation (or parole) at the time of the offense for which they were in state prison.^[7]

Approximately 4,300 child molesters were released from prisons in 15 U.S. states in 1994. An estimated 3.3 percent of these 4,300 were rearrested for another sex crime against a child within 3 years of release from prison. Among child molesters released from prison in 1994, 60 percent had been in prison for molesting a child 13 years old or younger. The median age of victims of those imprisoned for sexual assault was less than 13 years old; the median age of rape victims was about 22 years. Child molesters were, on average, five years older than violent offenders who committed their crimes against adults. Nearly 25 percent of child molesters were age 40 or older, but about 10 percent of inmates with adult victims were in that age group.^[7]

A sex offender registry is a system in place in a number of jurisdictions designed to allow authorities to keep track of the residence and activity of sex offenders (including those released from prison). In some jurisdictions (especially in the United States), information in the registry is made available to the public via a website or other means. In many jurisdictions, registered sex offenders are subject to additional restrictions (including housing). Those on parole (or probation) may be subject to restrictions not applicable to other parolees or probationers.^[10] These include restrictions on being in the presence of minors, living in proximity to a school or daycare center, or owning toys (or other items of interest to minors). Israel’s sex offender registry is accessible only to security officials, rather than to the general public.^[11]

Megan's Law, in the U.S., is designed to sanction sex offenders and reduce their recidivism rate. The law is enacted and enforced on a state-by-state basis. Most states also restrict where convicted sex offenders can live after their release, prohibiting residency within a designated distance of schools and daycare centers (usually 1,000–2,000 feet (300–610 m)). Guided by the 2007 Adam Walsh Child Protection and Safety Act, sex offenders must avoid of such areas as schools, bus stops, gyms, recreation centers, playgrounds, parks, swimming pools, libraries, nursing homes, and places of worship by 500 to 2,500 feet (150 to 760 m). However, residence stipulations vary from state to state. Some states (such as Arkansas, Illinois, Washington and Idaho) do not require sex offenders to move from their residences if a forbidden facility is built or a law is enacted after the offender takes up residency. Many aspects of the laws are criticised by reformists and civil right groups like National RSOL^[12] and Human Right Watch,^[13][14] and treatment professionals as Atsa.^[15][16]

Committing to a residence requires a convicted sex offender to be notified of registration regulations by local law enforcement if convicted after January 1, 2005. The offender must act upon the notification within five business days of receipt. If and when an offender is released from incarceration, they must confirm their registration status within five business days. Registration data includes the offender's sex, height, weight, date of birth, identifying characteristics (if any), statutes violated, fingerprints and a current photograph. An offender's email addresses, chat room IDs and instant-messaging aliases must be surrendered to authorities. In Colorado, an offender must re-register when moving to a new address, changing their legal name, employment, volunteer activity, identifying information used online or enrollment status at a post-secondary educational institution. A web-based registration list may be found on county websites, which identifies adult convicted sex offenders who are sexually-violent predators convicted of felony sexual acts, crimes of violence or failure to register as required. Legally, "any person who is a sexually violent predator and any person who is convicted as an adult...has a duty to register for the remainder of his or her natural life".^[17] Exceptions to this include deferred sentencing for the offense or petition of the court for termination of registration.^[18]

Some sex offenders are deemed too dangerous to society to be released, and are subjected to civil confinement — indefinite continuing incarceration, which is supposed to, but does not always, provide meaningful treatment to the offender.

Behavior modification programs have been shown to reduce recidivism in sex offenders.^[19] Often, such programs use principles of applied behavior analysis. Two such approaches from this line of research have promise. The first uses operant conditioning approaches (which use reward and punishment to train new behavior, such as problem-solving)^[20] and the second uses respondent conditioning procedures, such as aversion therapy. Many of the behaviorism programs use covert sensitization^[21] and/or odor aversion: both are forms of aversion therapy, which have had ethical challenges. Such programs are effective in lowering recidivism by 15–18 percent.^[22] The use of aversion therapy remains controversial, and is an ethical issue related to the professional practice of behavior analysis.

In 2007, the Texas State Auditor released a report showing that sex offenders who completed the Texas Sex Offender Treatment Program (SOTP) were 61 percent less likely to commit a new crime.^[23]

Chemical castration is used in some countries and U. S. states to treat male sex offenders. Unlike physical castration, it is reversible by stopping the medication. For male sex offenders with severe or extreme paraphilias, physical castration appears to be effective. It results in a 20-year re-offense rate of less than 2.3 percent (versus 80 percent in the untreated control group), according to a large 1963 study involving a total of 1036 sex offenders by the German researcher A. Langelüddeke.^[24] This was much lower than otherwise expected, compared with overall sex offender recidivism rates. Although considered cruel and unusual punishment by many, physical castration does not otherwise affect the lifespan of men (compared with uncastrated men).

Therapists use various methods to assess individual sex offenders' recidivism risk. Risk assessment tools consider factors that have been empirically linked by research to sexual recidivism risk. Researchers and practitioners consider some factors as "static", such as age, number of prior sex offenses, victim gender, relationship to the victim, and indicators of psychopathy and deviant sexual arousal, and some other factors as "dynamic", such as an offender's compliance with supervision and treatment.^[25] By examining both types of factors, a more complete picture of the offender's risk can emerge, compared with static or dynamic factors used alone.^[26]

It is argued that in the U.S., sex offenders have been selected as the new realization of moral panics about sex, stranger danger, and national paranoia, the new folk devils or boogeymen. People convicted of any sex crime are "transformed into a concept of evil, which is then personified as a group of faceless, terrifying, and predatory devils", who are, contrary to scientific evidence, perceived as a constant threat, habitually waiting for an opportunity to attack.^[27] Consequently, sex offenders are brought up by media on Halloween, despite the fact that there has never been a recorded case of abduction or abuse by a registered sex offender on Halloween.^[27]

Academics, treatment professionals,^[15][28] and law reform groups such as National Association for Rational Sexual Offense Laws^[29] and Women Against Registry^[30] criticize current sex offender laws as based on media-driven moral panic and "public emotion", rather than a real attempt to protect society.^{[27][31][32][33][34][35]} This can motivate legislators to pass knee-jerk laws^[36] to address public hysteria, echoing a "populist punitiveness" perspective.^[37] Many lawmakers feel that they will attract votes by appearing to be "tough on sex offenders".^[38] One discrepancy pointed out by critics is that John Walsh, father of Adam Walsh and supporter of the Adam Walsh Act, has admitted having a relationship with a 16-year-old girl while being in his early 20s and aware of age of consent being 17 in New York,^[39] meaning that, had he been convicted, John Walsh himself could be required to register as a sex offender. Since passage of the Adam Walsh Act, Walsh himself has criticized the law, stating "You can't paint sex offenders with a broad brush."^[40]

Critics point out that contrary to media depictions, abductions by predatory offenders are very rare^[41] and 93% of child sexual abuse is committed by someone the child knows: a family member, a family friend, someone in a position of authority.^[42] According to the U.S. Department of Justice, sex offender recidivism is 5.3%,^[43] the lowest for any type of crime except homicide.^[44]

Critics say that, while originally aimed at the worst offenders, as a result of moral panic the laws have gone through series of amendments, many named after the victim of a highly publicized predatory offense, expanding the scope of the laws to low-level offenders, and treating them the same as predatory offenders, leading to the disproportionate punishment of being placed on a public sex offender registry, with the consequent restrictions on movement, employment, and housing.^[30][41] As a result of the media narrative of sex offenders, highlighting egregious offenses as typical behavior of sex offenders, and distorting the facts of cases,^[45] the panic has been said to increased, leading legislators to attack judicial discretion,^[45] by making registration mandatory based strictly on offense of conviction, without considering the likelihood to re-offend or the actual severity of the crime, thus punishing less serious offenders under the harsh sex offender laws.