It is said (by some students of recidivism rates for sex offenses) that there is a "low base rate problem." What is this problem? The problem -- it is said -- is that if reported crimes are the measure of rates of recidivism, "sex offenders hav[e] a low base rate (defined as the overall rate of recidivism of an entire group of offenders)"; i.e., the problem is that the recidivism rate for sex offenders appears to be low in both absolute and relative terms.

But the semingly low recidivism rate may be deceptive. There are various reasons why this possibility exists.

First, for example, recidivism has a wide variety of possible definitions. See, for example, the discussion at http://www.csom.org/pubs/recidsexof.html:

Defining Recidivism

Although there is common acceptance that recidivism is the commission of a subsequent offense, there are many operational definitions for this term. For example, recidivism may occur when there is a new arrest, new conviction, or new commitment to custody. Each of these criteria is a valid measure of recidivism, but each measures something different. While the differences may appear minor, they will lead to widely varied outcomes.

• Subsequent Arrest—Using new charges or arrests as the determining criteria for "recidivism" will result in a higher recidivism rate, because many individuals are arrested but for a variety of reasons, are not convicted.
• Subsequent Conviction—Measuring new convictions is a more restrictive criterion than new arrests, resulting in a lower recidivism rate. Generally, more confidence is placed in reconviction, since this involves a process through which the individual has been found guilty. However, given the process involved in reporting, prosecution, and conviction in sex offense cases, a number of researchers favor the use of more inclusive criteria (e.g., arrests or charges).
• Subsequent Incarceration—Some studies utilize return to prison as the criterion for determining recidivism. There are two ways in which individuals may be returned to a correctional institution. One is through the commission of a new offense and return to prison on a new sentence and the other is through a technical violation of parole. The former is by far the more restrictive criterion, since an offender has to have been found guilty and sentenced to prison. Technical violations typically involve violations of conditions of release, such as being alone with minor children or consuming alcohol. Thus, the use of this definition will result in the inclusion of individuals who may not have committed a subsequent criminal offense as recidivists. When one encounters the use of return to prison as the criterion for recidivism, it is imperative to determine if this includes those with new convictions, technical violations, or both.

Furthermore, for example, some or many sex crimes may not be reported. See again, e.g., the discussion at the same source:

Underestimating Recidivism

Reliance on measures of recidivism as reflected through official criminal justice system data obviously omit offenses that are not cleared through an arrest or those that are never reported to the police. This distinction is critical in the measurement of recidivism of sex offenders. For a variety of reasons, sexual assault is a vastly underreported crime. The National Crime Victimization Surveys (Bureau of Justice Statistics) conducted in 1994, 1995, and 1998 indicate that only 32 percent (one out of three) of sexual assaults against persons 12 or older are reported to law enforcement. A three-year longitudinal study (Kilpatrick, Edmunds, and Seymour, 1992) of 4,008 adult women found that 84 percent of respondents who identified themselves as rape victims did not report the crime to authorities. (No current studies indicate the rate of reporting for child sexual assault, although it is generally assumed that these assaults are equally underreported.)

Moreover, for example, "sex crimes" and "sex offenses" are broad categories, and recidivism rates may vary widely for sub-classes of such offenses. Thus, even if the overall recidivism rate for sex crimes is low, it is possible that the recidivism rate for some species of sex crimes -- such as child molestation -- is very high. Id.

Nevertheless, consider some raw numbers. For example, the source cited above states: 

The following studies have found low base rates for sex offender populations:

• Hanson and Bussiere (1998) reported an overall recidivism rate of 13 percent.
• Grumfeld and Noreik (1986) found a 10 percent recidivism rate for rapists.
• Gibbens, Soothill, and Way (1978) reported a 4 percent recidivism rate for incest offenders.

Is it possible that recidivism rates for sex offenses have been grossly exaggerated? Some internet sources say or suggest that this is the case.

See, e.g., Sentencing Law and Policy (blog), (December 15, 2004) (reporting that one study found that "[s]ex offenders re-offend at lower rates than those convicted of other felonies. After five years, 15% of sex offenders return to prison for new offenses compared to 43% of offenders convicted of property crimes.").

See also, e.g., Eric Lotke, Politics and Irrelevance: Community Notification Statutes, electronic reprint from 10 Federal Sentencing Reporter No. 2 (September/October 1997)(footnotes omitted):

A. The myth of high reoffense rates.

The belief that sex offenders repeatedly reoffend fuels the rush toward community notification. "Statistics show that 95% of the time, anyone who molests a child will likely do it again," declared the Indiana senator proposing sex offender registration in that state. A Florida senator referred to "sexual predators who start to look for their next victim as soon as they are released from prison," and a California legislator warned the public that sex offenders "will immediately commit this crime again at least 90 percent of the time."

Scholarly research does not support these claims. Elizabeth Alexander's comprehensive meta-analysis of 7,753 sex offenders found reoffense rates of 10.9% among treated sex offenders and 18.5% among untreated offenders. Lita Furby, whose work prompted the popular belief that "nothing works" to reduce sex offender recidivism, found overall reoffense rates of 12.7% among the 15,361 people she researched. Most recently, a national report commissioned by the Office of Canada's Solicitor General found overall reoffense rates of 13.4%. Thus, it appears that only a minority of identified sex offenders reoffend. Over 80% never offend again, almost the opposite of what many politicians claim, and substantially lower than for many other types of crime.

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