The criminological questions posed in the expert opinion commissioned by Germany’s Federal Constitutional Court addressed the prevalence of (consensual) sibling incest, the consequences of incest in terms of its effects on individuals and families, the so-called "natural aversion to incest," genetic risks, and, finally, trends in police and prosecution statistics on incest. Answers to these questions were based on a secondary analysis of research in the fields of sexuality, criminology, sociology, psychology, and medicine, on an analysis of police and law enforcement statistics, and on the evaluation of the data gained from the Freiburg Cohort Study.
Incidence, Prevalence, and the Number of Unreported Cases of Sibling Incest
Empirical knowledge regarding the frequency of incest is important when seeking to answer the question of whether criminal prohibition contributes substantially to the protection of legal interests. To this extent, legal systems with and without a criminal prohibition of incest could provide a quasi-experimental base.
However, the secondary analysis of empirical research on the incidence (frequency of incestuous acts, duration and intensity of incestuous relationships) and prevalence of sibling incest (percentage of a particular population that has committed, at least once, an incestuous act or that has been exposed to incestuous acts) indicates that there is insufficient data. This lack of data prevents us from drawing definite conclusions about the frequency of incestuous acts between siblings and the effects of the duration and the intensity on the relationship. Older as well as more recent research on human sexuality does not differentiate as to whether sexual acts occur within or outside the family. Initially, research on human sexuality focused on the general distribution of sexual preference and orientation; in recent decades, however, scholars in this field have primarily been concerned with questions pertaining to public health policy and the problems of treating sexual aberrations (especially pedophilia).
That said, the problem of incestuous acts within the family (and close social proximity) has been addressed by sexual abuse studies from the 1970s onwards. In this line of research, incest is largely equated with sexual abuse. Consequently, there are limitations based on age (research concentrates on sexual activity during childhood and adolescence) and there is a focus on victimization (i.e., of the child or adolescent sexual partner). Consensual sexual acts involving adult members of the family (or involving persons of similar age) are at most marginally included if not excluded at the outset. Surveys concentrate on sexual assaults of children and adolescents committed by older (adult) members of the family. Conventional criminological surveys of unreported cases (analyses of self reports and victim surveys) only marginally capture sexual acts, and, if sexual acts are addressed at all, only those sexual acts are included that are marked by violence or other means of intimidation. As a result, no assertions based on representative empirical studies can be made about the prevalence and incidence of incest in Germany or in other countries.
Some predominantly North American studies that focus for the most part on sexual abuse - studies that cannot be generalized due to the sample procedure - give some clues about sexual contacts between siblings that are not marked by violence and coercion. Based on these studies, the prevalence of sibling incest (consensual sexual intercourse) can be estimated as 2 to 5 %. Most of these cases consist of one or occasional incidents of sexual contact between siblings.
The current state of research, therefore, does not allow for a clear differentiation (and thus an assessment of the relevance of the threat of punishment for the frequency of incest) between countries where incest is not a crime (e.g., France, Spain, South American countries) and countries where incest is placed under threat of prosecution.
Effects of Incest on Personal Development
The threat posed to psychological developmental plays a role in the assessment of both the degree of wrongfulness associated with sibling incest as well as of the need for criminal protection against it: a topic of lasting controversial debate. However, only a few studies – comparable to the deficits in research on the prevalence and incidence of sibling incest – deal with the psychological consequences of such relationships based on quantitative data. These studies distinguish between sexual acts before puberty and those that take place after the onset of puberty. Siblings can engage in sexual interaction consensually before puberty, and this interaction can occur in the context of normal, playful sexual discovery. Because of this presumption, sibling incest – in contrast to father-daughter incest – is often not regarded as a criminal offense or as aberrant sexual behavior but as normal, experimental behavior among children. There is evidence, however, that an incestuous experience begun between siblings prior to puberty, even if it is initially consensual, can develop into a relationship in which one sibling dominates and abuses the other. This kind of relationship is thought to have a lasting negative effect on the identity and self-esteem of those involved. If psychological trauma and long-term risks to psychosexual development result from sibling incest, the extent of the damage suffered by the persons involved depends on the degree of coercion employed and the duration of the incestuous relationship.
It is unclear whether psychological trauma can result from a relationship between two persons who fall in love and voluntarily engage in sexual activity. The findings concerning the consequences of sibling incest do not currently allow us to verify specific hypotheses about psychological development. It could, however, be of significance to know how siblings in incestuous relationships handle the developmental tasks of adolescence. Among these developmental tasks are the handling of sexuality and the reorganization of social relationships. Of central import are the resources necessary to cope in an age-appropriate way with these developmental tasks; it would be useful to know whether these resources are available or whether incestuous relationships are entered into precisely because they are not. The fact that incest families tend not to offer a protective atmosphere speaks for a deficit of resources.
Furthermore, a correlation has been reported between educational methods in which the child’s emotions are neglected and sibling incest. This correlation suggests that siblings who enter into sexual relationships - at least those of longer duration - are trying to compensate for deficits. As these studies have been conducted on the basis of selective clinical groups, however, the incidence of consequential psychological damage of sibling incest cannot be determined. The question of whether incest between adult siblings leads to problems with respect to developmental psychology could only be exhaustively answered if it were possible to compare incest couples and "non-incest" couples. Comparative (longitudinal) analysis of this sort does not exist at the moment.
Incest and the Consequences for the Family
Whether sibling incest should be considered harmful for the psychological development of other family members is a question that encompasses a social dimension. Third parties find themselves confronted with a phenomenon that represents a social taboo. Thus, indirect damage resulting from incest in the family, such as marginalization and/or social isolation, is possible. Frequently, however, families in which incest occurs are burdened by multiple social and psychological problems even before the occurrence of incest. The results of clinical studies of adolescent perpetrators of incest show that violence and dysfunction within the family are significant etiological variables for the occurrence of incest. Accordingly, sibling incest is regarded as a consequence, not a cause, of a dysfunctional family structure. This assumption is also supported by the few empirical studies on the criminology of incest conducted largely in the first half of the 20th century. These studies point out that cases of incest registered by the police come solely from socially and economically marginalized groups and that families in which incest takes place were markedly conspicuous with regard to social and personal problems (illness, crowded housing conditions, unemployment, alcohol problems, reduced intelligence) even before the incestuous activity began.
"Natural" Aversion to Incest?
The incest case heard by Germany’s Constitutional Court is characterized by the fact that the siblings did not grow up together but met only as young adults. Thus, an issue of particular relevance to the determination of blameworthiness is whether it was possible for the siblings to develop an aversion to incest. The question of whether humans have an innate aversion to incest or whether the incest taboo is merely a norm perpetuated by society is highly controversial. The starting point of this discussion is the "Westermarck" hypothesis that humans who grow up together later develop an instinctive sexual aversion to one other. The crucial factor for this aversion is not genetic kinship. Rather, it is growing up in the immediate proximity of siblings (or other persons) that prevents incest. Aversion to incest, therefore, works because siblings who had close physical contact during early childhood see each other not as potential sexual partners but as siblings.
Observations that incestuous relationships do not constitute the norm but rather are isolated cases among humans and animals led to the hypothesis that aversion to incest is a consequence of genetics. The biological explanation for this conclusion would be the prevention of inbreeding and the reduction of the risk of genetic problems. Evolutionary scientists assume that the human neural system developed a special kin-recognition system that allows humans (1) to choose a sexual partner according to the theory of selection and fitness and (2) to inhibit sexual contact with genetically related humans as the offspring of such a union could exhibit a larger number of harmful genetic predispositions that might curb the genetic variability necessary for adaptation. More recent research supports the effectiveness of such a kin-recognition mechanism, however, only in those cases in which siblings grow up together.
Genetic Risks
Because of the importance of a thorough understanding of genetic risks for the justification of the criminal prohibition of incest, the secondary analysis was extended to research on the occurrence of genetic defects and the diseases related to them. Offspring of incestuous relationships exhibit a greater stock of homozygous genes that are characterized – in contrast to heterozygous genes – by a lower variability. A higher degree of heterozygosity means a greater capacity to react to variable environmental conditions and, thus, increased vitality. Children of incestuous relationships have a lower degree of heterozygosity. Even if they do not develop a genetic disease, their lower degree of heterozygosity causes a decrease in their general fitness, called an inbreeding depression. Genetic risks are studied mainly in those regions of the world where marriage between close relatives is in line with social custom.
Research has shown that offspring of incestuous relationships have a higher risk of developing genetic diseases than do offspring of non-incestuous relationships. The closer the kinship relation between sexual partners, the higher the probability of a genetic anomaly for the offspring. In general, genetic diseases and hereditary abnormalities affect about 2 to 5 % of live births and account for some 50 % of deaths of children in Western countries. A survey of 38 populations in Asia, Africa, Europe, and South America estimates the risk of a transfer of genetic abnormalities to children of incestuous relationships as being 1.7 to 2.8 % higher compared to relationships between unrelated partners. The risk for diseases that occur later, during the first year of life, is 7 to 31 % higher compared to the rest of the population. In Jerusalem, for instance, the occurrence of genetically-caused malformation was observed in Arab babies of incestuous relationships. The result of the study was that 8.7 % of the babies of incestuous relationships had malformations (for instance, cardiac defects or Down’s syndrome) compared to 2.6 % in a control group. Based on studies, it can be concluded that there is a higher risk of genetic damage although the risk is within a range not far above the average risk in the normal population.
Sibling Incest and Prosecution
The assessment of the criminal prohibition of incest also depends on how and to what extent the prohibition is actually implemented. The prosecution statistics, however, do not distinguish between the various forms of incest regulated in § 173 StGB. The total number of convictions for violations of § 173 StGB clearly decreased in the second half of the 20th century. This development, on the one hand, accounts for the influence of the reforms of § 173 at the beginning of the 1970s. It also shows that, on average over many years, the prosecution of incest leads to fewer than 10 convictions per year. Of these convictions, only a few involve sibling incest, as the data of the Freiburg Cohort Study shows. Four birth cohorts (Baden-Württemberg, 1970, 1973, 1975, 1978) were evaluated based on police and judicial registrations involving § 173 para. 2 sen. 2 StGB. For these birth cohorts (N: 3.600.000), there are currently 11 judicial dispositions of cases involving § 173 para. 2 sen. 2 StGB. The dispositions are distributed over 20, 17, 15, and 12 years of risk periods of the respective birth cohorts (age 18 and above). From this follows that, under current conditions from a birth cohort in Germany up to the age of 35, a maximum of three to four persons are registered for sibling incest. Thus, § 173 para. 2 sen. 2 StGB does not play a major role for police and judicial practice nor for the structure of abnormalities in birth cohorts. According to the data in the prosecution statistics, most of those individuals convicted are adult males. More in-depth empirical studies of incest perpetrators registered by the police or the judicial system have not been conducted in recent times. The surveys of cases of incest from the first half of the 20th century show that sibling incest is rarer in police and judicial practice than father-daughter incest. The isolated cases of sibling incest that were identified in the surveys are indeed cases of "older brother-younger sister incest."
Some individual analyses were conducted on the prosecution of incest cases in accordance with § 173 II StGB during the first half of the 20th century. The research interest in focus was the description of the perpetrators and the victims, especially their social backgrounds. The empirical studies of the cases of incest heard before court during the relevant period of time indicated in particular that the perpetrators were members of socially and economically marginalized groups. Given the lack of data on the prevalence of incest, however, it remains unclear whether this result is contorted as a consequence of a selective prosecution at the expense of the lower social strata.
The available empirical studies do not contain any information about the reasons why these cases became publicly known. On the basis of the data from the Freiburg Cohort Study, we can conclude that cases of § 173 para. 2 sen. 2 StGB come to the attention of prosecution authorities almost exclusively because a victim decides to file a complaint alleging violent assault. In nine of 13 cases in which § 173 para. 2 sen. 2 StGB was indicated, charges of rape or sexual assault (§§ 177, 176 StGB) were also instituted. Most of the incest cases included in the Freiburg Cohort Study were discontinued (ten cases, including four discontinuations on the basis of §§ 153, 170 StPO; three convictions; and three with unknown outcomes). According to the prosecution statistics, the result of judicial dispositions in incest cases is, overwhelmingly, a criminal fine.
Conclusion
In summary, the criminal prohibition of consensual incest between adults is relevant in only very few cases. While it is true that there are no representative empirical studies on the incidence and prevalence of sibling incest, some small-scale regional studies have shown that only 2 to 5 % of the surveyed population group had engaged in incestuous activity with a sibling. These experiences are largely limited to nonrecurring encounters or relationships of a short duration. Most of the “incest” cases that come to the attention of authorities involve other (aggravating) factors – factors that are the subject of special offense definitions in all legal systems.
If the incest law were abolished, no serious gaps in protection would result. In Germany, around 10 incest-related convictions are registered annually. The data from the Freiburg Cohort Study shows that by the time they reach the age of 35, only three to four persons born in a particular year (almost one million people are born each year) are registered by the judicial authorities for incest offenses. In addition, it has been shown that even in legal systems without a criminal prohibition of incest, the social incest taboo prevails. The "natural" aversion to incest is often explained by genetics and by a neuronal kin-recognition system. A natural avoidance of incest is so effective (excluding those regions of the world where certain forms of marriage among relatives are socially expected) that sexual relationships and marriage take place with partners from outside the family. Furthermore, while genetic risks are higher for offspring of incestuous relationships, this is only marginally so, and these risks can be addressed by means of genetic counseling. Therefore, the decriminalization of incest is not inconceivable. Decriminalization is worth considering, not only because cases such as the one at issue are very rare, but also because they tend to exhibit anomalies indicative of the fact that a "natural" aversion to incest did not develop because the partners grew up separately.