PRINCETON – Last month, the German Ethics Council, a statutory body that reports to the Bundestag, recommended that sexual intercourse between adult siblings should cease to be a crime. The recommendation follows a 2012 decision by the European Court of Human Rights upholding the conviction of a Leipzig man for having a sexual relationship with his sister. The man has served several years in prison, owing to his refusal to abandon the relationship. (His sister was judged to be less responsible and was not jailed.)

Incest between adults is not a crime in all jurisdictions. In France, the offense was abolished when Napoleon introduced his new penal code in 1810. Consensual adult incest is also not a crime in Belgium, the Netherlands, Portugal, Spain, Russia, China, Japan, South Korea, Turkey, Côte d’Ivoire, Brazil, Argentina, and several other Latin American countries.

The Ethics Council took its investigation seriously. Its report (currently available only in German) begins with testimony from those in a forbidden relationship, particularly half-brothers and sisters who came to know each other only as adults. These couples describe the difficulties created by the criminalization of their relationship, including extortion demands and the threat of loss of custody of a child from a previous relationship.

The report does not attempt to provide a definitive assessment of the ethics of consensual sexual relationships between siblings. Instead, it asks whether there is an adequate basis for the criminal law to prohibit such relationships. It points out that in no other situation are voluntary sexual relationships between people capable of self-determination prohibited. There is, the report argues, a need for a clear and convincing justification for intruding into this core area of private life.

The report examines the grounds on which it might be claimed that this burden of justification has been met. The risk of genetically abnormal children is one such reason; but, even if it were sufficient, it would justify only a prohibition that was both narrower and wider than the current prohibition on incest.

The prohibition would be narrower, because it would apply only when children are possible: The Leipzig man whose case brought the issue to attention had a vasectomy in 2004, but that did not affect his criminal liability. And the goal of avoiding genetic abnormalities would justify widening the prohibition to sexual relationships between all couples who are at high risk of having abnormal offspring. Given Germany’s Nazi past, it is difficult for Germans today to treat the desirability of that goal as anything but permitting the state to determine who may reproduce.

The Council also considered the need to protect family relationships. The report notes that incest between siblings is rare, not because it is a crime, but because being brought up together in a family or family-like environment (including Israeli kibbutzim that rear unrelated children collectively) tends to negate sexual attraction.

The report does recognize the legitimacy of the objective of protecting the family, however, and makes use of it to limit the scope of its recommendation to sexual relations between adult siblings. Sexual relations between other close relatives, such as parents and their adult children are, the report argues, in a different category because of the different power relations between generations, and the greater potential for damage to other family relationships.

The taboo against incest runs deep, as the social psychologist Jonathan Haidt demonstrated when he told experimental subjects about Julie and Mark, adult siblings who take a holiday together and decide to have sex, just to see what it would be like. In the story, Julie is already on the Pill, but Mark uses a condom, just to be safe. They both enjoy the experience, but decide not to do it again. It remains a secret that brings them even closer.

Haidt then asked his subjects whether it was okay for Julie and Mark to have sex. Most said that it was not, but when Haidt asked them why, they offered reasons that were already excluded by the story – for example, the dangers of inbreeding, or the risk that their relationship would suffer.

Perhaps not coincidentally, when a spokesperson for German Chancellor Angela Merkel’s Christian Democrats was asked to comment on the Ethics Council’s recommendation, she also said something completely beside the point, referring to the need to protect children. The report, however, made no recommendations about incest involving children, and some of those caught by the criminal law did not even know each other as children.

When Haidt pointed out to his subjects that the reasons they had offered did not apply to the case, they often responded: "I can’t explain it, I just know it’s wrong.” Haidt refers to this as "moral dumbfounding.”

In the case of the incest taboo, our response has an obvious evolutionary explanation. But should we allow our judgment of what is a crime to be determined by feelings of repugnance that may have strengthened the evolutionary fitness of ancestors who lacked effective contraception?

Peter Singer is a Professor of Bioethics at Princeton University and Laureate Professor at the University of Melbourne.

Copyright: Project Syndicate.