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Oxford Uehiro Prize in Practical Ethics: When is Sex With Conjoined Twins Permissible?

This essay was the runner up in the Oxford Uehiro Prize in Practical Ethics Graduate Category

Written by University of Oxford student James Kirkpatrick

It is widely accepted that valid consent is necessary for the permissibility of sexual acts. This requirement explains why it is impermissible to have sex with non-human animals, children, and agents with severe cognitive impairments. This paper explores the implications of this requirement for the conditions under which
conjoined twins may have sex.[1] I will argue that sex with conjoined twins is impermissible if one of them does not consent. This observation generalises to prohibitions on a wide range of everyday activities, such as masturbation, blood donations, and taking drugs to cure one’s headache. While these implications are
highly counterintuitive, it is dificult to articulate the relevant moral difference between these cases.

Conjoined twinning occurs when a single embryo formed by a sperm and egg splits early in the development, but not completely. This results in two genetically
identical clones that share the same set of genes, as well as some body parts. While conjoined twins are typically classifi ed by the point of fusion, for our purposes, they may be divided into two broad categories: homogenitally conjoined twins, who shared a single set of genitalia, and heterogenitally conjoined twins, with two distinct sets of genitalia.

Permissible sexual activity is widely taken to require the valid consent of each of the participants of the sexual act.[2] Valid consent requires the satisfaction of the following three conditions.[3] First, an agent’s consent must be informed: she must have all of the information of the relevant facts about what the act involves. Second, consent must be given voluntarily: the decision to either consent or not consent must be made by the agent herself; an agent who has been coerced or threatened does not count as having given valid consent. Third, the agent must have the capacity to consent: she must grasp the value and consequences of the act, understand the information about what she is consenting to, and use that information to make an informed decision. If any of these conditions are absent, then any resulting sexual action is non-consensual.

Given that valid consent is necessary for permissible sexual activity, let us examine cases in which consent is lacking from one twin. First, consider cases involving heterogenitally conjoined twins, such as Chang and Eng Bunker.[4] These cases are complicated by the question of whether having sex with heterogenitally conjoined twins counts as having sex with both twins. Consider the following case:

Case 1. A and B are heterogenitally conjoined twins. A validly consents to have sex with C. B does not want to have sex with C and does not give her consent. A and C go ahead and have sex anyway.

Prima facie, having sex with A does not seem to entail having sex with B, given their separate genitalia. For any two non-conjoined individuals, having sex with one of them does not entail having sex with the other. Similarly, if A and B are minimally fused, as with Chang and Eng Bunker, then it seems implausible that B and C would count as having had sex. After all, Chang and Eng fathered a total of 21 children between them, and their descendants are considered to have come from just one of the twins. In such cases, B’s consent is not necessary for the permissibility of the sex act. However, if A and B are more extensively fused, say through the more substantial sharing of internal organs, then B and C may well count as having sex. In such cases, the sex act would be impermissible, since the valid consent of one participant of the sex act is withheld. Nevertheless, even if B and C do not count as having sex, Case 1 still involves the serious moral wrong of being involuntarily exposed to other agents’ sexual intercourse. (This is presumably why having sex in public and exposing unwilling others to pornography is often considered to be wrong.) Consequently, even if the sex is completely consensual, a moral wrong may still be committed.

Cases involving homogenitally conjoined twins, such as Abby and Brittany Hensel, are more straightforward, since having sex with one twin entails having sex with the other.[5] Since the valid consent of every participant of a sexual act is a necessary condition for that act’s permissibility, the consent of both twins is required for permissible sex. Consider the following case:

Case 2. A and B are homogenitally conjoined twins. A validly consents to have sex with C. B does not want to have sex with C and does not give her consent. A and C have sex anyway.

This sexual act is impermissible for the following reasons. By having sex with A, C is also having sex with B, since A and B share genitalia. After all, B’s claim to ownership over the genitalia of A and B’s shared body is just as strong as A’s. Furthermore, given that B does not consent to having sex with C, B and C have had non-consensual sex. This is a serious wrong and is clearly impermissible. Consequently, on the balance of reasons, the sexual act is impermissible.

The observation that having sex with someone who is a homogenitally conjoined twin entails having sex with their twin has some surprising implications. Consider the following case:

Case 3. A and B are homogenitally conjoined twins. B is in a permanent vegetative state (PVS) and so is severely  mentally incapacitated. A validly consents to sex with C. A and C have sex.

This sexual act is impermissible for the following reason. Sex with A is permissible only if both A and B consent to the sex act. After all, by parity of reasoning with Case 2, in virtue of having sex with A, C also has sex with B. However, it is widely held that having sex with someone in a PVS is a serious moral wrong and so is impermissible. Such a person is not in a position to validly consent to sex, since she is not in a position to know the relevant facts about the act and so she is decisionally capacitated. Given that B is in a PVS, she is not capable of giving valid consent. Consequently, the sexual act is impermissible. The upshot of this argument is that A can never have permissible sex so long as B is in a PVS.

This argument generalises to other cases in surprising ways. If A and B are conjoined twins, for any act in which A might engage, if it also requires B’s consent and B is severely mentally incapacitated or otherwise does not consent, that act is impermissible.[6] Troublingly, B’s lack of consent will rule out A’s desired course of action in a wide range of mundane everyday actions. Consider the following examples:

Masturbation. A wants to masturbate. If A masturbates, she sexually touches B. But it seems strongly impermissible to involuntarily touch someone sexually (say who is in a PVS or otherwise does not consent to being sexually touched). Consequently, A should not masturbate.

Blood Donation. A wants to donate blood. If A donates blood, she involuntarily takes blood from B. But it seems strongly impermissible to involuntarily take the blood of someone (say who is in PVS or otherwise does not consent to having their blood taken). Consequently, A should not donate blood.

Headache. A has a headache and wants to take paracetamol. (B does not have a headache.) If A takes paracetamol, then she introduces a drug into the bloodstream of B. But it seems impermissible to involuntarily introduce a drug into someone’s bloodstream (say who is in PVS or otherwise does not consent to having a drug introduced into their bloodstream). Consequently, A should not take the drug.

Risk Imposition. A wants to undertake a risky action, which imposes upon her a non-negligible risk of serious harm. If A undertakes this action, she also exposes B to that risk. But it seems impermissible to involuntarily expose someone (say who is in PVS or otherwise does not consent to having a risk imposed on them) to a non-negligible risk of serious harm. Consequently, A should not undertake the risky action.

In each case, it is impermissible for A to follow her desired course of action: it seems that being conjoined to B severely limits A’s freedom to do certain things, namely, to masturbate, to give blood, to take drugs to cure her headache, and to self-impose non-negligible risks. This generalisation poses a puzzle: these implications are highly counter-intuitive, but it is difficult to articulate the relevant moral differences between these cases.

Let us finish by illustrating the difficulty in driving a wedge between these cases by considering how one may argue that it could be permissible for A to masturbate. One might invoke the doctrine of double effect to explain the asymmetry between these cases. According to the doctrine of double effect, it is sometimes permissible to \cause a harm as a side effect. . . of bringing about a good result even though it would not be permissible to cause such a harm as a means to bring about the same good end” (McIntyre 2014). By appealing to this doctrine, one may argue, for example, that it is permissible for A to masturbate. Even though she foresees that masturbating would entail sexually touching someone who is in a PVS, this would be an unintended consequence of her action rather than a means to bring about her pleasure. This is different from merely sexually touching someone in a PVS, since this would presumably be to use that agent’s body purely as a means to an end, rather than an end in itself. The impermissible sexually touching is not a foreseen yet unintended side effect but the desired end in itself.

However, by parity of reasoning, this entails the permissibility of having sex with conjoined twins in Case 3. After all, C has consensual sex with A, while foreseeing that this entails having non-consensual sex with B. Even though C foresees that having sex with A entails having non-consensual sex with B, he does not intend to have sex with B. Having sex with B is a foreseen, yet unintended consequence of C’s action, rather than being a means to bring about sex with A. This strikes me as an extremely implausible and repugnant application of the doctrine 4 of double effect, although I see no way of distinguishing between this application and the application in the previous paragraph. Consequently, the general appeal to the doctrine of double effect fails. Unfortunately, spatial constraints prevent a thorough investigation of other potential moral differences between these cases.

To conclude, this paper has argued that the actions of conjoined twins are significantly curtailed by their condition in surprising ways. While I remain optimistic that a satisfactory moral distinction between these cases will be found, it is hard to see how to resist this argument. Since I have not found a satisfactory resolution to this puzzle, I leave this an open challenge for others.


[1] Conjoined twins with two distinguishable functioning brains present philosophical and ethical problems about personal identity, the ethics of self-ownership, and organ donations; see Campbell and McMahan 2010; Liao 2006; Olson 2014; Savulescu 2012; Savulescu and Persson 2016. This
author is not aware of any discussion on the relation between sexual consent and conjoined twinning.

[2] See Archard 2007; Cahill 2001; Cowling and Reynolds 2017; Soble 2002.

[3] See Kleinig 2010; Miller and Wertheimer 2010; Wertheimer 2010.

[4] Chang and Eng Bunker were joined by the areas around their xiphoid cartilages and, while their livers were fused, they were otherwise independently complete.

[5] Abby and Brittany are dicephalic parapagus conjoined twins who have individual organs in the upper part of their body, but who share most organs at or below the naval with exception of the spinal cord.

[6] One caveat: A can appeal to such a principle of presumed consent|one permitting us to act on behalf of someone who is otherwise incapacitated, so long as we are acting in their own best interests|to justify going into surgery without B’s actual consent. However, such a principle would not justify A’s having sex in something like Case 3.


Archard, David. 2007. “The wrong of rape.” The Philosophical Quarterly 57(228):374-393.

Cahill, Ann J. 2001. Rethinking Rape. Ithaca, NY: Cornell University Press.

Campbell, Tim and Jeff McMahan. 2010. “Animalism and the varieties of con- joined twinning.” Theoretical Medicine and Bioethics 31(4):285-301.

Cowling, Mark and Paul Reynolds, eds. 2017. Making Sense of Sexual Consent. London: Routledge.

Kleinig, John. 2010. The nature of consent. in Miller and Wertheimer (2010) pp. 3-24.

Liao, S. Matthew. 2006. “The Organism View Defended.” The Monist 89(3):334-350.

McIntyre, Alison. 2014. Doctrine of Double Effect. In The Stanford Encyclopedia f Philosophy, ed. Edward N. Zalta. Winter 2014 ed. Metaphysics Research Lab, Stanford University.

Miller, Franklin and Alan Wertheimer, eds. 2010. The Ethics of Consent: Theory and Practice. Oxford: Oxford University Press.

Olson, Eric T. 2014. “The Metaphysical Implications of Conjoined Twinning.” The Southern Journal of Philosophy 52:24-40.

Savulescu, Julian. 2012. “Conjoined Twins, Cloning and Arti cial Intelligence.” URL: 

Savulescu, Julian and Ingmar Persson. 2016. “Conjoined Twins: Philosophical Problems and Ethical Challenges.” The Journal of Medicine and Philosophy 41(1):41-55.

Soble, Alan, ed. 2002. The Philosophy of Sex: Contemporary Readings. Oxford: Rowman & Little field.

Wertheimer, Alan. 2010. Consent to sexual relations. in Miller and Wertheimer (2010) pp. 196-197.

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2 Comment on this post

  1. It seems to me that this is a no win, stalemate. I would say that if two people are sharing the same room, a very small cramped room, there are considerations they each share more-so than a large multi-room mansion. I would lean toward the right of the one not to have sex than the one that does. The lesser damage would be done to the one wanting sex than the one that would be traumatized not wanting sex. The one not wanting sex would be frustrated at the very least and probably recover quickly. While the other could have far greater effects causing longterm mental and emotional damage. As the old saying goes, “the lesser of two evils.” The answer could be , get married and ride off into the sunset!

  2. The whole article fails for giving no basis for its ethical principles: How do you know that sex is evil without consent? I didn’t see a Bible quote on it. And if you reject the Bible as the basis, how do you know right from wrong? You just make it up? You have no proof for the basis of your argument. How do you know that “It is always evil for a person to do something to another for which that person does not consent?”

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