Moral Theology & Ethics


I have a case study for a class I am taking. The case is about Siamese twin girls joined at the abdomen and with a fused spine. Doctors soon determined that unless the twins were surgically separated both would die. Mary, the weaker twin, whose brain was underdeveloped, would never be able to survive separated from Jodie. Jodie, who was strong and alert, had an 80-90% chance of dying if surgery was not performed. She had a good chance of surviving in the event of surgery, although, in all likelihood she would be severely handicapped and need medical attention throughout her life. In similar cases in the past, the surviving twin has sometimes died within six months of surgery (a real case from UK in 2000)

I am looking for suggestions on where to research church teaching regarding choosing possibly life for one or possibly death for both. Both in scripture and in other documents of the church.

Any guidance is very much appreciated.



I am sure the principle of Double Effect would be involved but I think it would take a professional ethicist to sort it out.


Can you elaborate for me - what does the principle of double effect mean?

Really appreciate the reply.




PDE is pretty nuanced, but it would be a good place to start. Bare bones, your intended end can be justified if the foreseeable negative consequences of acting don’t outweigh the good end itself, so long as the bad effects are not intended and merely endured.


I apologize - but what is PDE?


The principle of double effect, I just abbreviated it


Germain Grisez (recently deceased) was considered a trustworthy moral theologian (Catholic). I am linking you to his third textbook in a series. Note, he would have been the first to tell you that moral theologians can be wrong, so take what he writes with a grain of salt. The series is The Way of the Lord Jesus. Oh, and he has his own system, elaborated in the first book. The answer might refer to that system. The question he answers is directly about Siamese twins.


Thank you.


The shared heart and liver were not exclusively the organs of either twin; the chosen means to fashion an adequate heart for the stronger was to cut away the shared heart from the weaker and the chosen means to provide a liver from the stronger was to cut it away from the weaker. That, of course, resulted in her death, but her death was an effect of chosen means that in no way contributed to the end sought.223 Therefore, in my judgment, the operation was not intrinsically wrong as it would have been had it involved a choice to kill the weaker twin. Nevertheless, as explained above, the surgery did involve the wrongful killing of the weaker twin inasmuch as the surgeon should not have accepted her death, there being little chance the operation would significantly benefit her sister.

If that is true I certainly don’t think it would be moral. In order for the separation to be moral under the principal of double effect it would have to be the case that the death of the weaker twin was not the direct cause of the stronger twins survival.

We know that case can be made with an embryo who implants in the tube. They die as a result of the tube being removed but their death is not cause of the mother’s survival.

A rule of thumb I came up with for principle of double effect cases (I don’t know if it’s valid) is to remove the bad effect. Is the good effect still achieved?

Applied to etopic pregnancy then if a tube was about to rupture with no baby in it you would not hesitate to remove the tube. That is why I think the death of the baby can be seen as indirect in that scenario.

I’m inclined to lean towards the separation being more similar to removing the baby without the tube. My understanding is that that is understood to be immoral.


I really appreciate this detail - Thanks!


You’re welcome. I’ll actually supply the conditions in full:

The act itself must be morally good or at least indifferent.
The agent may not positively will the bad effect but may permit it. If he could attain the good effect without the bad effect he should do so. The bad effect is sometimes said to be indirectly voluntary.
The good effect must flow from the action at least as immediately (in the order of causality, though not necessarily in the order of time) as the bad effect. In other words the good effect must be produced directly by the action, not by the bad effect. Otherwise the agent would be using a bad means to a good end, which is never allowed.
The good effect must be sufficiently desirable to compensate for the allowing of the bad effect“ (p. 1021).



One note, the term “Siamese Twins” is not used because it is offensive to people with this condition. The correct term is “Conjoined”.

I would advise you to ask the National Catholic Bioethics Center


I apologize - I tried to go back and correct it, but haven’t figured out how to do that.

Thanks for the correction and advise.


Actually - I just went back and checked - that is how the case was given to me. I will notify my professor.


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