Monday, January 8, 2007

Are Conjoined Twins Biologically One Organism? [Serge]

I appreciate the interaction between Scott and commenter Daniel, and I'll patiently await Scott's response when he gets back, but I needed to comment on this aspect of the discussion which is purely biological:

To conclude, I would be interested in hearing what you have to say to my conjoined twins scenario. A single embryo at some point splits, but not completely enough to have it become a separate organism (or at least, not permanently). It then develops into identical twins who are (say) conjoined at the hips (they share the same pair of legs). If our only 'moral ingredients' are biological, then this is only one human being. From my perspective however, since personhood depends on relevant neural structure, I would say we have two human beings (in the moral sense--one in the biological sense) since there are two brains. So as I understand your perspective, destroying said twins before birth and cell splitting would be one murder, and destroying said twins after birth would be two murders (but why two if your only criterion is biologically human genetic code?).
If I understand Daniel's point correctly, he believes that conjoined twins are biologically one human being that happens to possess two separate brains, thus being considered two morally relevant human persons.

From a purely biological standpoint, Daniel is incorrect. Conjoined twins are two separate human organisms that have been fused together during the process of embryogenesis. Here is a citation in the medical literature which confirms this:

A review of over 1,800 publications concerning the embryology and pathologic anatomy of conjoined twins provides convincing evidence that they all result from the secondary union of two originally separate monovular embryonic discs.
We could conceivably reproduce conjoined twins surgically by taking two monozygotic twins after birth, and surgically fusing their hips together. In this case, would anyone claim that there is only one human organism? Clearly not. A similar mechanism is responsible for conjoined twins, although it occurs at about day 14 post fertilization in humans. We consider conjoined twins as two individual human organisms, because they are two individual human organisms.

To address the moral question, it would be wrong to behead one of the conjoined twins for the same reason it would be wrong to stab one in the heart. The result of the action would be the death of a human organism, albeit one that is connected to another human organism. That is what makes the action wrong.

Lastly, when Daniel speaks of a "biologically human genetic code" as conveying human value, he needs to be careful to avoid a straw man. The pro-life view as expressed here is that every human being, or living organism of the species Homo Sapiens, has intrinsic value and should not be intentionally killed without adequate reason (the language I am using is purposefully unambiguous). It is not the mere presence of a human genetic code, but the use of that code by an organism of our species.


  1. Serge, thanks for addressing these questions. While it's true my biological formulations may be undersophisticated, I nevertheless feel like you have failed to address my primary question, so let me see if I can 'tweak' the conjoined twins scenario to better bring out my question.

    As you probably know, fetal development can really go wrong. While some conjoined twins are conjoined at the hips, or the liver, or whatever, others are almost entirely fused (e.g. up to their upper-torso). I'll have this be case A.
    You probably also know that in some cases, one of the twins can be underdevelopped--to the point of lacking a heart, a brain, or simply being underweight. In some cases, a baby can be born with odd extra limbs (e.g. a leg sticking out of the side, or teeth in the back of the neck), even though the second 'twin' has been subsumed back into the first (or never really developped in the first place). Let's have this be case B.

    Since you seem to be more up-to-date on your biology than me, make whatever modifications to the two above scenarios for them to be medically plausible.
    In case A, I affirm that it is morally wrong to cut off the second twin's head. This is murder. In case B, I affirm that it is morally acceptable to cut off the second twin's limbs (the leg sticking out of the first twin's torso).
    Now, what accounts for this difference in ethical decision making? Both case A and case B are fused human organisms (two or one depending on how you look at it). If killing a human organism is always wrong (which is what I understand you and Scott to be saying), then chopping off limbs in case B is murder. To me, this is counterintuitive. Which is why I'll stick with my view: destroying a functional brain is murder, because the brain is the seat of the soul (they go hand in hand).

    Again, thanks for the interaction.
    I wish you the best.

  2. Serge is at home with his wife and new baby. He'll likely start posting again next week.


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