I recently finished reading The Language of God: A Scientist Presents Evidence for Belief by Dr. Francis S. Collins. If that title piques your interest, you should read the book. The book brings up many issues worthy of thought and pondering, but one which really stood out for me was the issue of the ensoulment of identical twins. I was disappointed that the book does not provide any answers, or even much coverage of the alternative viewpoints of this problem, so I went to investigate the matter myself.

Ensoulment is important to consider because many pro-life proponents state that abortion is murder. If the conceptus has a soul at the time of abortion, then the abortion would be homicide (I don’t reckon that anyone would attempt to argue that an entity with a soul is not human). An entity’s lack of a soul might strengthen the argument that it is not human, and thereby potentially nullify the proposition that abortion is murder.

How twins come about

A web search for “how twins” produces some informative links which describe the biological processes involved in the creation of twins.

In the case of the creation of a single baby, a sperm fertilizes an egg, the egg implants, and off the whole thing goes on its way to being born. The fertilized egg is called a zygote.

Twins are formed in one of two ways. Dizygotic twins are formed when two individual sperms fertilize two individual eggs. In the case of monozygotic twins, a single sperm fertilizes a single egg, just like in the case of a single baby. At some point during early development (a point which varies from case to case), the collection of the cells of the baby splits apart, and the two resultant parts continue on, growing as two distinct people (two babies are born).

The twin ensoulment problem

Any intellectually satisfactory theory of ensoulment must be able to explain the ensoulment of monozygotic twins. There are several possible explanations, including:

1. The single zygote is ensouled exactly at the point of fertilization, and the soul also splits into two souls at the point when the zygote or blastocyst splits.
2. The single zygote is ensouled exactly at the point of fertilization, and at the time of splitting, one part retains the soul, while the second part is ensouled at that moment with a second, distinct soul.
3. The single zygote has no soul at the point of fertilization, and at the time of splitting, both parts are ensouled. Alternatively, each part is eventually ensouled some time after the split, perhaps at different times.
4. The single zygote is ensouled with two souls, and at the time of splitting, each part retains one soul.

The Catholic position on ensoulment

The Declaration on Procured Abortion (from the Sacred Congregation for the Doctrine of the Faith), published in 1974, states:

[Paragraph 13] expressly leaves aside the question of the moment when the spiritual soul is infused. There is not a unanimous tradition on this point and authors are as yet in disagreement. For some it dates from the first instant; for others it could not at least precede nidation.

A recent Catholic blog post states that there are two main theories of ensoulment:

The first is called the “immediate animation, immediate ensoulment theory”; the second, the “immediate animation, delayed ensoulment theory” (also called the “serial ensoulment theory”).

Using material from notes for a medical ethics course at the International Catholic University, the blog post concludes that the [more] Catholic position is to hold the “immediate animation, immediate ensoulment theory”, despite the often raised point that Saint Thomas Aquinas believed in a delayed ensoulment. Unfortunately, although the course notes acknowledge that the existence of monozygotic twins poses a problem for the immediate ensoulment theory, no harmonization between this theory and monozygous twinning is proposed. The issue is essentially left untreated:

Inasmuch as monozygous twinning appears to be an aberration, rather than the norm, considerations of its ensoulment may also depart from the norm.

There does not appear to be at this time an official Catholic teaching on ensoulment.

Conclusion: ensoulment is not pertinent

Given room to speculate on the ensoulment of monozygotic twins, my inclination is to believe that the zygote is ensouled with two souls at conception. Although present science doesn’t seem to permit us to determine whether a single zygote will or will not split and become twins, God himself knows. Just as a single human individual would be ensouled at the time of conception, when all of his or her potentialities have begun the processes of actualization, so would a pair of people be ensouled, even though their unitary physical form would appear to us as indistinguishable from that of a single person.

Even though an official Catholic position on ensoulment in general remains as yet unspecified, paragraph 13 and footnote 19 of the Declaration on Procured Abortion make it clear that this uncertainty and lack of knowledge do not make abortion morally licit.

[Supposing] a belated animation, there is still nothing less than a human life, preparing for and calling for a soul in which the nature received from parents is completed

[On] the other hand, it suffices that this presence of the soul be probable (and one can never prove the contrary) in order that the taking of life involve accepting the risk of killing a man, not only waiting for, but already in possession of his soul.

[Even] if a doubt existed concerning whether the fruit of conception is already a human person, it is objectively a grave sin to dare to risk murder.