As an identical twin myself, this question has always intrigued me. To the best of my knowledge, the Church takes no official position on this matter. Catholics are free to explore different theories.
We know that human life begins at conception. The
Catechism of the Catholic Church (CCC) states, “Human life must be respected and protected absolutely
from the moment of conception” (CCC 2270,
emphasis added). In the case of identical twins, as in the case of singletons, conception (fertilization) occurs only once therefore, each twin’s soul
must exist at their shared conception.
We also know that, for some time after fertilization, embryos have the ability to generate all the cells and tissues of
more than one person. In his
presentation concerning cloning, Cardinal Alfonso Lopez Trujillo, states, “The totipotentiality of a cell consists in its ability to generate all the cells and tissues of a complete organism, including (if satisfactory circumstances exist) the development of an individual. In the human, each embryonic cell remains totipotent for a few days after fertilization. Homozygous germination (the phenomenon of identical twins) is the result of an incidental embryonic fission of the totipotent cells that make up the embryo in the first stages of its development” (footnote 8).
Science is presently unable to explain why “embryonic fission” (the division of an embryo resulting in identical twin embryos) occurs. It is important to consider both the cause itself as well as the timing of the cause. My identical twin brother, Jeff, and I propose the following theories:
1. At- or Pre-conception Factors: One or Two Souls May Exist at Conception
Due to presently unknown genetic or environmental factors existing prior to or at conception, the zygote (fertilized egg) is destined for embryonic fission. Therefore, when conception occurs, one of two things happens: (a) two souls exist within a single zygote, one for each embryo at embryonic fission; or (b) one soul exists, destined for “soul fission” (the division of a soul resulting in identical twin souls) - a new soul cannot come into play, as each embryo’s soul must have existed since conception, and each was conceived only once and at the same time.
2. Post-conception Factors: One Soul Exists at Conception
In this case the zygote is no different at conception than that of a singleton’s zygote because factors causing embryonic fission do not yet exist. Therefore, only one soul must exist. Later, presently unknown environmental factors cause embryonic fission and, as with theory 1(b) above, soul fission must occur.
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continued…)
