When Does the “Soul” Enter the Fetus?

There are two assumptions that the Abrahamic faiths make regarding fetal development:

  1. There is a soul.
  2. The soul enters the zygote at conception

What is the soul?

The attributes most commonly assigned to soul are the mind, the conscience, the will and the emotions.

The Search for the Soul:

Dr. Duncan “Om” MacDougall performed an experiment to determine the weight of the soul. He weighed his subjects at the moment of death and found that the soul weighed 21 grams. There is a problem however. He only had six subjects. Of these, only one lost 21 grams upon death and stayed at the new weight. He dismissed two of them due to technical difficulties. Another lost weight and then gained some. The last two lost weight and later lost more.

Modern neuroscience is exploring the age old question of psychophysical dualism, i.e., is there a soul? It has been found that consciousness, thoughts, behaviors and emotions can be predicted by the brain area that has been activated.

Tests of emotional regulation have shown that the inhibition of emotions is performed by areas in the prefrontal cortex. A brain region corresponding to the sense of self  has been located in the prefrontal cortex. Damage to the anterior prefrontal cortex in children affects their ability to develop moral reasoning.

Many experiments have been conducted in neuroscience in order to understand free will. A complete model has not been developed. There are still a lot of contentious issues. One of which is free will may be an illusion. It has been proposed by one scientist that prior intention and deliberation are functions of the medial prefrontal cortex.

So far science has been able to explain the mind, the conscience and the emotions without the need for the supernatural concept of soul. A physical explanation of the will should be developed in the near future. The brain structure which most closely resembles the soul is the prefrontal cortex.

The Soul and the law:

Since most of our soul-like qualities can be explained materially, the existence of a soul is a matter of faith. To base abortion laws on a religious concept, such as a soul violates the establishment clause of the US constitution.

What feature makes us human?

Since we cannot establish the existence of the soul, much less when one would enter the body, what is it that makes us uniquely human? It is the brain structure known as the prefrontal cortex. As we saw previously, it is responsible for our sense of self, ourmoral reasoning, the inhibition of emotion and possibly will.

When do we become human?

It makes sense to me to say the “soul” enters the body when the prefrontal cortex is developed. The prefrontal cortex doesn’t just appear fully formed. It gradually develops.

Some of the most rudimentary features start to appear at 10 weeks. Other features don’t appear until between weeks 32 and 34. Somewhere between 10 weeks and 34 weeks, we could say that enough of the prefrontal cortex exists to say that the fetus has a “soul” and is human.

For me, when there is enough of a prefrontal cortex for the fetus to have a sense of self, I would say the fetus is human. I don’t know in which week that occurs, but for the rest of this answer, I will arbitrarily pick week 25 as being the point at which the fetus has developed a sense of self. This means that before week 25, abortion should remain legal because the fetus isn’t yet human.

What does this mean for abortions after week 25?

Now that we have established that that the fetus is human at week 25, I would be comfortable arguing about whether abortion should be illegal beyond that point. Now we can talk about back alley abortions, the fate of unwanted children, viability of the fetus, and all of the other considerations that come up around this issue.

The data for the US from 2003 shows that only 1.4% of abortions occur at or after 21 weeks. So if abortion were made illegal after week 25, it would only affect a small amount of the population. If exceptions were made for rape, incest and the life of the mother, etc. I would strongly consider it.


Until the prefrontal cortex has achieved a significant amount of development, the fetus has no “soul” and is not yet human. I argue that it is only after this point in fetal development that one can meaningfully talk about killing a human and consider making it illegal. If you try to make the curettage of a zygote illegal, you have lost me. There is no point in talking.

Keep in mind 25 weeks was picked arbitrarily for this answer. A neuroscientist would have to weigh in on where to draw the line.

©2016 Stephen L. Martin


Duncan MacDougall (doctor)

Non-materialist neuroscience

What Are the Three Parts of Man?


Neuroscience of free will

Affective neuroscience

Prenatal development of neurons in the human prefrontal cortex: I. A qualitative Golgi study

Late termination of pregnancy

©2016 Stephen L. Martin

13 thoughts on “When Does the “Soul” Enter the Fetus?

  1. We become human when the male sperm fertilizes the female egg in the natural process called sexual reproduction.

    The new embryo takes on human nature at the moment of conception because that is the moment when a complete set of human DNA commands human life into existence.

    The new human embryo will not become a cat, dog or pig because human DNA naturally renders a human being.

    And that human being comes into existence at the exact moment the human DNA comes into existence, at the moment of conception.

    This is not abstract theory. The moment of conception can be seen by the human eye using modern instrumentation.

    1. Christians didn’t consider the baby human until birth until 1970’s, when they switched it to conception. See http://www.christianitytoday.com/edstetzer/2012/november/morning-roundup-11512-pro-choice-evangelicals-president.html
      and http://www.bpnews.net/16151/analysis-sbc-sets-the-record-straight-on-conventions-abortion-stance and http://www.etsjets.org/files/JETS-PDFs/33/33-2/33-2-pp215-225_JETS.pdf

      “From a sociological viewpoint, people don’t act like an early stage fetus is a person. Even from nature’s viewpoint, it doesn’t seem like a fetus is a person – as many as a third of all fetuses are miscarried. When a miscarriage happens at an early stage, if it is noticed at all, the parents may be saddened, but the fetus generally isn’t named and given a burial. At a later stage of development, such things are not uncommon.

      Murder is a legal construct and is defined by the legislature. Many people believe that Roe v. Wade was wrongly decided and abortion should be considered murder. They base this on moral and theological grounds. They base this on the appearance of a fetus – certainly late stage fetuses do look like babies. A majority of moral people do not believe that abortion should be outlawed for a variety of reasons: they don’t believe personhood attaches until viability or birth; outlawing abortion will not end it, but will make more it more dangerous; it is not society’s business to interfere with a woman’s body and autonomy, etc. However, there is widespread agreement that personhood does attach at some point before birth.” excerpt from https://www.quora.com/Why-do-sane-people-not-regard-abortion-as-murder/answer/Richard-Potter-17

      1. Actually what we are debating is a definition. The definition of what constitutes human life. My definition is based on qualities that are uniquely human: our sense of self, moral reasoning, the inhibition of emotion and will. These happen to be the qualities that Christians have said is the hallmark of the soul.

        You are defining a human life as that which becomes a human being or as a complete set of human DNA.

        We aren’t debating science. We are debating terms using science to bolster our arguments. But neither argument is more scientific.

        When my ex-wife miscarried, we were sad, but we didn’t hold a funeral. If it had been an infant, we would have had a funeral. This says something about how people view a fetus.

        Did you read the articles about evangelicals changing their positions on abortion?

  2. Law values potential, back to the althing. Moral systems do too, but not, for the most part, as something with its own standing. For instance, I could imagine a legal argument which advocated for the preservation of a fertilized egg because the egg could reasonably be expected to grow into something which would care for its parents in their dotage. I can’t think of a stand-alone moral argument on the same (instrumental) grounds.
    Law also asks for things like the 25 week benchmark. It needs an arbitrary point for adjudication to begin.
    Morality shuns that kind of fixed point; it is a method rather than the means.

  3. Until the fetus has a prefrontal cortex, it is not fully human. There are qualities we consider to be human and they include: sense of self, moral reasoning, the ability to regulate our emotions and will. The only animals besides man that are self aware are some of the great apes. Rather than act on emotional impulse, man has the ability to regulate his emotions. Man and some of the great apes have a sense of fair play and morality. Man has free-will.

    I maintain that until the fetus has these capabilities, it is not murder. It is not murder to cut off some of your skin. Aborting a pre-conscious fetus is removing human tissue and letting it die like cutting off skin and letting it die, but it is not the murder of a human being.

    If you base it on DNA would masturbation for men or menstruation for women be murder?

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