This essay is one of the editor’s picks on abortion and infanticide. Read related articles here. 

Senator Marco Rubio is right. The life of a human being begins at conception—not at implantation, “viability,” or birth. This is a scientific fact.

It is not, as CNN journalist Chris Cuomo ignorantly insisted in a televised confrontation with Rubio, a claim of “faith” with no scientific basis. To our surprise, however, the distinguished bioethicist Arthur Caplan has intervened to try to rescue Mr. Cuomo in a fight he is losing and losing badly. According to Professor Caplan, Senator Rubio has the science wrong. But he doesn’t. And Professor Caplan fails to show that he does.

Red Herrings

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Caplan first appeals to a resolution issued by the National Academy of Sciences in 1981, in response to a congressional bill asserting “that actual human life exists from conception.”

It would have been clearer to say that what begins at conception is an actual human life—the life of a human individual, a human being. The bill’s vaguer wording allowed the NAS to respond with a blatant dodge: it simply noted that “human life” (as opposed to a human life, the life of a new human individual) is passed on continually across generations. That is true—but irrelevant to the issue at hand. So is the NAS resolution’s next point: that when the embryo becomes “a person” is a philosophical (or theological) question on which science is silent.

That is also true. Science reveals empirical facts. It cannot tell us who, if anyone, is a “person,” morally speaking—which beings, if any, have fundamental dignity and basic moral rights. There are correct answers to these questions—they are not merely subjective issues—but they are not answered by application of scientific methods of inquiry. We cannot determine whether there even is such a thing as human rights, or whether slavery, or Hitler’s genocide against Jews, was morally wrong, by conducting laboratory experiments or constructing mathematical models.

What science can and does reveal is whether a human zygote, embryo, or fetus is a newly conceived human being. This was the question on which Senator Rubio appealed to science. And, contrary to Professor Caplan’s complaint, Rubio got it right. Science shows that the human zygote, embryo, and fetus, like the infant or adolescent, is indeed a human being—a living individual of the human species. And—despite its red herrings and irrelevancies—the Academy did not deny this truth. The 1981 resolution provides no support at all for Professor Caplan’s charge against Rubio.

The Scientific Facts

Readers may wonder why Professor Caplan reached back to a resolution passed by a learned society some thirty-four years ago in seeking authority to support his case against Senator Rubio. Professor Caplan remarks: “Since that time [1981], scientists and physicians have remained more or less mum on the issue of when life begins.” But that is simply incorrect. Before and after 1981, there have been countless scientific monographs and scholarly articles—in embryology, developmental biology, and genetics—explicitly affirming that a human being at the earliest stage of development comes to be at fertilization. Here are three of many, many examples:

“Human life begins at fertilization, the process during which a male gamete or sperm unites with a female gamete or oocyte (ovum) to form a single cell called a zygote. This highly specialized, totipotent cell marked the beginning of each of us as a unique individual.” “A zygote is the beginning of a new human being (i.e., an embryo).” Keith L. Moore, The Developing Human: Clinically Oriented Embryology, 7th edition. Philadelphia, PA: Saunders, 2003. pp. 16, 2.


“Fertilization is the process by which male and female haploid gametes (sperm and egg) unite to produce a genetically distinct individual.” Signorelli et al., Kinases, phosphatases and proteases during sperm capacitation, CELL TISSUE RES. 349(3):765 (Mar. 20, 2012)


“Although life is a continuous process, fertilization (which, incidentally, is not a ‘moment’) is a critical landmark because, under ordinary circumstances, a new, genetically distinct human organism is formed when the chromosomes of the male and female pronuclei blend in the oocyte” (emphasis added; Ronan O’Rahilly and Fabiola Mueller, Human Embryology and Teratology, 3rd edition. New York: John Wiley & Sons, 2000, p. 8). (Many other examples could be cited, some of which may be found here. )

That is the authority of science. On request, we can cite dozens more examples. The authorities all agree because the underlying science is clear. At fertilization a sperm (a male sex cell) unites with an oocyte (a female sex cell), each of them ceases to be, and a new entity is generated. This new entity, initially a single totipotent cell, then divides into two cells, then (asynchronously) three, then four, eight and so on, enclosed all the while by a membrane inherited from the oocyte (the zona pellucida). Together, these cells and membrane function as parts of a whole that regularly and predictably develops itself to the more mature stages of a complex human body.

From the zygote stage onward this new organism is distinct, for it grows in its own direction; it is human—obviously, given the genetic structure found in the nuclei of its cells; and it is a whole human organism—as opposed to what is functionally a part of a larger whole, such as a cell, tissue, or organ—since this organism has all of the internal resources and active disposition needed to develop itself (himself or herself) to the mature stage of a human organism. Given its genetic constitution and epigenetic structure, all this organism needs to develop to the mature stage is what human beings at any stage need, namely, a suitable environment, nutrition, and the absence of injury or disease. So it is a whole human organism—a new human individual—at the earliest stage of his or her development.

This is why it is correct to say that the developing human embryo is not “a potential human being” (whatever that might mean) but a human being with potential—the potential to develop himself or herself (sex is established from the beginning in the human) through the fetal, infant, child, and adolescent stages and into adulthood with his or her identity intact.

So the man known to the world as Arthur Caplan is the same human being—the same living individual of the human species—who was once a Columbia graduate student, and before that a gifted Brandeis undergraduate, a rambunctious teenager, a precocious toddler, a newborn infant, a seven-month old fetus, a four-week old embryo, and a newly conceived human being. He was never a sperm cell or an oocyte; those were (genetically and functionally) parts of his parents. But he was once a zygote—a single-celled human individual, distinct from the gametes whose union brought him into being.

In Search of Bright Lines

Professor Caplan advances three arguments to try to show that Senator Rubio gets the science wrong. First, he suggests that science uncovers no bright line—no discrete point at which a new life comes to be all at once. Does conception occur, he wonders, when the sperm penetrates the oocyte, or when the genes they contain mix, or when the new single genome begins to function, or . . . ? Caplan thus mentions several events at or close to the time of fertilization, claims they form a continuum, and then concludes that science shows no “clean boundaries.” But this point casts no doubt on the proposition that the life of a new human being begins at conception. Caplan’s argument merely asserts, without proving, that no discontinuity occurs at fertilization.

And the events he considers all happen within hours of when sperm meets the egg. So he gives no reason to think that human beings begin (e.g., that he himself or anyone else began) at implantation or “viability” or sentience or birth or a few months after birth when self-awareness has clearly been acquired.

We know that a discrete event, an abrupt change, does occur at fertilization. Before fertilization, there are two parental sex cells, each internally oriented toward meeting and fusing with a cell of its opposite number. (None of us was ever literally a twinkle in anyone’s eye.) Afterward, there is one entity, genetically distinct from both sex cells; and it functions as an organism, developing itself (i.e., developing by an internally directed process) toward more mature stages of human life. This marks a change in kind (and not a mere change in degree of development).

Biological entities, in other words, are defined by their characteristic behavior; and that is enough to show that what is generated at conception is a new human being. But by applying the same principle to a more detailed biological picture, we can answer the question to which Caplan shifted: Which event surrounding the meeting of sperm and egg constitutes their transformation into a zygote?

When a sperm penetrates an oocyte’s outer membrane, the resulting structure may look similar, but its characteristic behavior is opposed to that of a typical oocyte. (For one thing, it repels rather than attracts sperm.) So this penetration, when successful, is the point at which oocyte and sperm cease to be and give rise to a new entity—a new organism. This is confirmed by the fact that the new structure immediately begins to engage in activities aimed at its survival as a whole, and its organized differentiation and growth into later stages of human life. It exhibits, that is, holistic (i.e., integral) organic functioning: the behavior of a single organism. We therefore agree with those scientists who conclude that a new human organism comes to be when a sperm cell successfully penetrates an oocyte.

Some have argued (less convincingly, we think) that two sex cells have not given way to a new human being until their genetic materials have actually intertwined. Their disagreement is over at what precise point the two sex cells cease to be and a new single entity begins. And if they are right, the difference is a matter of hours, not days or weeks. Either way, a human being exists long before a woman knows she is pregnant (and even before the blastocyst stage, when cells could be extracted for research). Rubio was right.

Arguments from Mortality

Professor Caplan offers another argument, but again it is beside the point. He appeals to high estimated rates of embryonic death before implantation (and of miscarriages) to argue that “many embryos that result from conception—indeed, the majority of them—lack the capacity to become living human beings.”

Let’s sort the facts from the errors. Yes, fertilization can fail. When it does, the result isn’t a human embryo—a new individual member of the human species—but what anyone who knows anything about human embryology and teratology recognizes as a disorganized growth: a complete hydatidiform mole, for example, or a tumor. Instead of organization for growth in the direction of maturity, there is disorganization: in some cases, for instance, a random mass of skin cells and hair. The result (mole, tumor, cyst) is not a human being. No one would object to discarding it.

So from the fact that fertilization sometimes fails, nothing follows for the debate between Rubio and Cuomo (or Caplan). That fertilization can produce moles or tumors when it fails says nothing about the nature of the organized, self-directing entity that results when it succeeds.

What about Caplan’s suggestion that high mortality rates among embryos means that most of them “lack the capacity to become human beings”?

The argument rests on a basic error. It is like saying that babies suffering from genetic diseases that will cause death in infancy are not human beings because they will die before learning to walk or talk. An embryo, as opposed to a teratoma or complete hydatidiform mole, is already a human being—identifiable as such by its (genetic and epigenetic) organization and behavior now.

It is true that many human embryos die before birth from health or environmental problems. That does not mean that they were not human beings—any more than high infant mortality rates from natural causes in antiquity (and in some places, alas, even today) meant (or mean) that ill-fated newborns are not human beings.

Professor Caplan makes one more observation which he suggests undermines Senator Rubio’s claim, but it too turns out to be beside the point. There are rare cases in which twins or triplets are gestating and “one of those lives is absorbed into the body of another—fetal resorption.” This phenomenon is well-known to embryologists, developmental biologists, and anatomists, but it does not mean that the embryos that were lost by resorption were, before their deaths, something other than embryonic human beings—human individuals in the embryonic stage of development.

Imagine three human beings coming to be—however that happens in fact—and one of them then dying. If the cells of the deceased are somehow absorbed into the survivors (perhaps in connection with some biotechnological treatment), does that undermine the status of any of the three as a human being? The answer is obviously no.


Peter Singer, who defends the morality of abortion and even infanticide, can hardly be pinned as a pro-life propagandist. But he happily concedes—indeed, insists—that the debate about abortion is not about when a human life begins. Abortion obviously takes the life of a human being. Marco Rubio is right about that, and Chris Cuomo has no idea what he’s talking about.

The real question is whether human beings have inherent worth and dignity—and a right to life—or whether their value and right to life depends on factors such as age, size, stage of development, or physical health. Do all human beings have a right to life, or are some “not yet persons” (the unborn, the newly born), or “no longer persons” (those suffering from severe dementia or in minimally conscious states), or lifelong “non-persons” (those congenitally severely cognitively disabled)? Are all human beings equal in worth and dignity? Pro-lifers say yes. Professor Singer and other honest, informed abortion advocates say no.

Science cannot settle that dispute. It cannot tell you that it is wrong to kill the physically handicapped on the ground that they are, as the Nazis said, “useless eaters.” For that matter, it cannot tell you whether people may be enslaved or pillaged on account of their language or race.

But for those who reject sorting human beings into “superiors” and “inferiors”; for those who embrace the principle at the heart of our civilization—the equal dignity of all human beings—science can reveal something crucial indeed: namely, who is a human being. And pace Professor Caplan, Senator Rubio is on the firmest possible scientific ground when he says that science shows that the child in the womb, from the very point of successful fertilization, is indeed a human being.