The world began to face the prospect of human cloning when the journal Nature published Dolly the sheep's "birth announcement" in the form of a letter authored by Wilmut and colleagues.1 But despite all the attention given the issue, including two presidential commissions,2,3 the psychological consequences of cloning have been little addressed. There being no actual human clones akin to Dolly, any such exploration would have to rely on reasoning by analogy, a necessarily risky method—but the only one available. This Viewpoint shows how one can approach cloning by analogues to it—that is, by reference to situations and phenomena that are "clone-like" in various ways.
I will address only reproductive cloning—cloning to make babies—and not cloning pursued to derive embryonic stem cells to aid in the study and treatment of disease. The current scientific consensus is that attempting to clone a human being would be unethical. Many mammalian clones do not survive to term, and those that do are often plagued with various anomalies.4
But what if we eventually understand and could prevent such problems? Would there be any other cause for concern? In particular, would a clone run any psychological risks over and above those involved with simply being human?
Cloning can occur naturally when, at its first cell division, a fertilized egg, or zygote, cleaves into 2 single-celled embryos. Monozygotic twins are the result. Intentional embryo splitting in vitro,5 in which one of the early embryos is implanted and the other frozen for later implantation, would result in the birth years apart of individuals with identical DNA. Many of the issues discussed here regarding the sort of clones that are the focus of this article would also apply to such "delayed twins." Unsubstantiated claims aside, contemporaneous monozygotic twins are the only human clones that we know currently exist.
When people talk about cloning, they almost always mean cloning by somatic cell nuclear transfer, or nuclear transfer (NT) for short. NT involves removing the haploid nucleus of an egg, inserting the diploid nucleus of a somatic cell, and then chemically or electrically inducing the egg to divide.
Complete genetic identity between a person and self-clone would result from cloning only if a woman were to donate an egg for enucleation, as well as the nucleus from one of her somatic cells to be inserted into that egg. Even though their nuclear DNA would be identical, other progenitor (donor)-clone pairs would not be a perfect genetic match, including any in which the nuclear donor was male. That is because some DNA is extranuclear. This mitochondrial DNA is inherited only from the mother. It constitutes a relatively small percentage of a person's total DNA, but that doesn't make it unimportant.6
Further complicating the picture is the phenomenon of epigenesis, or gene expression. Although identical twins have completely identical genes, environmental influences, both before and after birth, can affect the way the genome is read. The more time that monozygotic twins live apart, the older they get, and the more divergent their lifestyles, the greater the epigenetic difference one finds between them.7Those who claim that human clones are fated to not be viable as embryos and fetuses, and hence to die before birth, base their prediction on faulty epigenesis—in particular, the fact that maternal "imprinting" marks are removed by the cloning process.8 Epigenetic differences would surely be present between progenitor and clone because their environmental history would necessarily be different, both physically and psychologically. Despite this, identical twins are still the best genetic and biologic analogue available for use in trying to anticipate clone psychology.
