Human Reproduction

Through most of the past century, improved reproductive technology has consisted in large part of better ways of not reproducing. Better contraception has been accompanied by striking changes in human mating patterns: a steep decline in traditional marriage, a corresponding increase in nonmarital sex and, perhaps surprisingly, extraordinarily high rates of childbirth outside of marriage.1 While the long-term consequences of reliable contraception will continue to play out over the next few decades, they will not be discussed here. This chapter deals with more recent developments in the technology of human reproduction.


Eugenics, the idea of improving the human species by selective breeding, was supported by quite a lot of people in the late nineteenth and early twentieth centuries.2 Currently it ranks, in the rhetoric of controversy, only a little above Nazism. Almost any reproductive technology capable of benefiting future generations is at risk of being attacked as “eugenics” by its opponents.

That argument confuses, sometimes deliberately, two quite different ways of achieving similar objectives. One is to treat human beings like show dogs or racehorses – have someone, presumably the state, decide which ones get to reproduce in order to improve the breed. Such a policy involves forcing people who want to have children not to do so and perhaps forcing people who do not want to have children to do so. In addition, it imposes the eugenic planner’s desires on everyone; there is no reason to assume that the result would be an improvement from the point of view of the rest of us. A prudent state might decide that submissiveness, obedience to authority, and similar characteristics were what it wanted to breed for.


The alternative is what I think of as libertarian eugenics. The earliest description I know of is in a science fiction novel, Beyond This Horizon by Robert Heinlein, arguably one of the ablest and most innovative science fiction writers of the twentieth century.

In Heinlein’s story, genetic technology is used by couples to control which of the children they could produce they do produce. With the assistance of expert advice, they select among the eggs produced by the wife and the sperm produced by the husband the particular combination of egg and sperm that will produce the child they most want to have, the one that does not carry the husband’s gene for a bad heart or the wife’s for poor circulation, but does carry the husband’s good coordination and the wife’s musical ability. Each couple gets its own child, yet characteristics that parents do not want their children to have are gradually eliminated from the gene pool. Since the decision is made by each set of parents for their own children, not by someone for everyone, it should maintain a high degree of genetic diversity; different parents will want different things. And since parents, unlike state planners, can usually be trusted to care a great deal about the welfare of their children, the technology should mostly be used to benefit the next generation, not to exploit it.

Heinlein’s technology does not exist but its result, in a crude form, does. The current, more primitive, method is for a woman to conceive, obtain fetal cells by extracting amniotic fluid (“amniocentesis”), have the cells checked to see if they carry any serious genetic defect – in particular, the extra copy of chromosome 21 that produces Down syndrome – and abort the fetus if they do.3

A version that eliminates the emotional (some would say moral) costs of abortion is now coming into use. Obtain eggs from the intended mother, sperm from the intended father. Fertilize in vitro – outside the mother’s body. Let the fertilized eggs grow to the eight-cell level. Extract one cell – which at that point can be done without damage to the rest. Analyze its genes. Select from the fertilized eggs one that does not carry whatever serious genetic defect they are trying to avoid. Implant that egg back in the mother.

At present there are two major limitations to this process. The first is that in vitro fertilization (IVF) is still a difficult and expensive process. The second is that genetic testing is a new technology, so only a small number of genetic characteristics can actually be identified in the cell. Some genetic diseases, yes; musical ability or intelligence, no. The use of IVF is, however, increasing; for the cohort of Danish women born in 1978, 6% of their babies were produced with the help of artificial reproductive technologies such as IVF. Given current rates of progress, the second limitation is likely to be rapidly reduced over the next decade or two. We will then be in a world where at least some people are able to deliberately produce “the best and the brightest” of the children those people could have had. That ability will be greatly increased when and if we get the ability to determine the genetic structure of egg and sperm before they are combined, greatly increasing the number of alternatives that parent can choose among.4

So far I have been considering a reproductive technology that already exists, although at a fairly primitive level: selecting among the fertilized eggs produced by a single couple. We come next to some newer technologies. The one that has gotten most of the attention is cloning, producing an individual who is genetically identical to another.5 One form of cloning is natural and fairly common; identical twins are genetically identical to each other. The same effect has been produced artificially in animal breeding: get a single fertilized egg, from it produce multiple fertilized eggs, implant them to produce multiple genetically identical offspring. In agriculture, cloning to reproduce particularly desirable varieties of grapevines or apple trees – grafting – is a technology that has been practiced for more than 2,000 years.

The form of cloning that has recently become controversial starts instead with a cell from an adult animal and uses it to produce a baby that is the identical twin of that adult. Much of the initial hostility to the technology seemed to be rooted in the bizarre belief that cloning replicates an adult – that, after I am cloned, one of me can finish writing this chapter while the other puts my children to bed. That is not how cloning works – although we will discuss something very similar in a later chapter, where the copying will be into silicon instead of carbon.

Another technology, a little further into the future, is genetic engineering. If we knew enough about how genes work and how to manipulate them, it might be possible to take genetic material from sperms, eggs, or adult cells contributed by two or more individuals and combine it, producing a single individual with a tailor-made selection of genes.

Sexual reproduction already combines genes from our parents in us. Genetic engineering would let us choose which genes came from which, instead of accepting a random selection. It would also let us combine genes from more than two individuals without taking multiple generations to do it, as well as splicing in useful genes from other species. Primitive versions of the technology have already been used successfully to insert genes from one species of plant or animal into another.

Another possibility is to create artificial genes, perhaps an entire additional chromosome.6 Such genes would be designed to do things within our cells that we wanted done – prevent aging, say, or fight AIDS – but that no existing gene did. Constructing them would be a project at the intersection of biotechnology and nanotechnology.

Current and near-future technologies to control what sort of children we have depend on IVF, a technology originally developed to make it possible for otherwise infertile women to have children. It also makes possible artificial cloning of eggs, by letting the fertilized egg divide and then separating it into two. It makes possible cloning of adult cells, by replacing the nucleus of a fertilized egg with a nucleus from an adult cell. And it may yet make possible genetic engineering and artificial genes. It has also already made it possible for surrogate mothers to bear children produced from other women’s fertilized eggs.

Other new technologies may make possible reproduction by a different sort of infertile parents: same-sex couples. At present a pair of women who wish to rear a child can, in at least some states, adopt one. Alternatively, one of the women can bear a child using donated sperm. But they cannot do what most other couples desiring children do: produce a child who is the genetic offspring of both of them. The closest that can be managed with traditional technology is to use sperm donated by a father or brother of one to inseminate the other, producing a child who is, genetically speaking, half one of them and a quarter the other.

That situation is changing. Techniques have been developed for producing artificial sperm containing genetic material from an adult cell. They may make it possible in the fairly near future for two women to produce a child who is, in the full sense, theirs. At some point an analogous technology might make possible artificial eggs, permitting two men, with the assistance of a borrowed womb, to produce a child who is, in the same sense, theirs.7


If your goal is to genetically manipulate a human being, you need to insert a gene into every relevant cell, or start with a single-celled embryo.

Matt Ridley, Genome, p. 247

The genetic engineering we have been discussing is applied to a single cell, a fertilized ovum. Doing that is an elegantly simple way of changing things, since the altered characteristics of a single cell will be passed on to every cell of the body built from that cell and possibly to new bodies descended from that one. For the same reason some find the application of such technologies to humans frightening, a way of permanently changing at least part of the human race.

The technical term for this is germ-line genetic engineering. Surprisingly enough, it is not the only way of altering living things by changing their genes. The alternative is to alter genes in the cells of an already existing organism. This raises an obvious problem. Altering the genes in a single cell is a difficult and chancy procedure. A single human body contains about 100 trillion cells.8 How can one possibly alter enough of them to make a difference?

This problem was solved a very long time ago, and not by humans. Viruses reproduce by hijacking the mechanism of a cell, modifying it, and using it to produce more viruses. Since viruses hijack us, it seems only fair for us to hijack them.

A retrovirus contains a message written in RNA which reads, in essence: ‘Make a copy of me and stitch it into your chromosome.’ All a gene therapist need do is take a retrovirus, cut out a few of its genes …, put in a human gene, and infect the patient with it. The virus goes to work inserting the gene into the cells of the body and, lo, you have a genetically modified person.

Ridley, Genome, p. 247

This form of genetic engineering has already been used to combat severe combined immune deficiency – SCID – the genetic disease that makes children’s bodies unable to defend themselves against infection.9 It used to be that such children could be kept alive only in a sterile environment and died young. Later a method was discovered of treating the disease by monthly injections of the protein that the defective genes were failing to make. Currently, that approach is combined with genetic therapy that repairs some of the defective genes and so reduces the victim’s dependency on the injected protein. Similar cures are being developed for a variety of other diseases.

SCID is a rare disease. Cancer, on the other hand, is one of the chief causes of death in modern societies, second only to heart disease and soon expected to overtake it. And cancer is a genetic disease.

My development from a fertilized ovum to an adult was made possible by cell division, starting with a single cell. So is the process of healing wounds by building new tissue to replace the old. Once I reach my full size, most of those cells must stop dividing, since otherwise I will keep growing. Cells are provided with mechanisms to make them divide – oncogenes – which get turned off when division is no longer necessary. They are provided with additional mechanisms to stop them dividing, in case the oncogene gets somehow stuck on “divide.” And, just to play safe, they have a third mechanism to make the cell self-destruct if the first two fail.

Mutation changes genes. If enough changes happen in the same cell to make all three mechanisms fail, you get cancer; that, at least, is the current theory. For three different things to break in the same cell is, of course, very unlikely. But with 100 trillion cells, even very unlikely things can happen. Which suggests a possible tactic for curing cancer. Genetically modify the cancer cells in a way that fixes at least one of the three things that is wrong with them. If you fix one of the first two, the cancer cells stop dividing. If you fix the third, they die.

The possibility of retrofitting cells with new genes also suggests a tempting, and disturbing, possibility for high-tech law enforcement. Suppose we conclude that some of the causes of criminal behavior are genetic; perhaps that there is a gene, more likely a group of genes, for psychopathy. Instead of sentencing a criminal to be imprisoned we sentence him to have his genes revised – a brand-new, high-tech version of the old dream of reforming criminals instead of deterring them.

When we are finished altering some of the genes in every cell in his body – I am assuming a more advanced version of the technology than we have at present – is he still the same person? Have we reformed or replaced him?

Maybe Huxley Got It Right

So far I have been discussing ways of changing what people are like by changing their genes. Another possibility is by changing their environment – their very early environment. There is now good evidence that subtle features of the prenatal environment, the mother’s womb, have significant and interesting effects on how the occupant turns out.

Look at one of your hands and compare the length of the first and third (“ring”) fingers. On average, the greater the relative length of the third finger, the higher the level of testosterone (and the lower the level of estrogen) in the womb you occupied. Finger length does not matter very much, but finger length is not all that is affected; the relative length of those two fingers also correlates with children’s relative scores on numeracy and literacy tests.10 It has long been observed that, on average, males seem to do relatively better at mathematical learning, females on verbal. Apparently the difference is at least in part due to their different uterine environments. Not only does a womb with a male fetus have, on average, a higher level of testosterone than a womb with a female fetus, but among males or among females the level of testosterone correlates with relative mathematical and verbal abilities.

In Brave New World, Aldous Huxley described a future dystopia in which the state produced different sorts of people for different purposes, each designed to be good at and contented with a particular role in life. They did it by controlling not genes but uterine chemistry, using artificial wombs for the purpose. He may have been on to something.

Why Bother?

New technologies make it possible to do new things; there remains the question of whether they are worth doing. In the case of reproductive technology, the initial driving force, still important, was the desire of people to have their own children. From that we get IVF and the use of surrogate mothers to permit a mother unable to bring her fetus to term to get someone else to do it for her. The desire to have your own children also provides a possible incentive for cloning – to permit a couple unable to produce a child of both (because one is infertile) to produce instead a child who is an identical twin of one – and for technologies to allow same-sex couples to reproduce.

A second and increasingly important motive is the desire to have better children. In the early stages of the technology this means avoiding the catastrophe of serious genetic defects. As the technology gets better, it opens the possibility of eliminating less serious defects – the risk of a bad heart, say, which seems to be in part genetic, or alcoholism, which may well be – and selecting in favor of desirable characteristics. Parents want their children to be happy, healthy, smart, strong, beautiful. These technologies provide ways of improving the odds.

One can imagine the technologies used for other purposes. A dictatorial government might try to engineer the entire population, to breed some inconvenient characteristic, say aggressiveness or resistance to authority, out of it.11 A less ambitious government might use cloning to produce multiple copies of the perfect soldier, or secret policeman, or scientific researcher, or dictator – although multiple identical dictators might be asking for trouble.

Such scenarios are more plausible as movie plots than as policies. It takes about twenty years to produce an adult human; few real-world governments can afford to plan that far ahead. And while a clone will be genetically identical to the donor, its environment will not be identical, so while cloning produces a more predictable result than sexual reproduction, it is far from perfectly predictable.12 Getting your soldiers, secret police, scientists, or dictators the old-fashioned way has the advantage of letting you select them from a large population of people already adult and observable.

One further argument against the idea is that if it is an attractive strategy for a dictatorial state, it ought already to have happened. Selective breeding of animals is a very old technology. Yet I know of no past society that made any serious large-scale attempt at selective breeding of humans in order to produce in the ruled traits desired by the rulers.13 Insofar as we have observed selective breeding of humans it has been at the individual or family level, people choosing mates for themselves or their children in part on the basis of what sort of children they think those mates will help produce.

A more serious danger is the exploitation of cloned children on a smaller scale. In a version sometimes offered as an argument against cloning humans, an adult produces a clone of himself in order to disassemble it for body parts to be used for future transplants. One obvious problem with that scenario is that even if the cloning were legal, the disassembly would not be – in the United States at present or in any reasonably similar society. But one can imagine a future society in which it was. On the other hand, the process again involves a substantial time lag, and becomes increasingly less useful as improved medical technology reduces the problems of transplant rejection.

There has been at least one real-world case distantly analogous to this, however. Looking at it suggests that producing a human being at least partly to provide tissue for transplant may not be such an ugly idea after all. In 1988, Anissa Ayala, then a high school sophomore, was diagnosed with a slow progressing but ultimately fatal form of leukemia. Her only hope was a treatment that would kill off all her existing blood stem cells and replace them by a transplant from a compatible donor. The odds that a random donor would be compatible were about 1 in 20,000.

Her parents spent two years in an unsuccessful search for a compatible donor, then decided to try to produce one. The odds were not good. A second child would have only a 25% chance of compatibility. Even with a compatible donor the procedure had a survival probability of only 70%. The mother was already forty-two, the father had been vasectomized. The alternative was worse; Anissa’s parents took the gamble. The vasectomy was successfully reversed. Their second daughter Marissa was born – and compatible. Fourteen months later she donated the bone marrow that – as she put it five years later in a television interview – saved her sister’s life.

Marissa was produced by conventional methods; the controversial element, loudly condemned by a variety of bioethicists, was producing a child in the hope that she could donate the bone marrow required to save another. But cloning, had it been practical, would have raised the odds of a match from 25% to 100%.

For another potentially controversial use of cloning, consider parents whose small child has just been killed in an auto accident. Parents have a very large emotional investment in their children, not children in the abstract but this particular small person whom they love. Cloning could let them, in a real although incomplete sense, get her back – in the form of a second child very nearly identical to the first.14

Reasons Not To Do It

I disagree with your principles, so will require you to die for mine.

Voltaire’s view, bioethically revised

Reproductive technologies – most recently cloning, earlier contraception, IVF, and artificial insemination – have aroused widespread opposition. One reason, the idea that such a technology might be particularly useful to a dictatorial state, I have already dismissed as implausible. There are at least three others.

The first is the “yecch” factor. New technologies involving things as intimate as reproduction feel weird, unnatural, and for many people, frightening and ugly. That was true for contraception, it was true for IVF and artificial insemination, it is strikingly true for cloning, and it will no doubt be true for genetic engineering when and if we can do it. That reaction may slow the introduction of new reproductive technologies but is unlikely to prevent it, so long as those technologies make it possible for people to do things they very much want to do.

A second reason is that new technologies usually do not work very well at first. Judging by experience so far with cloning large mammals, if someone tries tomorrow to clone a human it will take many unsuccessful tries to produce one live infant and that infant may suffer from a variety of problems. That is a strong argument against cloning a human being today. But it is an argument that will get weaker and weaker as further experiments in cloning other large mammals produce more and more information about how to do it right.

The final reason is the most interesting of all. It is the possibility that individual reproductive decisions might have unintended consequences – perhaps seriously negative ones.


Consider a simple example: gender selection. Parents often have a preference as to whether they want a boy or a girl. The simplest technology to give them what they want – selective infanticide – has been in use for thousands of years. A less costly alternative – selective abortion – is already being used extensively in some parts of the world.15 And we now have ways to substantially alter the odds of producing male or female offspring by less drastic methods.16 As such techniques become more reliable and more widely available, we will move toward a world where parents have almost complete control over the gender of the offspring they produce. What will be the consequences?

For the most extreme answer, consider the situation under China’s one-child policy, imposed on a society where families strongly desire at least one son. The result is that a substantial majority of the children born are male; some estimates suggest about 120 boys for 100 girls. A similar but weaker effect has occurred in India even without a restriction on number of children; recent figures suggest about 107 boys for 100 girls. With better technologies for gender selection17 the ratios would be higher. The consequence is likely to be societies where many men have difficulty finding a wife.

The problem may be self-correcting – with a time lag. In a society with a high male-to-female ratio women are in a strong bargaining position, able to take their pick of mates and demand favorable terms in marriage.18 As that becomes clear, it will increase the payoff to producing daughters. There is not a lot of point to preserving the family name by having a son if he cannot find a woman willing to produce grandchildren for you. A high ratio of men to women might also result in a shift in mating patterns in the direction of polyandry: two or more husbands sharing the same wife. Even without changes in marriage laws there is still the possibility of serial polyandry. A woman marries one man, produces a child for him, divorces him, and marries a second husband.19

Class Genes

What about technologies allowing parents to choose among the children they might have, or even to add useful genes, perhaps artificial, that neither parent carries? Lee Silver, a mouse geneticist and the author of a fascinating book on reproductive technology,20 worries that the long-term result might be a society divided into two classes: generich, the genetically superior descendants of people who could afford to use new technologies to produce superior offspring, and genepoor.

There are two reasons this is not likely to happen. The first is that human generations are long and technological change is fast. We might have a decade or two in which higher-income people have substantially better opportunities to select their children. After that the new technology, like many old technologies, will probably become inexpensive enough to be available to almost anyone who really wants it. It was not that long ago, after all, that television was a new technology restricted to well-off people. Currently, about 97% of American families below the poverty line own at least one color television.

The second reason is that human mating is not strictly intraclass. Rich men sometimes marry poor women and vice versa. Even without marriage, if rich men are believed to carry superior genes – as, after a generation or two of Lee Silver’s hypothetical future, they would be – that is one more reason for less rich women to conceive by them, a pattern that, however offensive to egalitarian sensibilities, is historically common. Put in economic terms, sperm is a free good, hence provides a low-cost way of obtaining high-quality genes for one’s offspring. I doubt we will get that far, but if we do we can rely on the traditional human mating pattern – monogamy tempered by adultery – to blur any sharp genetic lines between social or economic classes.


Everything not forbidden is compulsory.

Sign over the entrance to the ant colony in The Once and Future King

In our society people are not supposed to become sexually active until they become adults. In practice, it doesn’t work that way, leading to problems with which anyone who reads newspapers, watches television, or worries about his or her own children is familiar. The essential problem is that we are physically ready to reproduce before we are emotionally or economically ready. That has become increasingly true as the age of physical maturity has fallen – by about two years over the past century, probably as a result of improved nutrition. With the continuing progress of medical science, we may soon be able to reverse that change.

Suppose a drug company announces a new medication – one that will safely delay puberty for a year, or two years, or three years. I predict that there will be a considerable demand for the product. Are parents who artificially delay the physical development of their daughters guilty of child abuse? May schools pressure parents to give the medication to boys about to reach puberty, as many now do for other forms of medication designed to make children behave more as schoolteachers wish them to? If schools do require it, are parents who refuse to artificially delay the development of their sons guilty of child abuse – or at least subject to the same pressures as parents who today refuse to put their sons on Ritalin?

While we are at it, what about the application of a similar technology to other species? Cats are lovely creatures, but kittens are much more fun. If only they stayed kittens a little longer ….


1   For some possible explanations, see Friedman (2000) Chapter 13.

2 Including George Bernard Shaw, H. G. Wells, John Maynard Keynes, Harold Laski, and the Webbs on the left, Winston Churchill on the right. Opponents included G. K. Chesterton, the Catholic church, and Josiah Wedgewood, a radical libertarian M.P. (Ridley, 199,9 pp. 292–295).

3 A still cruder form, exposing sickly infants, predates Heinlein by several thousand years.

4 The obvious problem is that examining an egg or sperm is likely to damage it. Heinlein’s ingenious solution – I do not know if it was his own invention or not – was to take advantage of the fact that each sperm contains half of the father’s genes, having been produced by a process of division from a cell that contained all of them. The full genotype is reproduced in every cell, so we can, given suitably advanced gene-mapping technologies – which now exist and will soon be cheap – get it by destructively analyzing a few of them. We then analyze one-half of the split, destroying it in the process, and deduce what must be in the other half. We now know the genetic content of a sperm that we have not examined and so have not injured. An analogous process could be applied to eggs. In both cases, it probably requires that the final stage in producing egg or sperm occur outside the body where we can keep track of what is happening.

5 Strictly speaking, clones created from an adult cell are identical only in the nuclear DNA; the mitochondrial DNA, which comes from the egg, is different unless the egg the nucleus is inserted into is from either the same individual as the cell or that individual’s maternal ancestor (mother, mother’s mother, …) or someone sharing an ancestor in the direct maternal line (mother’s daughter, mother’s mother’s daughter, …). In the rest of the discussion I will ignore this complication for purposes of simplicity.

6 Lee Silver discusses this possibility in Remaking Eden.

7 In the case of two women, if the child is entirely theirs it must be a daughter, since neither has a Y chromosome to contribute. In the case of two men, it might be either a son or a daughter, since a male carries one X and one Y chromosome.

Lee Silver (1998) describes two other technologies that could be used to produce children for same-sex couples. Each, however, gives a child who is only genetically 25% the product of each parent. In one case the child is a chimera – an organism produced by fusing two fertilized eggs, giving an individual half of whose cells come from one egg and half from the other. In the other case the child is, genetically speaking, the grandchild of the parental couple – the intervening generation having been aborted and the cells necessary to produce a fertilized egg harvested.

8 Different sources give different estimates; I am using 100 trillion, here and later, since it is a conveniently round number.

9 One such child became famous as the “bubble boy.”

10 The tests were the Standardized Assessment Tests given, in the U.K., at the age of 7. Many news stories confused them with the U.S. SAT exam, given to students who are applying to college. An abstract of the original article is webbed.

11 See Heinlein, 1948, for a fictional account of an unsuccessful attempt to breed aggression out of the species.

12 This was one of the issues underlying Levin’s The Boys From Brazil, 1991, a novel about a secret project to produce multiple clones of Adolf Hitler, later made into a successful movie.

13 Two possible examples of small-scale projects along those lines are the reported attempt by King Frederick William of Prussia to breed tall men to tall women in order to produce tall recruits for his regiment of “Potsdam Giants” and attempts by Hitler to get the best of German men and women to mate.

14 A real-world example – involving a kitten not a baby – was reported in late 2004.

15 Accounts of sex selective abortion, of female infanticide in India and China, of future demographic effects on China associated with an aging population and a high m/f ratio.

16 One firm doing sperm sorting claims an 88% success rate for producing female offspring, 73% for male. Another uses IVF and preimplantation genetic diagnosis to get a success rate claimed to be near 100%.

17 An article on the effect of the availability of inexpensive blood tests to determine the gender of the fetus.

18 For a discussion of the relevant economics, see Friedman, 1986, chapter 21.

19 Two interesting fictional discussions of these issues are Heinlein, 1966, and Schulman, 1983. The latter describes a world with a very high male-to-female ratio where women are drafted into temporary prostitution.

20 Silver, 1998.