NICHOLAS D. KRISTOF: Birth Without the Bother?

Earlier this year in Gujarat, India, I came across a most unusual kind of outsourcing: womb-rental.

Americans looking for a surrogate mother to bear a child can save a fortune and avoid regulations by paying an Indian woman $4,000 or $5,000 to carry their fetus. An embryo that has been created in vitro by the American parents is implanted in the Indian woman’s uterus and she goes through the pregnancy and delivers the baby — and then hands it over to the Americans.

Ultimately, that kind of surrogacy could be mixed with genetic screening of embryos — to weed out babies of the “wrong” gender or with the “wrong” characteristics — to save busy couples the bother of pregnancy or the nuisance of chance.

Yes, all this gives me the willies, too. So some of the most monumental decisions we will face in the coming years will involve where we draw the line making some genetic tinkering legal and some illegal.

One of the crucial evolving technologies is P.G.D., or preimplantation genetic diagnosis. This allows a couple to test embryos that have been created in vitro when they are roughly three days old.

P.G.D. is now used principally to test for serious genetic diseases, including Down syndrome and Tay-Sachs. But it could equally be used to test for milder risks.

Five years ago, I tested my own DNA for 130 common genetic markers (a perk of journalism is the chance to test new technologies) and found that I have markers that give me slightly increased risk of blood clots, schizophrenia, Type 2 diabetes and Alzheimer’s. On the other hand, I didn’t have many other common genetic risk factors, including those associated with colon cancer, melanoma or breast cancer.

Everybody has some of these troublesome genetic predispositions. But in the future we could use P.G.D. to screen out these kinds of genetic risks.

Nonmedical screening would also be possible. Dr. Dean Hamer, a prominent geneticist, believes that the VMAT2 gene is the “God gene,” associated with spirituality. What if religious families prefer embryos with a genetic disposition for faith?

Michael Sandel, the Harvard philosopher, begins his new book on genetics, “The Case Against Perfection,” with the story of a deaf couple who sought a child who would be deaf as well. “Is it wrong to make a child deaf by design?” he asks, then refining the question: “Is there still something wrong with parents picking and choosing the kind of child they will have?”

Yes, there is.

Like Professor Sandel, I worry that our scientific capabilities may surpass our wisdom. Look at the dog kingdom. All of today’s dogs descended from wolves, and in less than 15,000 years we ended up with Chihuahuas and Great Danes. We may do the same to our own descendants.

As Liza Mundy notes in her fascinating new book, “Everything Conceivable: How Assisted Reproduction is Changing Men, Women and the World,” the main driving force in the new technologies is simply the profit motive.

“What is at work in assisted reproduction,” she writes, “is often not science but business.”

So where do we regulate and draw the line? My vote is to allow genetic technologies aimed at combating disease or infertility, but to bar any effort that goes beyond the curative to enhance the germ line DNA of our offspring.

International womb-rental troubles me but in the end would pass muster. It helps infertile American couples who might not otherwise be able to afford a baby, and the Indian women are thrilled with the chance to earn what for them are substantial sums, at less risk than with their other options.

Likewise, I would tolerate egg trafficking, a booming industry that offers women money to have their eggs extracted. Infertile couples need eggs — and why shouldn’t the donors be paid?

As for genetic screening, I would accept P.G.D. to cull embryos at risk for medical problems, even those that strike only in old age like Alzheimer’s. And my vote is to allow parents to use P.G.D. to choose the sex of a child in the U.S., although I would feel differently in countries like China or India where the son preference could create a huge shortage of girls.

What should cross the line into illegality is fiddling with the heritable DNA of humans to make them smarter, faster or more pious — or more deaf. That is playing God not just with a particular embryo but with our species, and we should ban it.