THIS YEAR, THE 50th ANNIVERSARY of the discovery of the structure of DNA has kindled many debates about the implications of that knowledge for the human condition. Arguably the most emotionally charged is the debate over the prospect of human genetic enhancement, or "designer babies." It's only a matter of time, many say, before parents will improve their children's intelligence and personality by having suitable genes inserted into them shortly after conception. A few commentators have welcomed genetic enhancement as the latest step forward in the age-old struggle to improve human life. But many more are appalled. They warn that it is a Faustian grab at divine powers that will never be used wisely by us mortals. They worry that it will spawn the ultimate form of inequality, a genetic caste system. In his book "Our Posthuman Future" (just released in paperback), the conservative thinker Francis Fukuyama warns that genetic enhancement will change human nature itself and corrode the notion of a common humanity that undergirds the social order. Bill McKibben, writing from the political left, raises similar concerns in his new jeremiad "Enough: Staying Human in an Engineered Age." But whether they welcome or decry it, almost everyone agrees that genetic enhancement is inevitable if research proceeds on its current course. Genetic enhancement is a major concern of the President's Council on Bioethics; its chairman, Leon Kass, and several of its members, including Fukuyama, are outspoken worriers. As it happens, some kinds of genetic enhancement are already here. Anyone who has been turned down for a date has been a victim of the human drive to exert control over half the genes of one's future children. And it is already possible to test embryos conceived in vitro and select those that are free of genetic defects such as cystic fibrosis. But when it comes to direct genetic enhancement-engineering babies so they will carry genes for desirable traits-there are many reasons to be skeptical. Not only is genetic enhancement not inevitable, it is not particularly likely in our lifetimes. This skepticism arises from three sources: futurology and its limits, the science of behavioral genetics, and human nature itself. The history of the future should make us raise an eyebrow whenever the experts tell us how we will live 10, 20, or 50 years from now. Not long ago, we were assured that by the turn of the century we would live in domed cities, commute by jet-pack, and clean our homes with nuclear-powered vacuum cleaners wielded by robot maids. More recently we were promised the paperless office, interactive television, the Internet refrigerator, and the end of bricks-and-mortar retail. It's not just that these developments have not yet happened. Many of them, like domed cities, never will happen. Even in mundane cases, technological progress is far from inexorable. Air travel, for example, is barely faster or more comfortable today than it was when commercial jets were introduced 50 years ago. Why are technological predictions usually wrong? Many futurologists write as if current progress can be extrapolated indefinitely-committing the fallacy of climbing trees to get to the moon. They routinely underestimate how much has to go right for a development to change our lives. It takes more than a single "eureka!" It takes a large number of more boring discoveries, together with the psychological and sociological imponderables that make people adopt some invention en masse. Who could have predicted that the videophones of the 1960s would sink like a stone while the text messaging of the 1990s would become a teenage craze? Finally, futurologists tend to focus their fantasies on the benefits of a new technology, whereas actual users weigh both the benefits and the costs.