There is general agreement among the scientists at dinner that, when it’s safe and practical, heritable edits ought to be used to fix bad single-gene mutations, such as Huntington’s disease and sickle-cell anemia. But they recoil at the idea of using gene editing for human enhancements, such as trying to give our kids more muscle mass, or height, or perhaps someday higher I.Q.’s and cognitive skills. The problem is that the distinction is difficult to define⁠—is preventing obesity a cure or enhancement?⁠—and even more difficult to enforce. “Look at what parents are willing to do to get kids in college,” Feng Zhang says. “Some people will surely pay for genetic enhancement.”

“A big problem with enhancement is equal access,” Sontheimer adds. “Should rich people be allowed to buy the best genes they can afford?” That could lead to the dystopia described in Aldous Huxley’s 1932 novel Brave New World, in which the modification of embryos produces a caste system ranging from intelligence-enhanced leaders to stunted menial laborers. Our world is already suffering from widening gaps in wealth and opportunity, and a free market for genetic enhancements could produce a quantum leap in these inequalities and also, literally, encode them permanently. “In a world in which there are people who don’t get access to eyeglasses,” Feng Zhang says, “it’s hard to imagine how we will find a way to have equal access to gene enhancements. Think of what that will do to our species.”