In 2018, the first genetically modified babies were reportedly born in China, made possible by the invention of CRISPR technology in 2012. This controversial advancement overturned the preexisting moral consensus, which had held for over 50 years before: While gene editing an adult person was morally acceptable, modifying babies, and thus subsequent generations, crossed a significant moral line. If this line is passed over, scientists will be left without an agreed-upon ethical limit. What do we do now?

John H. Evans here provides a meta-level guide to how these debates move forward and their significance to society. He explains how the bioethical debate has long been characterized as a slippery slope, with consensually ethical use at the top, nightmarish dystopia at the bottom, and specific agreed-upon limits in between, which draw the lines between the ethical and the unethical. Evans frames his analysis around these limits, or barriers. He examines the history of how barriers were placed, then fell, then were replaced by new ones, and discusses how these insights inform where the debate may head. He evaluates other proposed barriers relevant to where we are now, projects that most of the barriers suggested by scientists and bioethicists will not hold, and cautiously identifies a few that could serve as the moral boundary for the next generation.