Leonard Hayflick in the US during the early 1960s showed that normal populations of embryonic cells divide a finite number of times. He published his results as 'The Limited In Vitro Lifetime of Human Diploid Cell Strains' in 1964. Hayflick performed the experiment with WI-38 fetal lung cells, named after the Wistar Institute, in Philadelphia, Pennsylvania, where Hayflick worked. Frank MacFarlane Burnet, later called the limit in capacity for cellular division the Hayflick Limit in 1974. In the experiment, Hayflick refuted Alexis Carrel's hypothesis that cells could be transplanted and multiplied indefinitely from a single parent cell line.

In 1999, John Ancona Robertson, a researcher who studied bioethics and law, published “Ethics and Policy in Embryonic Stem Cell Research,” hereafter “Ethics and Policy,” in the Kennedy Institute of Ethics Journal. In the article, Robertson analyzes the ethical debates around research that uses human embryonic stem cells, or ESCs, and categorizes the different kinds of concerns within those debates. Researchers obtain human ESCs from human embryos. ESCs are pluripotent, which means they have the capability to transform into various cell types, such as skin cells or muscle cells. Robertson explains that ESCs have great medical potential, but since obtaining them requires destroying embryos, some people oppose the use of ESCs in research. After reviewing the main concerns present in debates over ESC research, he concludes that ESC research is still morally permissible. With “Ethics and Policy,” Robertson argued that ESC research should go forward and be federally funded in the US at a time when many politicians, bioethicists, and scientists opposed such research.