Slate's bioethics correspondent has made a scathing attack on this month's decision by the UK's fertility regulator to allow pre- implantation genetic diagnosis for flawed embryos. William Saletan says that the decision by the Human Fertilisation and Embryology Authority shows that the "slippery slope" is a reality and that it can be measured in three ways.

The first is "penetrance", the probability that a gene will lead to a disease. The old standard was that a 90% probability would justify PGD. Now a 30-80% is enough. The second scale is treatability. The old standard was that screening was only allowed when treatments would be "awful or unreliable". But now a mere risk of failure, not a certainty of failure, is enough. The third scale is age of onset. Originally PGD was allowed only for diseases which were present in a child when it was born. Now the diseases for which PGD is allowed can show up at the age of 40. The HFEA even asks whether PGD should be used to screen out diseases which will not develop until a person is 70 or 80.

Saletan complains that the criteria for destroying selected embryos are not only changing and slippery but subjective. "Significant anxiety" in the carriers of the gene is also reason enough for PGD, according to Dame Suzi Leather, the head of the HFEA. And to rebut the HFEA's claim that it is trying to set boundaries for reproductive technology, he cites an internal briefing paper which says that the HFEA had "no intention following this discussion to define limits for conditions that should not be tested for using PGD."