I. General Remarks on Slippery Slope Arguments

A slippery slope argument is a figure of reasoning with which an action that is morally acceptable in itself is conclusively shown to be morally impermissible because it has morally unacceptable consequences. In general, two variants of slippery slope arguments can be distinguished on the basis of two possible consequences: the causal variant and the conceptual variant.

The causal variant postulates that the introduction of a practice that is morally acceptable in itself will, with a certain probability, lead to the establishment of a morally unacceptable practice in the future. Therefore, contrary to first appearances, it is better not to introduce a practice that is morally acceptable in itself. A recurring weakness of causal slippery slope arguments is that in most cases it is difficult or even impossible to seriously state precise probabilities for the occurrence of the undesirable consequence. Even if plausible probabilities can be identified in individual cases, a causal slippery slope argument is only valid if the negative consequence is sufficiently important and its probability of occurrence is high enough to outweigh the foreseeable benefits associated with the introduction of the morally acceptable practice.

The conceptual variant of the slippery slope argument often occurs in a certain variation, the so-called vagueness variant: If action (1) is permissible, then a minimally deviating action (2) is also permissible, which in turn justifies an action (3) that goes beyond it, and so on. In this way, one finally arrives at an action (n) that is morally unacceptable in itself. However, since action (n) is logically connected to action (1) via a chain of minimally deviating actions, the argument goes, action (1) is already impermissible. Conceptual slippery slope arguments can fail because the minimal differences between the actions in question are ethically relevant, so that only the introduction of action (1) is justified, but not the introduction of actions (2) to (n).

Whether or not a slippery slope argument has the theoretical shortcomings outlined above must be examined on a case-by-case basis.

II. Slippery Slope Arguments in the Case of PGD

It is argued against the permissibility of PGD for the early detection of diseases with the subsequent possibility of killing the embryo in the event of a genetic defect that this would also make the use of PGD for other purposes (such as the selection of an embryo based on certain characteristics such as eye colour or intelligence) more likely (causal) or that it should also be consistently permissible in other cases (conceptual). However, since PGD should not be established beyond the early detection of diseases, PGD should also not be used for the purpose of early detection of diseases. As outlined above, the causal variant of this dam-break argument is only successful if plausible probabilities for the imminent expansion of PGD can be asserted and the negative consequences (weighted by their probability of occurrence) outweigh the positive consequences of PGD (weighted by the probability of their occurrence). Furthermore, if the causal or conceptual dam-break argument is to work, the expansion of PGD must actually be a negative consequence. This, for its part, is asserted by means of a dam-break argument:

If some parents-to-be were to use PGD for the purpose of selecting an embryo that is to grow up to be as efficient as possible, then, according to a causal dam-break argument, social pressure would arise on other parents-to-be to also inspect the genome of their embryos and select them according to efficiency-indicating criteria. This is because in a society with genetically selected people (depending on the predictive accuracy of PGD), performance is likely to be higher overall than in a society without such selection, which would tend to put all those who have not been selected at a competitive disadvantage. As a result, existing class differences in a society would be exacerbated and perpetuated, as socio-economically better-off groups would apply PGD to their embryos, in addition to the already existing advantages. Against the background of these foreseeable consequences, a liberal legal regulation that would allow the possibility of PGD for purposes other than the early detection of diseases would be ethically inadmissible. Even if the question of a concrete probability of occurrence (or even just a probability range) of the competitive dynamic is difficult to answer, such a development does not seem completely remote. Yet the follow-up question as to whether a more unequal society, because it was partly created on the basis of performance-orientated PGD, would be advantageous (higher GDP, higher social expenditure, etc.) or disadvantageous (competition, low esteem, etc.) for the socio-economically less advantaged groups is highly speculative. However, it is precisely this information that could influence the ethical evaluation of the use of PGD for purposes other than the early detection of diseases, so that the dam-break argument against PGD for the purpose of higher performance remains in limbo.