Wash away your sins. An era of moral decay. Keep your nose clean. Corruption. There are endless examples of metaphors equating morality with cleanliness, and vice with filth. Business school researchers at Toronto and Northwestern have found that this is not just a matter of language; being associated with something immoral leads to an urge to physically clean oneself.
Zhong and Liljenquist have conducted three studies of the “Macbeth effect,” the need to cleanse oneself after a threat to one’s moral purity (Science 8 September 2006: Vol. 313. no. 5792, pp. 1451 – 1452; abstract). New Scientist of 7 September 2006 describes the experiments.
There seems to be an active mental mapping between morality and cleanliness. Ethics is abstract, and we activate our corporeal instincts when thinking about it. Perhaps this is the only way we can think of morality in an extensive way. If that’s true, then ethical concepts that can’t be modeled physically cannot exist.
I don’t know if one can disprove this hypothesis. A candidate concept has evidently been thought of; it then remains to show that there is no physical correlate. However, language is pliable enough that one will always be able to draw link to a bodily metaphor. Whether this is persuasive will be a subjective judgment. Perhaps neural mapping and brain imaging will eventually be able to help.