[W]henever we heard an unflattering portrait of our own side our first question to ourselves was not “Is this true?” but “What are they trying to hide about themselves by accusing us of this?” Once this mental defense system had been perfected, few criticisms could hit home.
Markus Wolf, Man Without a Face: The Autobiography of Communism’s Greatest Spymaster, New York, 1999, p. 40
[C]onsider the convention against the use of ad hominem arguments in science and many other arenas of disciplined discussion. The nominal justification for this rule is that the validity of a scientific claim is independent of the personal attributes of the person or the group who puts it forward. Construed as a narrow point about logic, this comment about ad hominem arguments is obviously correct. But it overlooks the epistemic significance of heuristics that rely on information about how something was said and by whom in order to evaluate the credibility of a statement. In reality, no scientist adopts or rejects scientific assertions solely on the basis of an independent examination of the primary evidence. Cumulative scientific progress is possible only because scientists take on trust statements made by other scientists—statements encountered in textbooks, journal articles, and informal conversations around the coffee machine. In deciding whether to trust such statements, an assessment has to be made of the reliability of the source. Clues about source reliability come in many forms—including information about factors, such as funding sources, peer esteem, academic affiliation, career incentives, and personal attributes, such as honesty, expertise, cognitive ability, and possible ideological biases. Taking that kind of information into account when evaluating the plausibility of a scientific hypothesis need involve no error of logic.
Why is it, then, that restrictions on the use of the ad hominem command such wide support? Why should arguments that highlight potentially relevant information be singled out for suspicion? I would suggest that this is because experience has demonstrated the potential for abuse. For reasons that may have to do with human psychology, discourses that tolerate the unrestricted use of ad hominem arguments manifest an enhanced tendency to degenerate into personal feuds in which the spirit of collaborative, reasoned inquiry is quickly extinguished. Ad hominem arguments bring out our inner Neanderthal.
Nick Bostrom, ‘Technological Revolutions: Ethics and Policy in the Dark’, in Nigel Cameron & Ellen Mitchell (eds.), Nanoscale: Issues and Perspectives for the Nano Century, Hoboken, New Jersey, 2007, pp. 141-142