Ad hominem circumstantial
Ad hominem circumstantial involves pointing out that someone is in circumstances such that he is disposed to take a particular position. Essentially, ad hominem circumstantial constitutes an attack on the bias of a person. The reason that this is fallacious is that pointing out that one's opponent is disposed to make a certain argument does not make the argument, from a logical point of view, any less credible; this overlaps with the genetic fallacy (an argument that a claim is incorrect due to its source). Noting the opponent's bias is not necessarily irrational, but neither is it strictly correct according to logic. This illustrates one of the differences between rationality and logic.
Examples:
"Tobacco company representatives are wrong when they say smoking doesn't seriously affect your health, because they're just defending their own multi-million-dollar financial interests."
"He's physically addicted to nicotine. Of course he defends smoking!
Of course, such statements could also be reworded to avoid the logical fallacy:
"Tobacco company representatives may be biased when they say smoking doesn't seriously affect your health, because of their own multi-million-dollar financial interests. Thus, such statements may be wishful thinking, or even outright lies, on their part."
"He's physically addicted to nicotine. Therefore, his defense of smoking may be biased."
