If the argument is sound under careful, critical consideration, and you approve of its motivation, then allow the emotional appeal to move you.
My instinctive feeling is that there is often a real dichotomy between emotional and rational argument-based appeals. When constructing your message, if you use one approach heavily enough you do so at the expense of the other.
If that's true, then dismissing all the emotional parts of an "emotional appeal" will often not leave the strongest "rational", non-emotional argument behind. Sometimes removing all the emotion won't leave behind an argument at all! To judge non-emotionally, you'd be better off looking for non-emotional messages the same source has targeted at other audiences which are perceived to shun emotional appeals, such as scientifical publications.
Now, why is there a dichotomy in the first place? I don't think it's mainly due to choice between different styles of argument (emotional vs. non-emotional). I suspect it's due to bias in the listeners.
We Lesswrongers know that rationality (truth) and emotions are orthogonal and complementary, rather than opposite and exclusive. But most people have the Spock mental model of "rationality" or "cold sober facts". So I conjecture that when such people hear an emotional appeal, and in the middle of it a cold hard fact, the mere presence of that fact and of the appeal to rationality detracts from the power of the main emotional-appeal story - even if the fact is an excellent and convincing one. And conversely, when people are listening to a fact-based "rational" appeal, inserting emotion will lower their confidence and agreement - even when the emotion is a wholly appropriate one.
I submit this as my instinctive appraisal, I don't have hard data like deliberate studies to back it up.
In a comment elsewhere, BrandonReinhart asked:
I suspect that we percieve a dichotomy between emotional appeal and a well-reasoned, well-evidenced argument.
I have a just-so story for why our kind can't cooperate: We've learned to distrust emotional appeal. This is understandable: the strength of an emotional appeal to believe X and do Y doesn't correlate with the truth of X or the consequences of Y. In fact, we are surrounded by emotional appeals to believe nonsense and do useless things. The production and delivery of emotional appeal is politics, policy, and several major industries. So, in our environment, emotional appeal is a strong indicator against rational argument.
In order to defend against irrationality, I have a habit of shutting out emotional appeals. I tune out emotive religious talk. I remain carefully aloof from political speeches. I put emotional distance between myself and any enthusiastic crowd. In general, my immediate response to emotional appeal is to ignore the message it bears. It's automatic now, subverbal -- I have an aversion to naked emotional appeal.
I strongly suspect that I'm not only describing myself, but many of you as well. (Is this true? This is a testable hypothesis.)
If we largely manage to broadly ignore emotional appeal, then we shut out not only harmful manipulations, but worthwhile rallying cries. We are motivated only by the motivation we can muster ourselves, rather than what motivation we can borrow from our peers and leaders. This may go some way towards explaining not just why Our Kind Can't Cooperate, but why we seem to so often report that Our Kind Can't Get Much Done.
On the other hand, if this is a real problem, it suggests a solution. We could try to learn an alternative response to emotional appeal. Upon noticing near-mode emotional appeal, instead of rejecting the message outright, go to far mode and consider the evidence. If the argument is sound under careful, critical consideration, and you approve of its motivation, then allow the emotional appeal to move you. On the other hand, I don't know if this is psychologically realistic.
So, questions:
I hypothesize that we are much more averse to emotional appeals than the normal population. Does this stike you as true? Do you have examples or counterexamples?
How might we test this hypothesis?
I further hypothesize that, if we are averse to emotional appeals, that this is a strong factor in both our widely-reported akrasia and our sometimes-noted inability to work well together. How could we test this hypothesis?
Can you postpone being moved by an emotional appeal until after making a calm decision about it?
Can you somehow otherwise filter for emotional appeals that are highly likely to have positive effects?