The opposite is also true: a "negative halo effect" can be easily observed, wherein "bad" traits are also similarly grouped and feed on each other.

An interesting part of halo effects is that people seem to understand them on an instinctual level - not enough to get rid of them, but enough to exploit them...

I've drawn an extremely strong correlation in a particular online game between having a marijuana reference in one's handle and being bad at the game; being bad is not strongly linked with marijuana references, but that's only because they're in the extreme minority of the population; if you're sporting a "420", you're almost definitely underperforming. I've never bothered forming a hypothesis as to why this is, but it is.

So one day I decide to try a little experiment - for funsies, nothing rigorously scientific, just a "see what happens" thing - and predict aloud that an ally with a name referencing marijuana would perform poorly compared to the other players in the game. I turned out to be right (he was even worse than expected), but the interesting part was his response:

He implied my prediction was wrong, evidenced by that he was writing his college thesis on the effects of THC on the body.

The only way this statement makes sense is if we trace it through an expectation on his part that he can rebut the argument using the halo effect. "I am extremely accomplished academically," I could almost read on the screen, "Therefore, I am not a poor performer in this online video game."

Recently I contemplated writing an "Atheist's Bible", to present the most important beliefs of atheists. Eventually I realized that this Atheist Bible would not mention atheism. "Atheism" is just the default belief state we were born with. Atheism isn't having reasons not to believe religion; it's not having reasons to believe religion. If one knows how the world works, there are no gaps for religion to fill.

The French Encyclopedia of the late 18th century was by design an atheist work; it carried out this design by not mentioning religion.

Yes! I can't believe I don't see this repeated in one form or another more often. Fallacies are a bit like prions in that they tend to force a cascade of fallacies to derive from them, and one of my favorite debate tactics is the thought experiment, "Let's assume your entire premise is true. How might this contradict your position?"

Usually the list is longer than my own arguments.

Last November, Robin described a study where subjects were less overconfident if asked to predict their performance on tasks they will actually be expected to complete. He ended by noting that "It is almost as if we at some level realize that our overconfidence is unrealistic."

I think there's a less perplexing answer: that at some level we realize that our performance is not 100% reliable, and we should shift down our estimate by an intuitive standard deviation of sorts. That way, we can under-perform in this specific case, and won't have to deal with the group dynamics of someone else's horrible disappointment because they were counting on you doing your part as well as you said you could.