We'll listen to a person just long enough for them to make a single misstep or mistake, or say some other thing we disagree with, and then POW, we use that one little thing to dismiss everything they've ever said and done. "Oops, he used a debatable statistic in his second paragraph. No sense in finishing this article." Or, "No, don't listen to him. He once shook hands with a politician I hate." Or, "He thinks the tax burden has disproportionately shifted over the past couple decades onto the middle class? What does he know? He owns an Xbox."
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They're barely even human.
This is also why we love assigning labels to people. It saves us the work of finding even one thing about them to disagree with, by just assigning them all the flaws of whatever group they're in. If you already know that you disagree with everything a "Conservative" or a "Liberal" or an "Xbox owner" says, it saves you the trouble of actually listening to them. "How could rational thought come out of such small heads?" you reasonably cry.
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