The two messages were equally effective: in both cases, 6.5 percent of the stingy alumni ended up donating. Then we combined them, because two reasons are better than one. Except they weren’t. When we put the two reasons together, the giving rate dropped below 3 percent. Each reason alone was more than twice as effective as the two combined. The audience was already skeptical. When we gave them different kinds of reasons to donate, we triggered their awareness that someone was trying to persuade them—and they shielded themselves against it. A single line of argument feels like a conversation;
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