In my personal opinion we should always answer, because (scientific) curiosity trumps all reasons not to. However, we should carefully analyze the message before replying and be careful about what we say.<p>As a hobby sci-fi author with a long-term interest in the topic, I'd like to point out one thing in the debate that is often forgotten. When it comes to aliens and their motivations, all bets are off. Even if the aliens were capable of FTL traveling, this does <i>not</i> indicate that they are far more advanced than us, that they are probably benevolent, that they would consider us inferior, or anything like that. They could be like us. They could be totally different. They could be more intelligent than us, but they could also be more stupid than us. Technological advances are unpredictable and on the cosmic scale occur in a ridiculously short time span. So they might just be a hundred years advanced in comparison to us, for example, and judging from our own development this does not indicate any major advance in society or biological evolution.<p>Unfortunately, that also means that seemingly highly advanced aliens might seek out to destroy all other life in the galaxy for religious reasons, for instance. Or they might want to do the opposite. We don't know. Therefore, unless there were specific reasons to distrust them, we should reply, just as we tend to reply phone calls from unknown numbers.