halacha - What is the effect on me of saying amen?

I understand that if I hear another person make a bracha (and perhaps even if I don't actually hear it), I may? must? respond "amen". I also understand that saying "amen" can allow me to be yotzei (fulfill my obligation) through that other person's bracha, so, e.g., we don't all have to make kiddush individually. I understand that the speaker has to have some intention to include others in that latter case.

My question is about how these interact:

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  Suppose I overhear someone make a bracha that he intended only for himself (e.g. he's about to eat something). If I say "amen" am I just being polite, am I fulfilling a mitzvah even though I get no benefit from the bracha itself, or do I now have to eat?

  
  
  


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  Suppose I overhear someone make a bracha that he intended to include others in, though not necessarily me specifically (e.g. motzi at the next table). If I say "amen" am I now yotzei through him because he intended to include others? What if I don't want to be yotzei through him?

  
  


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  Suppose I hear someone who intended to include me make a bracha. If I say "amen" with the intention of being yotzei, am I? What if I don't want to be -- do I say "amen" and I'm not, do I not say "amen", or am I yotzei whether I want to be or not?

  
  

Not wanting to be yotzei in these last two cases could be either because I've already made the bracha myself (e.g. I lit my own chanukiyah before joining friends) or because I intend to make my own bracha later. So either way, I want to avoid a bracha l'vateilah.

When does saying "amen" make me yotzei, when does it not, and when does it give me the option?

Answer

First of all, I have to correct a common misconception in your question: that one must say Amen in order to fulfill one's obligation in the blessing. This is only true if the one saying the blessing is not fulfilling his obligation then as well. In the vast majority of cases where the one saying the blessing is fulfilling his obligation with you, you can fulfill your obligation by simply listening to the entire blessing. (Rambam Brachot 1:11, see also R. Akiva Eiger to OC 219:5) I note that the former case (fulfilling someone else's obligation without doing so yourself) is not allowed in general for food blessings except those which directly relate to Mitzva food (eg. Kiddush, Matza at the Seder). (Rambam Brachot 1:10, Shulchan Aruch OC 167:20)

Additionally, in order for a listener to fulfill his obligation, both the listener and the speaker must intend for that to happen. Additionally the listener must hear every single word (not just the end). Both these statements are true even if the listener said Amen. (OC 215:3)

Now in terms of your second question, the answer will depend on the one saying the blessing's intention: if his intention is to fulfill the obligation of anyone who needs (as is the general assumption in the case of a Chazzan) then you can fulfill your obligation if you hear the whole thing etc. But if his intention is just for a specific group of people, then only those people can fulfill their obligation. (parallel to OC 589:9 and MB sk 17) As for how to determine the intent of the one saying the blessing at the other table, I'm not sure and you might just have to ask him. It seems to me to be a good practice to always have anyone who wants to fulfill their obligation in mind when you say blessings in case someone tries to fulfill his obligation without realizing he needs to be careful for this. As above, if you ever don't want to fulfill your obligation with this blessing just don't intend to; problem solved.

Finally as to your first question, the Shulchan Aruch rules (OC 215:2) that if one hears an entire blessing from a believing Jew even if one does not intent to fulfill one's obligation with it, one is obligated (חייב) to say Amen. (I phrased that very minimalisticly; there are other instances where Amen should be said but I don't want to complicate this answer any further.) Moreover, the Gemara says (Brachot 53b) that the one who answers Amen is greater than the one who said the blessing! The Rashba (Shu"t 5:53) explains this to mean that a praise of God is not complete without others confirming it, similar to a contract which needs signatures of witnesses to make it complete. In this way, the one answering Amen is doing more than the one saying the blessing could have done on his own.