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It seems as though the act of crying when sad does nothing to relieve that sadness. Is there an advantage to crying from an evolutionary perspective, or is it the end result of a different process? Also, why is there the same physiological reaction of crying when in sadness or in pain?

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Purely anecdotally it stirs empathy in peers so could lead to protection from other members of the group when an individual is vulnerable. –  Rory M May 3 '12 at 17:57
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Just a note, not every behaviour (or generally phenotype) has to be advantageous. This question just assumes that there has to be one, and while I agree with Rory about the probably effect of this one, it’s not a valid assumption in general. –  Konrad Rudolph May 3 '12 at 19:57
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@RoryM Yes but why does it stir empathy in peers rather than, say, blinking quickly? –  Jez May 3 '12 at 22:07
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I've seen various odd theories, but most commonly, it's just assumed to be a social signal. There's no inherent survival advantage to turning up the corners of your lips when you feel happy, either, but there is probably an advantage to letting other members of your group know how you're feeling. There may even be specific advantages to having this happen automatically and in ways that are slightly difficult to fake (tears or duchenne smiles). –  octern May 3 '12 at 22:08
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As for why we use tears rather than some other kind of signal: the "choice" could have been a fluke of evolution (people coordinated around the first good signal that happened to develop). There's also some work suggesting that our physiological and behavioral responses to pain and sadness are mediated by proinflammatory hormones, so maybe it's not a coincidence that being sad superficially resembles having hay fever... –  octern May 3 '12 at 22:11
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First, here is a link to an article that discussion the answer to your question:
Why Cry? Evolutionary Biologists Show Crying Can Strengthen Relationships

The original article to which it refers can be found here:
Emotional Tears as Biological Signals

To speculate a bit on Jez's question, crying in adults could be a result of our possible Neoteny. Crying in infant animals is extremely common as a way to elicit attention and sympathy from the mother. If we are neotenous apes, crying could be a left-over behavior that was seized by evolution, instead of being discarded.

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It seems to me that your neoteny hypotesis just shifts the question. How did crying evolve in human infants? Animal pups only have the "vocalization" part of it, by the do not shed tears... (also, as Wikipedia reports, human newborns often cry without weeping, probably because of insufficient development of their nervous control) –  nico May 4 '12 at 6:54
    
@nico You're right, it's not a very satisfying hypothesis. I don't have the answers to your questions, but they are good questions. –  Preece May 4 '12 at 7:00
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