Can someone explain to me how the human instinct to cry works? What kind of activity is happening in the brain to cause this reaction? And why? What kind of biological purpose is this supposed to serve?
This is a WAG, but I think crying occurs when the emotional circuits in your brain get overloaded due to stress or confusion. As for a biological purpose, I can imagine it being a warning signal to parents that a child’s needs should be addressed before some harm is done (starvation, dehydration, exposure, etc…)
I don’t know for sure, but a psychiatrist might be along soon enough to give a more accurate answer…
crying is a means to influence others. People rarely cry when alone and when they do it is a short event. Cying is for the benefit of those around the cryer.
How does it go from being a warning signal to parents to a physical reaction to an emotional state, though? Weird.
Where’d you get that idea, Rhapsody?
I believe some of the most profound crying is done in private. I cry watching a sad movie all by myself.
Maybe it’s just me, though. 
Peace,
mangeorge
Me too. I find it hideously embarassing crying in public. (FWIW I’m male).
Datapoint:
I also am more likely to cry when alone than with an audience.
But Rhapsody may have a point, in that crying, if not meant to influence another, is meant to communicate with another. It signals honesty of emotion. Emotions are important to us, and it’s important to communicate about them, even if just to ourselves. At least, our biology apparently thinks so.
Chemically, crying is believed to remove chemicals from the body that build up during stress. May also release endorphins.
http://www.washingtonpost.com/wp-srv/WPlate/2000-01/12/029l-011200-idx.html
I’ve also been told (but cannot cough up a cite for it), that crying is a defense mechanism, a surrender reflex, “showing throat”, communicating “I am not a threat” to an adversary. I’m willing to believe that for the purely personal and empirical reason that I will find myself crying when I’m fencing if I believe I am outmatched. I don’t feel particularly sad about it; it’s an involuntary reflex.
Rhapsody-
Never lost a close family member, have you? Leads to a lot of crying in private. For a long time. For no real discernible benefit to the cryer re: other people’s reactions.
Really.