Would you give up your life to save your child's life? A lot of people would say yes and in fact a significant number of them would say that it is something a parent must do. Would you give up somebody else's life to save your child's life? This is trickier, of course, and less prone to emotional hyperbole. Obviously an awful lot of people would choose that a random stranger die instead of their own child; in fact probably the great majority would. How about if the person you have to sacrifice is your partner instead?
Wendy linked this blog post about parenthood which was written from the perspective of a father. He said that after his child was born he suddenly realized that he would be willing not only to die to save his child but also to throw his wife under a bus if that is what it took. Presumably this means he would be willing to murder absolutely anyone to save his child which I think was meant to be heartwarming but mostly managed to be terrifying. I understand the desperate desire to prevent any harm from coming to my child but the idea that I would murder my wife to prevent my daughter's death is completely mindboggling. Even if it wasn't murder but rather just a 'you can only save one of them from certain doom' sort of made up situation I would save my wife. Would I sacrifice myself? I think the answer is no, but that is the hardest question.
I wonder if these values are things that come from inherent personality characteristics or are a product of environment. My father sometimes talked about how his father made it clear that the children came a distinct second behind his wife so maybe these values are passed along within social groups. I don't recall being told explicitly how my parents viewed this sort of situation (and honestly I don't know how they would answer) but I suspect these sorts of values will often be communicated implicitly. It is certainly true in the case of my grandfather, my father and myself that we would take extreme risks with our own lives to save our children. There is a world of difference between extreme risk and certain death though and nearly all parents will do the first but many would not do the second.
Of course much of this discussion is entirely moot because real life very rarely grants us scenarios where we can guarantee saving a life by sacrificing a life. Far more often when these things come up it is a momentary decision made to do something very dangerous in the hopes of saving someone who might survive anyhow. Even more relevant is that what we think we will do in these situations is often unrelated to what we will actually do - few people think they will commit murder when ordered to by an authority figure but in fact they often will. It might be that our answers in the cold, rational state of typing have no bearing at all on what we would really do in the heat of the moment. Saying "I would do the right thing!" is easy but when the knife is in your hand and adrenalin floods your body actually following through is hard.