after Mackie, I'm going to say asking anyone to defend their values is a joke.

I provided fairly extensive argument earlier in the thread.

Thats only a problem if you believe value derives from evolution, a proposition you presented but which I dont hold. — Ourora Aureis

If not evolution then from God?

You even mentioned social and cultural factors yourself, but then you immediately overule them with the gene propagation idea. — Ourora Aureis

I wrote that valuing human life is ingrained and the reason (you requested reasoning) I think it's ingrained is because it promotes gene propagation and the survival of our species.

I already presented why I think people hold these values, and its mostly a case of religious philosophy, not some innate emotional reaction that derives from their biology. — Ourora Aureis

I thought you were asking me why I value zygotes. Granted religious thought has been somewhat ingrained in me even though I've never really been religious.