Interesting question. At root, I think that we all have the same basic ethics -- we're social creatures, and some amount of shared ethos keeps any society going (don't kill - at least within your social group!, don't harm, etc). I think some is built in, though obviously there are individuals who are mis-wired. If nothing else, we have to have some empathy and compassion to survive as a society; empathy, 'the shared human condition', regulates our behavior towards others.

On a more personal, conscious level, my mom gave me three rules when I went out to play. "Don't hurt yourself, don't hurt others, and never make a mess bigger than you can clean up." Works for me.

There are other more complex, philosophical issues where I arrived at my stance through reading, debate, or whatever, but those are highly specific (for instance, I'm very hard line about freedom of speech, even when it's uncomfortable -- one of the few places I see in black and white).