Nature vs. Nurture

Since having children I have often contemplated the role of nature vs. nurture in the child-rearing process. There are numerous behaviors that have manifested themselves in my children that I have certainly never taught them, then there are many that I want to believe I have instilled.
For example, when I had my first baby I was intent to train him to be a good sleeper. My doctor, as well as the books by all of the 'experts', assured me that I could indeed train my child to sleep. Sure enough, when Eli was 5 weeks old he slept through the night for the first time and not long after that it became the routine. It was not easy, it did not 'just happen' but my consistent efforts worked and I was elated. However, to a person, when I told people what an easy baby I had they told me "JUST WAIT, your next one will be crazy, wild, impossible...nobody has two good babies". This was discouraging news because I had gotten quite used to my happy, healthy, good-sleeping baby. When I expressed my concern to my pediatrician he assured me that if I did all the same things with baby number two that I did with baby number one I would have another good sleeper. When Eli was 6 months old I got pregnant with baby number two. (Yes, that news led to a temporary meltdown but that is for another posting). My beautiful daughter Margaret Jane was born just after Eli had turned one year old. We brought our little princess home from the hospital and after 3 weeks she slept through the night for the first time. I felt a sense of confidence (and relief) unparalleled in my adult life. My little princess began sleeping through the night on a regular basis at about 5-6 weeks old. Nurture came through in flying colors and to date I have proved all of the critics wrong. You can, indeed, have two good babies. I have two sweet, smart babies that both sleep through the night - don't hate me.
Now that my babies are 2.5 and 1.5 I am exploring the nature vs. nurture concept all anew. On the one hand, Eli turned into a terrible two as if some mystical force gripped him around May 2008. Whining, temper tantrums, complete stubbornness. I had not taught my son to fall out on the floor, kick his legs, and demand his way. (or had I?) And I certainly did not teach him to yank every toy from the hands of his baby sister or knock her to the ground if he perceived she was getting more than him. Nature had taken over and it was ugly.
Earlier today, as I was exasperated after a long, tiring day of chasing around a one and two year old my daughter fell down and bumped her head and began to wail. Without missing a beat my son ran over to her and wrapped his arms around her, kissed her on the cheek and said, "it's okay baby, I'm sorry you are hurt".
Nature or Nurture?
Or how about this one: Eli just yanked a toy away from his baby sister and shoved her to the ground. Jason looked at him and asked with a half sarcastic tone, "Now, Eli, what would Jesus do?" Eli looked at his daddy and said, "Jesus would paddle my butt".
Let's call that nurture.