If you're asking about predisposition, then the behavior occurring in children is evidence that there's a natural inclination. We just try to socially condition against it.

There have also been many psychological experiments establishing that human nature is predisposed towards self-interest. Even when most people are decent, a majority still steal and cheat on a small scale as found by Dan Ariely:

"Across all of our experiments, we've tested maybe 30,000 people, and we had a dozen or so bad apples and they stole about $150 from us. And we had about 18,000 little rotten apples, each of them just stole a couple of dollars, but together it was $36,000. And if you think about it, I think it's actually a good reflection of what happens in society."

Now just imagine that those subjects were in a Lord of the Flies situation or were starving. You'd quickly see that predisposition in glaring focus.