Empathy is probably your best bet as far as a single variable goes. But otherwise we're talking about something that's incredibly complex on multiple levels, making it near impossible to address as a whole.

I like to envision human behavior and consciousness as a network of tensions and influences. (Perceived) material interests are one such tension, a particularly strong one. Strong enough that I feel confident saying that in general, people will tend to drift towards approximating an ethic that aligns with their material conditions.

The archetypes and behaviors modeled for us in our childhood and throughout our lives are a sort of structure that these forces interact with. We may have empathetic or selfish responses modeled for us by our parents, so those are the responses that spring to our minds when decisions arise. Good behavior modeling could mean the inherent tension towards self interest may be mediated or tempered by the limits of behavior we think to enact. Parents have a big impact on this early on, but so do later role models as well as media portrayals of people.

Social cohesion can be a big tension on people, incentivizing them to not act outside of group norms out of fear of being ostracized. Or on a more subconscious level, perhaps acting out of a "self" interest that benefits the social group, because the lines between Self and Other become blurred. Extending beyond the small self to consider the well-being of the large "Self", sometimes even at the expense of the small self.

Critical theory may be of interest to you.