Lead exposure in childhood is one thing we know leads to poorer impulse control; that is to say, being short-sighted and selfish.

I think it's most to do with exactly how much wealth a person was born into.

Someone who is constantly on the verge of being homeless, possibly even starving, will very soon get desperate enough to do things they aren't proud of to survive.

On the other hand, someone born into immense wealth will have very little understanding of people who are struggling. That'll make it very hard for them to empathize. And with all that power, their sheer indifference will have them crushing the poor under their boots like ants. This is highly exacerbated by the fact that it is in their best interest to support policy that transfers wealth upwards.

The anthropologist Sarah Blaffer Hrdy has theorized that empathy and altruism are acquired in early childhood by observing when one’s caretakers show empathy towards oneself, which both gives infants a sense of security in interacting with other people and which, because babies learn through observation and pattern recognition, establishes a standard in their minds that tells them humans are supposed to be selfless and kind to one another. This theory is relevant to us because, since most of us live in decaying neoliberal hellholes, many people’s parents are too busy working to give them appropriate care during their formative years and are not rich enough to pay for people who are qualified to do so in their stead. When it comes to people whose parents are rich, the selfishness that lead to them accumulating wealth could get passed on to their children by the same method. In other words, the conditions created by capitalism could be fucking with the ability of infants to become good people later in life

I think it's all about empathy.

The ability to experience empathy, like any other trait varies at the biological level (and probably other levels as well). It only makes sense some would be born with little ability to experience empathy, some with so much empathy they worry about ants, but most people in the middle.

Those with little empathy (and therefore little sympathy or compassion, as those come from empathy) don't allow morality to restrain their behavior so they tend to rise to power.

Most of the world's problems are because people who are effectively psychopaths are in charge everywhere. The psychopaths have created a world through capitalism that pits us against each other and brings out the worst in us.

A lot of those shitty things aren't normal in humans, but become much more common if you force them into violent, existential conflicts.

And insatiable greed is promoted as good at almost every level of neoliberal society, so that one should be self-explanatory.

People just say "human nature" as an excuse for their own conservatism

I don't have an answer to you but if you like this sort of discussions/topics, I highly recommend the book Behave by Sapolsky.

I would guess a person's life circumstances. Humans tend to reflect pain they're experiencing onto others. If you're hungry, that's a form of pain. If you're neglected, that's a form of pain. Even the sons of billionaires experience pain, probably in a way that's hard for reg folks to grasp.

Also, I think your dopamine regulation being all messed up might also contribute. Those people who seem to flail and rage because nothing can seem to satisfy them.

Also, everyone does terrible things and hurts others. You can be charitable and kind from one perspective and an oppressor in ways you won't allow yourself to see. We all do it.

We know that people who have less tend to be more generous than people who have more.

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.

The long branch is the long arm of altruism, the short branch is the short arm of altruism.