No doubt I, like most of us, learned ethics and morality from whoever raised us. Even toddlers quickly learn that if they hit another child, they're going to get whacked back. Later you learn that society imposes penalties on you for poor behavior. Mostly though I think that we live with a sort of "inner hell," that thing that makes you feel terrible if you transgress your own code of ethics. Like shooting the neighbor's dog for peeing on your garden. It doesn't necessarily have anything to do with religion or a belief in a deity, or even secular law. Secular law won't jail you for going up to an old lady and telling her how ugly you think her dress is. It's your inner hell that keeps you from being an asshole.
You studied psychology. Is a sociopath someone who lacks that "inner hell" and thus never feels guilt or shame?
Does that mean you believe there is no universal truth in human morality?
Mental illness isn't like pregnancy which is "either you are or aren't". It's a matter of degrees. Sociopaths, depending upon the degree do lack an "inner hell". A fully sociopathic person is like a
solipsist and does not feel the pain of others since they believe only their own pain (and pleasure) matters.
Re "inner hell": Most people have, as Freud saw it, an Ego (self), Superego (social conditioning) and Libido (base desires). Example; a man may really, really want to bang his neighbor, even against her will (Libido), but knows it's wrong (superego) and knows doing so is a one way trip to jail and/or divorce so, out of self-preservation (ego) he doesn't do it and sticks to free porn on the Internet.
Personally, my belief is it's a complex question since every person on the planet is the product of millions of years of evolution meaning a genetic component to behavior. Obviously a species that habitually murders its own won't survive. A species that has an innate sense of protecting its own kind, even at risk of its own life, is much more likely to survive. The individual dies and the species lives on. That's evolution in a nutshell.
There's also the cultural aspect where some cultures are more effective at survival and species propagation than others. That's social evolution; totally external to the body. Consistency is important here which is why a cultural touchstone is often a plus be it a Bible, Mein Kampf or The Communist Manifesto. People being people often outgrow physical touchstones such as books as they socially evolve.
In short, I think there are universal truths such as 2+2=4 and certain behaviors are best for the continuation and advancement of the species. I believe people are products of both their genetics and their culture. I also believe, even within the limitations of human genetics where we are all 99.5-99.9% genetically identical, that there is an infinite variety of human thought due to free will. Music is like that; musical notes are finite. The number of combinations of those notes is nearly infinite. Mankind is pretty good at writing music.