What sort of psychological changes could be made to a genetically engineered human

I'm in the rough outlining stages of this story, and found myself a bit stuck trying to turn this particular corner.

Firstly, here are the basics of this world:

In the future, it's a common practice to genetically alter embryos, rendering them immune to incurable diseases and protecting them from genetic defects.

As time went on, this procedure also included physically enhancing these future children, by ways of increasing height, increasing their ability to gain muscle, more appealing facial structures, etc.

However, in some of the seedier corners of society, people have experimented with changing the germline of offspring so as to make them not quite human anymore. This was done by testing the limits of what could be accomplished physically (keep in mind that this technology is still in its relatively early stages; the test-subjects would still look somewhat human-esque, not like some sort of eldritch horror). Since a physically-altered human already stirs plenty of unfavourable images in someone's mind, there's plenty of creative room to draw from there. The important thing is that, though potentially traumatized, they would still be fundamentally human on the inside, however skewed they may be physically.

At some point, these bootleg engineers realize they can experiment with the psychology of a human as well, making their subjects' behavior and thought processes fundamentally different from what we would normally experience. They could create a being with little regard for anything other than hedonism, or on the opposite end of the spectrum, a being that feels crushing numbness that no experience could ever lift the weight of.

The protagonist of this story is among the first people to be subjected to these experiments. However, I find it difficult to imagine what sort of subtle changes could be made to one's psyche that would make them appear slightly different, without it being hand-waved away as a normal point on the spectrum of human behavior. Somewhere beyond the box, if that makes sense.

As straightforwardly as I can put it, my question is:

Via altering their genetics, what kind of psychological changes could be made to a person that would make them subtly different from a human? Think trans-humanism, as in a sort of braching off in human's evolution.

EDIT: made the question less about mental illness, as it wasn't what I was trying to compare this to

posted over 5 years ago

Graviti‭

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