Previously, I talked about a bunch of different mental illnesses associated with a lack of empathy, but I didn't mention all of them. One of them that I forgot to talk about was specific forms of schizophrenia. It has been found that certain forms of schizophrenia, like the kind that was once diagnosed as paranoid schizophrenia, have led to severe impairments in the emotional empathy of people with the condition. The good thing is that schizophrenia has different ways to be treated and in modern times, untreated schizophrenia only affects about 0.32% of the population according to the World Health Organization - meaning that enough people get to be treated for this condition that untreated versions of this mental disorder affect a very small chunk of the population, much smaller than the other conditions that were previously mentioned.
In fact, there are a bunch of ways in the modern world to help treat people with schizophrenia, including medication, psychoeducation, family interventions, cognitive-behavioral therapy, and psychosocial rehabilitation. So, if you know anyone with schizophrenia, you should not give up on them. There are a bunch of different ways to get them proper medical help and help them get well if any part of their condition leads to them having problems in their life.
One issue is that people back in the past did not have any way to treat mental illnesses like paranoid schizophrenia and people with this condition just had to live with it. On top of that, studies have shown that inbreeding increases the chance that offspring will inherit paranoid schizophrenia and that possibility increases as the inbreeding coefficient goes up over multiple generations of inbreeding. To make things perfectly clear, inbreeding is not the only cause of this form of schizophrenia. There is not a singular cause for this condition and a variety of genetic factors not related to inbreeding and a bunch of environmental factors can play into whether someone becomes a paranoid schizophrenic or not.
The point I am getting at is that inbreeding is something that can lead to descendants having paranoid schizophrenia & this is actually a decent argument against certain forms of authoritarianism like feudalism. Feudalism was a form of authoritarian monarchy common in Europe and other places during the Middle Ages where people worked and fought for a king and nobles who essentially owned all property due to a system called manorialism. There are many issues with feudalism, but one is that it was common for royals to marry to try to secure alliances and intermarry with their own cousins to try and 'keep the bloodline pure'. Other nobles did this and this was common in many other authoritarian regimes like that of ancient Egypt. This system of inbreeding pretty much guaranteed that future generations of nobles and royals would essentially be far less empathetic than their predecessors and create a system where the fate of entire nations and the world rested in the hands of people who had difficulty caring about their fellow man. Inbreeding is also somewhat common in other places around the modern world.
In places with high amounts of classism and power based on birthright, inbreeding would be more common since you would create a shallow gene pool artificially. As more and more 'elites' only have offspring with other 'elites' while generally seeing poor people as less desirable to have kids with, these people create an artificial genetic bottleneck. Similar to the idea of 'blood purity', you have people who see themselves as better and purer than the people beneath them, making them more likely to prefer dating a close relative to someone outside of their social circle. Overtime in feudalist, theocratic, and absolutist regimes, this can lead to more inbreeding and more people with paranoid schizophrenia and other conditions that come with inbreeding like bipolar disorder.