How do they explain the genetic factors that cause predisposition? Just as the humans who eat too much sugar can more easily get type 2 diabetes, some humans w

How do they explain the genetic factors that cause predisposition? Just as the humans who eat too much sugar can more easily get type 2 diabetes, some humans who are exposed to 'X' can get mental diseases. (X is unknown or it varies). What if some humans are just more 'evolved' to live in the modern world than others? 10-12,000 years of living within dense populations, under despots, with periodic plagues, and in near constant war probably applies a lot of evolutionary selection pressures. Some of us whose ancestors were assimilated later probably have more genes that haven't been 'weeded out'. I've read some research that human brain size has decreased over the last 100,000 years. The theory is that as humans began to become less independent, they had less need for certain types of brain functions. Since the brain is an extremely resource intensive organ, less brain means less energy requirements. So, humans quickly lose areas of the brain that are less necessary. There are correlations between intelligence and depression. There are correlations between intelligence and sensory and spatial abilities. Maybe some of these things are related.