@feld They experience widespread mental health diseases such as depression, are more prone to developing genetic disorders, when they experience more typical health problems they have a harder time managing them due to worse side affects from drugs, lack of compatible blood and organ doners, specifically their relatives aren’t usually compatible when that’s usually who people get to donate to them. Contrast with children of cousin incest, there’s really nothing substantial in the scientific literature, at best you can look at populations where cousin incest is common like Pakistan and blame negative outcomes there on it but it’s just a correlation, and you can always find countries with higher rates of cousin incest and higher levels of wealth and quality of life that disprove it. But don’t take my word for any of this, it’s all out there on Google and pub med this isn’t esoteric knowledge.