@despairing,
I can give some numbers, since FA runs in my family. Of course this does not apply to everyone who comes from a family who has this problem, since my numbers are for my family only. I was drafted into doing the genealogy of my family. I went back 7 generations before I could not go back any further. Around 1700 people total. In the end every single generation had at least one person who never found anyone and died alone. About 170 people, so 10% of my family in the last 7 generations. But the one person died alone per generation applied regardless of how many children the parent had. Some ancestors of my family had 13 or more kids, and only one died alone. But others like my parents had two children, and one died alone. Some parents had several children, and all of them died alone. The only one constant was every generation in my family had at least one child who ended up FA. The ones I knew personally in my family, who have already died FA, or are on their way, all had one thing in common. Probable Avoidant Personality Disorder, Obsessive Compulsive Disorder, or both. This might be the determining factor if someone ends up FA, but unfortunately I can only say it applies to my family only. I wish I could find better numbers than this, since their is a major sampling bias, being my family only, but as far as I know, no one has ever studied this problem.