Two of my cousins are diagnosed with autism (daughter and grandson of my dad's brother). I didn't know this until I told everybody about my diagnosis because my dad and his brother were estranged and we rarely saw them. My uncle and his wife both thought that Dad and his father had Asperger's as well, but I think he died before the cousins were diagnosed.
One of Dad's "special interests" was genealogy, and he did a lot of work researching his family. Going back up his family tree were a LOT of stories of ancestors that did not get along well with other people or were just a bit strange. Grandpa was an English teacher with a photographic memory and could quote you any page of a book he had read, years later, unless he didn't enjoy it. All you had to say was, As You Like It, Scene 2, or ask what's on page 34 of his edition of Paradise Lost, and he could feed you the lines. My uncle had a photographic memory too and did computer work for NASA and IBM, although his wife didn't think he had autism because he was outgoing and sociable. My great-grandfather got into a feud so bad with a neighbor that he burned great-granddad's store to the ground. My great-great-great-grandfather was sent to the back of the lines by his captain in the Civil War. The captain went back to sort him out later, and ggg-grandfather shot him and deserted. He eventually was brought up on murder charges but exonerated on self-defense somehow (apparently he didn't get along well with people but was also very good at talking his way out of legal problems). He spent the rest of his life getting into quarrels with neighbors, suing them, and one time, stabbing somebody.
My mom thinks she's slightly ASD now, and thinks that maybe a couple of my nephews are too. We're debating how (or whether) to tell them.
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Diagnosed Bipolar II in 2012, Autism spectrum disorder (moderate) & ADHD in 2015.