Princess Diana may have lived with undiagnosed ADHD.
Her brother, Earl Charles Spencer, says she displayed symptoms of attention-deficit/hyperactivity disorder that went unrecognised during her lifetime.
Diagnosed with ADHD as an adult, Earl Spencer believes the condition may explain many of the late princess's lifelong struggles, including her emotional sensitivity and difficulties in relationships.
He told People magazine: "I was only found out to have very severe ADHD late in life, and I know it's exactly what Diana had.
"It's so similar.
"But Diana was as extreme.
"So I've got really upset when people have tried to say, because Diana could get very hurt and emotional, they would say, 'Oh, maybe she had bipolar or borderline personality' and all these.
"Who knows?
"I know it's not those, by the way, but these people throw out these theories.
"I think it's more humdrum in a way.
"I think it was ADHD, and rejection for people with ADHD goes very, very deep."
Earl Spencer also believes Diana had dyslexia, as she struggled with spelling and word order.
He said: "Look, Diana was born in 1961, and certainly the private school system in England was not tuned into children's needs as they are today.
"I think Diana suffered from dyslexia.
"She found spelling and word orders and letter orders quite tricky."
The claims are part of Earl Spencer's new book, Swan Song: Diana, My Sister, in which he shares personal memories of their relationship.
He describes how Diana would sometimes cut people out of her life suddenly and believes this behaviour was linked to her undiagnosed ADHD.
He said: "This isn't a criticism.
"It's a recognition that she was struggling with a condition that was totally undiagnosed, and she could turn on a pin.
"If she felt a friend had let her down, that was a really big deal and it was difficult to come back from that.
"I'm very straight in the book.
"There wasn't a single member of my family who Diana didn't fall out with and turn her back on for some time.
"But I suppose as family, we weren't going anywhere, so we came back into favour or whatever.
"But understanding the ADHD aspect of that is really helpful.
"And also the extraordinary pressure she was under...
"People said, 'Oh, she was paranoid.'
"No, she wasn't.
"She was sensitive to rejection, and she felt very let down by people.
"And because of her ADHD, because of her personality and her character, she felt these things very deeply."
Earl Spencer also reflected on the toll Diana's public life took on her.
He said: "And I think particularly so because she was giving so much of herself in her public life.
"I used to feel this extraordinary thing about Diana, that it was almost like people had no idea how much they were taking from her.
"And I'm talking about innocent members of the public, who adored her, but she felt the need to give.
"I think it made her feel better about life, but it was draining."
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