Princess Diana and her brother Charles Spencer both went through childhood trauma after their parents' divorce.
Spencer, 56, told the Sunday Times about the difficult time they went through as siblings when their father John Spencer split from his first wife Frances in 1969, according to People.
He said, "Diana and I had two older sisters who were away at school, so she and I were very much in it together and I did talk to her about it."
The 9th Earl Spencer is also a historian and writer, with a new book The White Ship, due to be released on September 17.
"Our father was a quiet and constant source of love, but our mother wasn't cut out for maternity. Not her fault, she couldn't do it," he said.
"While she was packing her stuff to leave, she promised Diana [then aged 5] she'd come back to see her. Diana used to wait on the doorstep for her, but she never came."
Spencer previously told People that the split was "tough on Diana".
He told The Sunday Times he's been going to therapy for 20 years and has done "a lot of very profound work on my unhappy childhood".
"Coming out the other side has been good.
"The one thing I've learned through all the stuff I've tackled is that very few people set out to be destructive."
Spencer, who has been married three times, shared that his childhood trauma gave him "a predisposition for rescuing people", but when he met Karen, a social entrepreneur whom he married in 2011, his mindset had changed.
"We got engaged quickly and I'm sure people thought, 'Here he goes again,' but I was confident," he said.
Karen herself told the Times, "The most appealing thing about Charles for me was how willing he was to work on himself, and continue to do so."
"I love that we are a supportive partnership. We're both ambitious, and there's been a lot of give and take."