That would be covered under mindset for me at least. It's a bad news, good news story.
The bad news is my wife would be a terrible passenger. She's big, can't hold still, and moves with jerky motions. I had a girlfriend when I was in my 20s that moved like that and it drove me crazy even though she was about 7 stone. My wife is over twice that.
The good news is my wife had zero interest in getting on the bike. She's great in many other ways too.