Jonathan Crane was born out of wedlock and suffered severe abuse as a child. His father took off before he was even born, and his mother was made to leave her new born son in the hands of her grandmother, a religious fanatic with less than pure intentions for the boy. As a child he was exposed to severe emotional and physical torment and torture from his great grandmother, the worst of course being the Keeny families chapel, where flocks of crows nested, and dressed in a suit contaminated with a homemade chemical designed to enrage nearby crows and force them to attack the source. It was because of this that Jonathan developed a crippling fear of his great grandmother.
Crane had an early start. He knew fear from a young age.
Edward Nashton was born into a broken home. His mother was absent and his father was abusive. When Edward was a young boy, he became excited at the idea of winning a puzzle contest at school. To increase his likelihood of winning, Edward sneaked into school during the night and practiced the puzzle until he could solve it with ease. He ended up winning, and was awarded a riddle book as a prize. Since that time, he has mastered puzzles, mind games, and riddles.
Nigma had an early start.
Now, I can see where you would argue that "that doesn't mean they were insane at the start!" But I could then make the same argument about Harley.
Described as having a genius-level IQ, Harleen attended Gotham State University on a scholarship for gymnastics, where she majored in Psychiatry under Dr Odin Markus. To get into the graduate program, she had Dr Markus meet with her in her dorm room, where she outlined what she believed would be a good graduate thesis: 'There are only two circumstances under which a person disregards the rules of society. When they commit a crime, or when they're in love.' To prove this thesis, she aimed to allow Dr Markus to observe an independent experiment on the matter. The test subjects: herself and her boyfriend Guy Kopski.
Here we can see that there is no proof that Harley was already insane to begin with, nor is there proof that she seduced Dr. Markus. In truth, Markus was the cause of her initial discovery of The Joker. Markus did more damage there at the start than Harley did.