“It is when people do wrong things wilfully that they are the more likely to do them again.”
Source: The Princess and the Goblin
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George MacDonald 127
Scottish journalist, novelist 1824–1905Related quotes

“I like to date schoolteachers. If you do something wrong, they make you do it over again.”
Source: It's Not Easy Bein' Me: A Lifetime of No Respect But Plenty of Sex and Drugs (2004), p. 12

Presumably a paraphrase of "A peculiar virtue in wildlife ethics is that the hunter ordinarily has no gallery to applaud or disapprove of his conduct" or of "Hunting for sport is an improvement ..." above.
Unlikely to be by Leopold, who knew that ethics involves not only doing the right thing, but also determining the right thing in the face of competing desirable criteria.
Misattributed

“Managers cannot learn from doing things right, only from doing them wrong”
Source: 2000s, A little book of f-laws: 13 common sins of management, 2006, p. 37 cited in: Andrew Carey (2008) Inside Project Red Stripe: Incubating Innovation and Teamwork at the Economist. p. 49.

“If it's wrong when they do it, it's wrong when we do it.”
Bennis Warren and Burt Nanus (1985) Leaders: The Strategies for Taking Charge. Harper and Row. p. 21
1980s

“Watch people like a hawk, and when they do something good, tell them.”
Personality Lectures

“People don’t like to have somebody knowing more than they do. It aggravates them.”
Pt. 2, ch. 12
Calpurnia
Variant: Folks don’t like to have somebody around knowin’ more than they do. It aggravates ‘em.
Source: To Kill a Mockingbird (1962)
Context: Folks don’t like to have somebody around knowin’ more than they do. It aggravates ‘em. You’re not gonna change any of them by talkin’ right, they’ve got to want to learn themselves, and when they don’t want to learn there’s nothing you can do but keep your mouth shut or talk their language.