Why do you consider "wrong" acts to be in our self interest? I've found that "good" acts actually feel good. Wrongs acts make me feel bad. So it's in my self interest to do good deeds and not bad.
Try this one. You go to the store and get gas, and a bunch of groceries. You run up a pretty big tab, but you need the stuff. However, it still puts a dent you your wallet, just the same. Anyway, you get home, put away the groceries, and take your receipt to balance your account, only to find that the cashier, forgot to add the cost of the fuel to your bill. You just got away with $55.00 worth of fuel (it wasn't your fault it was missed, and you didn't realize it at the time either, so you are innocent of any deception), and the cashier's oversite just benefitted you...but something is eating at you inside, because now, you know.
And you start thinking of that cashier trying to balance her cash drawer at the end of her shift, and you know that loss will come out of her paycheck...
Now, through no fault of your own, you benefitted from her error, but leaving things as they are (when you now realize the situation), you also know she will suffer for your gain.
Two choices:
"Too bad for her, you get the gas free and clear, with no legal obligation to compensate for it" (she'd never figure out where her cash till went short, let alone by whom).
OR -
"You go back to the store, bring the receipt and explain to the stunned cashier that she forgot to charge you for the gas." She can't even pull up how much it was because she cleared the computer and re-set the pump. So you tell her how much you pumped and pay her for it. The look on her face is priceless as she thanks you for helping her fix the problem she had no idea she had yet. But her eyes tell a different message..."thank you for your honesty, and not letting me take a fall for this."
Yeah, your wallet is a little bit tighter now, but you gained something even better, because every time you walk into that store, she will remember that you thought of her, first - and you will perceive that emotional warmth from her. You also notice that she pays it (some of that warmth) forward to the next customer, even though she isn't aware of it, and that customer walks out with half a smile on their face, but they don't realize why...
but you do...
v/r
Q