Kevin asked, "So does that mean it doesn't matter if we act like jerks?"

"It matters to the person in this branch that you're acting like a jerk to," said Zareenah.

"But what about globally? Does being a jerk in this branch increase the percentage of jerkish behavior across all branches?"

"I'm not sure about the math," said Dana. "But I definitely think that your choices matter. Every decision you make contributes to your character and shapes the kind of person you are. If you want to be someone who always gives the extra money back to the cashier, the actions you take now affect whether you'll become that person.

"The branch where you're having a bad day and keep the extra change is one that split off in the past; your actions can't affect it anymore. But if you act compassionately in this branch, that's still meaningful, because it has an effect on the branches that will split off in the future. The more often you make compassionate choices, the less likely it is that you'll make selfish choices in the future, even in the branches where you're having a bad day."