Have you ever experienced do-gooder exhaustion? When you've done the right things for so long, you just need to indulge your inner sinner? Or when you've given so much to others that you want to save something for yourself?

You aren't alone, and this isn't a defect in your moral virtue. Researchers at Northwestern University recently published three studies that show why one good deed can prevent another.

In the first study, participants were randomly assigned to write a story about themselves, either using the words "caring, generous, fair, and kind," or the words "disloyal, greedy, mean, and selfish." They then completed a time-filler task to mask the true outcome researchers were interested in: how the personal reflection would influence participants' generosity. After the "experiment" was over, participants were asked to make a small donation ($1-10) to a charity of their choice.

What effect did the story-writing have? Participants who wrote about selfishness gave, on average, $5.30. Those who wrote about generosity gave less. A lot less: $1.07, about as close to nothing as this study allowed.

A control group, who wrote a story about inanimate objects, donated on average $2.71. This shows that the selfish storytellers were indeed trying to redeem themselves-but there was also a "stinginess" effect on participants who felt pretty good about their past generosity .

Studies 2 and 3 found the same thing. In study 2, participants were asked to write a story either about themselves acting generously or selfishly, or about someone else acting generously or selfishly. Participants writing about themselves showed the same donation pattern as in study 1 ($1.11 vs. $5.56). But participants writing about someone else showed no difference. The take-home point: this effect is driven by self-reflection, not a more generic priming of generous or selfish behavior.

A third experiment found the same effect with a different test of prosocial behavior: how willing participants were to pay a price to be environmentally friendly. Participants who wrote about being generous were less willing to pay to go green than participants who wrote about being selfish.

It's easy to understand why someone feeling a bit guilty might decide to do some good. But why on earth would remembering your own generosity turn you into a Scrooge?

According to the researchers, it's all about maintaining a steady sense of moral self-worth. How good you feel about yourself depends on how good a person you think you are. Giving in to temptation, or refusing to do the right thing, lowers your moral self-worth. You feel worse and need to do something good to feel better.

But few of us have a self-image that paints us as Mother Teresa. We want to be good, but not martyrs. This means that doing good can lead to a discrepancy in self-worth just as much as being selfish. The research by Northwestern psychologists shows that we are motivated to reduce that discrepancy even when it's positive.

Why would we want to lower our moral self-worth, rather than cherish the warm glow of our own halo? Because always doing the right thing isn't what most of us are aiming for-and after awhile, it can start to feel bad. You may feel a little bit foolish for always putting others first. And always saying no to what you want can start to feel like self-induced punishment.

In other words, being good can feel bad-but being a little bit bad can make you feel better. Not just better, but more like yourself again.

This research is making me rethink my own cycles of being good and selfish. Back in my college term paper days, I developed a serious habit of shopping after I met a deadline. Now, it's like clockwork: turn in a project, hit the mall.

I'd always thought it was operant conditioning at work—I was rewarding myself for doing something good. Surely the promise of reward would keep me motivated for future Herculean tasks.

But now I realize, I may be trying to bring my self-worth back down to size. I don't want to feel like too much of a workaholic. Life needs balance, and everyone needs a little indulgence. Plus, when the credit card bill arrives, I'll feel just bad enough about myself to want to tackle that next project.

Study: Sachdeva, S., Iliev, R., & Medin, D.L. (2009). Sinning saints and saintly sinners: The paradox of moral self-regulation. Psychological Science, 20(4), 523-528.

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