If you figure that a good time to ask the boss for a raise is when she gets back from summer vacation in a few days, you’ve got it half right. Being in a good mood influences people’s judgment and decision making, meaning you have a greater chance of coming away with that extra five percent when the boss is in a good mood than when she has just been chewed out by the uber-boss. But here’s the other half: if the boss knows that you are trying to exploit her “incidental affect,” as psychologists call it, and is therefore reminded that her judgment may be clouded by incidental feelings, the effect may disappear.

So conclude Eduardo B. Andrade and Teck-Hua Ho of the Haas School of Business at the University of California, Berkeley, who tested how mood affects decision making—and how knowing that someone is trying to exploit that effect for his own ends can make the attempt backfire. They report their findings in the August issue of Psychological Science.

The world is full of examples of mood affecting judgment on issues that “should” turn on logic and rationality alone. When people are negotiating, being in a good mood makes it more likely that they’ll cooperate, while being in a bad mood makes it more likely they’ll be more competitive than cooperative. Let’s say you’re negotiating with someone—haggling over the price of a used car, say—and the seller responds, “that offer is enormously insulting!” Knowing the effect of anger on decision making, and really wanting the car, you backpedal, upping your offer. As Andrade and Ho write, “the angry feeling generated by a disliked offer is taken into account during the negotiation process.” So is a contented feeling: people feel they can ask more of someone in a good mood.

But what if the mood has nothing to do with the subject at hand—settling on a price for a car—and instead reflects something completely incidental, such as the boss's euphoria about how great his vacation was? In this case, the contentment has nothing to do with your request for a five percent raise. Your best strategy is nevertheless to ask for more when you know the boss is in a good mood, and less if he is in a bad mood.

But here’s where it gets tricky. If he knows that you know he is in a good mood, he is likely to take this into account, figure you’re trying to exploit his good humor, and overreact, telling you and your request for a raise to take a hike.