When Prime Minister David Cameron is giving an important speech, or in the midst of difficult negotiations, he relies on a simple mental trick known as the full bladder technique. The name is not a metaphor. Cameron drinks lots of liquid and then deliberately refrains from urinating, so that he is “desperate for a pee.” The Prime Minister believes that such desperation comes with benefits, including enhanced clarity and improved focus.

Cameron heard about the technique from Enoch Powell, a Conservative politician in the 1960s. Powell once explained why he never peed before a big speech. "You should do nothing to decrease the tension before making a big speech," he said. "If anything, you should seek to increase it."

Are the politicians right? Does the full bladder technique work? Is the mind boosted by the intense urge to urinate? These questions are the subject of a new paper by Elise Fenn, Iris Blandon-Gitlin, Jennifer Coons, Catherine Pineda and Reinalyn Echon in the journal of Consciousness and Cognition.

The study featured a predictable design. In one condition, people were asked to drink five glasses of water, for a total of 700 milliliters. In the second condition, they only took five sips of water, or roughly 50 milliliters. The subjects were then forced to wait 45 minutes, a “timeframe that ensured a full bladder.” (The scientists are careful to point out that their study was approved by two institutional review boards; nobody wet their pants.) While waiting, the subjects were asked their opinions on various social and moral issues, such as the death penalty, gun control and gay rights.

Once the waiting period was over, and those who drank lots of water began to feel a little discomfort, the scientists asked some of the subjects to lie to an interviewer about their strongest political opinions. If they were pro-death penalty, they had to argue for the opposite; if they believed in gay marriage, they had to pretend they weren’t. (To give the liars an incentive, they were told that those who “successfully fooled the interviewer” would receive a gift card.) Those in the truth telling condition, meanwhile, were told to simply speak their mind.

All of the conversations were videotaped and shown to a panel of seventy-five students, who were asked to rate the answers across ten different variables related to truth and deception. Does the subject appear anxious? How hard does he or she appear to be thinking? In essence, the scientists wanted to know if being in a “state of high urination urgency” turned people into better liars, more adept at suppressing the symptoms of dishonesty.

That’s exactly what happened. When people had to pee, they exhibited more “cognitive control” and “their behaviors appeared more confident and convincing when lying than when telling the truth.” They were less anxious and fidgety and gave more detailed answers.

In a second analysis, the scientists asked a panel of 118 students to review clips of the conversation and assess whether or not people were telling the truth. As expected, the viewers were far more likely to believe that people who had to pee were being honest, even when they were lying about their beliefs.

There is a larger mystery here, and it goes far beyond bladders and bathrooms. One of the lingering debates in the self-control literature is the discrepancy between studies documenting an ego-depletion effect – in which exerting self-control makes it harder to control ourselves later on – and the inhibitory spillover effect, in which performance on one self-control task (such as trying not to piss our pants) makes us better at exerting control on another task (such as hiding our dishonesty). In a 2011 paper, Tuk, et al. found that people in a state of urination urgency were more disciplined in other domains, such as financial decision-making. They were less impulsive because they had to pee.

This new study suggests that the discrepancy depends on timing. When self-control tasks are performed sequentially, such as resisting the urge to eat a cookie and then working on a tedious puzzle, the ego gets depleted; the will is crippled by its own volition. However, when two self-control tasks are performed at the same time, our willpower is boosted: we perform better at both tasks. “We often do not realize the many situations in our daily lives where we’re constantly exerting self control and how that affects our capacity,” wrote co-author Iris Blandón-Gitlin in an email. “It is nice to know that we can also find situations where our mental resources can be facilitated by self-control acts.”

There are some obvious takeaways. If you’re trying to skip dessert, then you should also skip the bathroom; keep your bladder full. The same goes for focusing on a difficult activity – you’re better off with pressure in your pants, as the overlapping acts of discipline will give you even more discipline. That said, don’t expect the mental boost of having to pee to last after you relieve yourself. If anything, the ego depletion effect might then leave you drained: All that willpower you spent holding it in will now hold you back.

F Fenn, Elise, Iris Blandón-Gitlin, Jennifer Coons, Catherine Pineda, and Reinalyn Echon. "The inhibitory spillover effect: Controlling the bladder makes better liars." Consciousness and Cognition 37 (2015): 112-122.