A polygraph is a sweat-inducing made-for-TV spectacle that even the government considers to have dubious results (there is a reason polygraph findings aren’t admissible in court). But individuals in national security careers have to put aside their concerns about the reliability of a lie detector machine. Whether you’re applying for a job with the Border Patrol or as a CIA agent, a number of cleared careers require a successfully completed polygraph examination.

If you’re facing a security clearance polygraph screening, it’s helpful to know what to expect—not so you can learn countermeasures, or try to “beat” the system, but so you can eliminate one of the greatest aspects for which the exam tests: fear.

There are two types of polygraph examinations used in security clearance and employment screenings: counterintelligence and lifestyle. Counterintelligence covers questions of espionage, sabotage and terrorist activities and is designed to root out contact with a foreign national or the compromise of classified information. Lifestyle deals more with the personal questions you answered on your SF-86, such as illegal activity, drug use, or falsification of the security clearance forms.

A full-scope polygraph combines the questions of both the CI and lifestyle polygraph.

“My professional opinion is that the government’s real motivator for use of the polygraph is scare value,” said Sean Bigley, national security attorney and managing partner of Bigley Ranish LLP. “The polygraph is more about getting scared people to admit what they would have otherwise omitted on their SF-86 than it is about actually digging up deception independently. Bearing that in mind, here is the reality: the scare tactic works subconsciously on many people.”

Just like your security clearance application, you are better off being truthful when you answer questions in a security clearance polygraph. Just as issues can be mitigated in a ‘whole person’ security clearance determination, seemingly self-incriminating polygraph responses may also be mitigated if you provide truthful answers.

Just ask former CIA Director John Brennan.

During a 2016 conference of the Congressional Black Caucus, Brennan was asked if a history of activism would prevent more diverse candidates from pursuing government careers. Brennan responded by recounting a story from his own security clearance polygraph screening. In 1980 Brennan was polygraphed as a part of his initial application process to work for the CIA. He was asked a standard counterintelligence polygraph question: “Have you ever worked with or for a group that was dedicated to overthrowing the U.S.?”

Brennan recalled that he had previously voted for a Communist Party candidate. He told the polygraph examiner he was neither a Republican or a Democrat, but had voted for the Communist Party candidate as a protest vote. He also noted that he was not a member of the Communist Party.

The line of questioning moved on, and Brennan expected to be kicked out of the running (this was 1980, during the peak of the Cold War). Rather than being denied a job or a clearance, Brennan passed the polygraph and got the job. Would he have passed if he had lied about his Communist Party vote? Not likely.

In addition to being truthful, here are four rules to keep in mind if you need to take a polygraph for a government position.

1. Follow your usual routine.

Many applicants worry that something like caffeine will hinder their performance on the polygraph. But it’s more likely to hurt you if you drink a cup of coffee every morning, and then skip it the morning of the polygraph. The same goes with prescription medications. If you normally take a particular medication every day, continue to do so the day of your security clearance polygraph. That said, if you have a pre-existing medical condition, advise your agency prior to taking the polygraph. Generally, pregnant women or those suffering from an illness such as a cold should not take a polygraph.

2. Don’t overthink.

The polygraph examination consists primarily of yes or no questions that should be simple. If you’re the kind of person who thinks there are no right or wrong answers, or who tends to doubt a choice after it’s made, this may affect you. The physiological receptors simply don’t have a great way to vet out a ‘maybe’ in a response.

“The term of art among polygraph examiners for people who tell the truth but register a lie is guilt grabber,” notes David Brown, author, Army veteran and regular contributor to ClearanceJobs.com. “Ironically, it can afflict those of outstanding integrity, who fail examinations because, due to an exceeding sense of responsibility, feel guilty for injustices that are totally unrelated to their actions or lives. Some are incorrectly flagged because they feel guilty for thinking about doing wrong.”

If this applies to you, go ahead and share that with the polygraph examiner. Just don’t find yourself going so far as to admit to something that isn’t true, when the reality is you’re thinking your way around an issue.

3. Don’t over-volunteer information.

If at any time during the polygraph you’re asked to speculate in providing a response, don’t do it. This isn’t a job interview, where you need to dazzle your examiner with your savvy and finesse. Sometimes “I don’t know” really is the correct answer. If you receive a question that comes out of left field, or that appears to purport information you didn’t include on your SF-86 (and which isn’t true), it’s okay to honestly plead your ignorance to your examiner.

If you’re asked to elaborate on a response provided on your SF-86, stick with the facts. “I might have been driving drunk before that disorderly conduct charge” is a more speculative (and incriminating) response than sticking with the facts: “I had been drinking, and was later arrested for disorderly conduct.” And again, if “I don’t know” is the real answer, then give that answer.

4. Don’t "study" to try to beat the polygraph.

You should expect to be asked if you’ve prepared for the exam or spoken with others about it. That’s why it’s best to avoid asking other applicants what their polygraph was like (their ease or discomfort is not likely to reflect your personal experience, anyway). And countermeasures such as controlled breathing will be detected by the professionals conducting the exam. It’s okay to review what to expect from news articles, or information sources provided by the agency’s website, but if you need to employ measures to try to “beat” the polygraph test you probably shouldn’t be working in a national security career in the first place.