Lie detector machines are great for the police. And plenty have used them…on TV. But do polygraphs, voice analyzing software and brain scans actually do what they promise to do?
How would you react to being asked whether you'd killed your neighbor and buried them under a rose bush in your back garden? Would it make you break out in a sweat or make your voice quiver? What about any activity in your brain? Judging from countless crime shows on TV, your body would probably do something that any seasoned detective could interpret as a sign of your lying - no matter how you answer.
And if the detective misses any of the tell-tale signs - you can be sure that the simple lie detector or polygraph will pick them up.
That's the idea anyway.
A lie detector or polygraph measures bodily reactions, such as skin conductivity, heart rate and blood pressure.
It can't tell itself whether you're lying or telling the truth - it's how you react to the different questions you're asked that does the trick.
You'll be asked control questions - questions to which your interrogator knows the correct answers - mixed in with pertinent questions about the suspected crime.
The questions are designed to encourage you to lie.
In the US, polygraphs are commonly used in combination with the control question technique. Some government agencies even use them to screen job candidates.
But the method is highly controversial - many scientists say it's unreliable.
"Classic lie detectors measure how excited a person is," says John-Dylan Haynes, director of the Berlin Center for Advanced Neuroimaging. "And they have a good hit rate. But you can learn to outfox a polygraph, too."
Fear, anger and surprise can influence the way a person reacts.
"Someone who is innocent may react just as strongly to a question about murder as a guilty person," says Hans-Georg Rill, an authorized expert for forensic psychology in Mainz.
And it's one reason why the method is illegal in Germany.
It's all in the questions
Interrogators can combine the use of polygraphs with another strategy, the so-called guilty knowledge test.
In the guilty knowledge test, the interrogator confronts a suspect with a question and several possible answers.
For example, they may ask: "How was Mister Smith killed? Was he shot, strangled, stabbed or poisoned?"
Interrogators are trained to believe that when a suspect displays a strong reaction to the correct answers, the suspect probably knows more about the crime than they say. It may even be taken as a sign that you were involved in the crime.
"Police in Japan use polygraphs and the guilty knowledge test very intensively and successfully," says Rill.
But the technology cannot detect lies - it can only suggest you know more than you admit.
Scanning the brain for lies
By scanning your brain, researchers say they can detect lies where they begin.
They will look for regions of the brain that are active when you lie - but which are at rest when you tell the truth.
The functional magnetic resonance imaging (fMRI) technique can reveal which regions of the brain are consuming the most oxygen at any given time and are therefore most active.
But it's not that simple, says Matthias Gamer, a neuroscientist at the University Medical Center at Hamburg-Eppendorf.
"There is no region in the human brain that is specifically associated with lying," he says.
That hasn't stopped some companies, such as "No Lie MRI," offering brain scans for commercial use.
"The technology [is] the first and only direct measure of truth verification and lie detection in human history!" claims the company on its website.
But Haynes doubts whether the product lives up to the promise.
"Current brain scanning methods are unfit for [lie detection]," says Haynes. "So I would question any company that offers such a service."
Your voice betrays you
You can also get software that is said to detect lies by analyzing the voice. The software analyses pitch, frequency, intensity and micro tremors, drawing conclusions on how stressed the speaker is. But, even here, scientists are skeptical.
"We conducted a scientific study of these programs," says Gamer, "and they didn't work."
Anders Eriksson of Gothenburg University and Francisco Lacerda of Stockholm University went a step further in 2007, writing that "these machines perform at chance level. Given […] the absence of scientific support for the underlying principles it is justified to view the use of these machines as charlatanry."
"Distinguishing between a lie and the truth is difficult," says Gamer, insisting there's no simple technology that can achieve such a complex task.
Hans-Georg Rill, however, says he can do it without technology. It's his job to evaluate the testimony of witnesses in court.
And he does it by analyzing the content of what a witness says.
Rill says there are common linguistic patterns, as well as in the story itself, that can indicate whether a person is lying or not.
"A witness will reveal more detail about an event they have experienced than one they have made up," he says.
The witness may even feel some guilt or attempt to play down the seriously of the crime.
Rill says his work "has nothing to do with intuition."
But outside the courts, in everyday life, it's probably best to rely on your intuition - we certainly think so. Your intuition may be as reliable as all the available technology. And it's a whole lot cheaper too.