In The Truth Machine, a science-fiction novel published in 1996, scientists invent a device that can detect lies with perfect accuracy. It abolishes crime, changes the world, and generally saves humanity from self-destruction. Which is nice.
Could such a machine ever be a reality? Not if our current technology is anything to go by. The polygraph has been around for almost a century, with wired-up offenders and twitching needles becoming a staple of criminal investigations. But there is no solid evidence that the signs it looks for – faster heart rates, shallower breaths and moist skin – can accurately indicate whether someone is telling a lie. Underpinned by fluffy theory and backed by a weak and stagnant evidence base, this lie-detection device is unlikely to get any better.
Inside the brain
Abandoning the polygraph, some scientists have turned to brain scanners. Two technologies have dominated the field. The first uses electronic sensors on a person’s scalp to measure an electrical signal, or “brainwave”, called the P300, which appears when we recognise something. By looking for this signal, you could potentially tell if someone is hiding knowledge about something they are already familiar with, like a murder weapon. This is certainly useful, but it is a long way from an all-purpose lie-detection method, and two of the key figures in the field have been arguing about how effective this is for many years.
The second technique is functional magnetic resonance imaging (fMRI), affectionately known as blobology for the colourful pictures it produces. It shows the location of firing neurons in an indirect manner, by tracking the blood flow that supplies them with nutrients and oxygen. Several fMRI studies have shown that some parts of the brain are consistently more active when people tell untruths rather than truths, particularly areas at the very front that help us to suppress unwanted actions. Successful lying, it seems, is mainly about repressing the urge to be honest.
But there is no “centre for dishonesty” in the brain, as such. The areas illuminated in fMRI scans have many functions. They can even be more active when people tell the truth, especially if they are trying to decide whether to be honest or not.
So, how accurate are the scans? In simple lab experiments, they can detect lies around 78 to 85% of the time. “We’re not that close to a perfect lie detector,” says Giorgio Ganis from the University of Plymouth, who uses fMRI to study deception. “There’s also a 15-20% chance of an innocent person being wrongly determined to be a liar.”
Tell me lies
What is particularly troubling is that these limitations crop up in simplified and artificial conditions, like volunteers lying about a playing card they have been given. So we know very little about how fMRI would fare at detecting lies in more realistic settings – for example, not a single study has scanned people’s brains when they lie during conversations.
There are also different types of lie. If you have been pulled over for speeding, you would need to come up with a tall tale spontaneously. If you were on trial for a crime, you would have more time to rehearse your story. Ganis found that these brands of lie produce different patterns of brain activity: rehearsed ones are accompanied by a weaker buzz in so-called action-repression areas, and a stronger one in memory centres.
Lie detection also depends on a person’s memories, which are subjective and fallible. In 2008, Jesse Rissman from Stanford University showed that fMRI scans can reveal if volunteers thought they had seen a face before, but not if they had actually done so. If people are convinced of their lies, or if they have simply forgotten crucial information, the scans will not pick that up.
