Promises are made to be broken, so it can be tough to tell which ones will be kept. But new-found patterns in brain activity can reveal whether someone intends to keep their word.
The finding raises the possibility of using brain scans to determine the true intentions of criminals who are up for early release on parole, according to Thomas Baumgartner of the University of Zurich in Switzerland.
He and his colleagues used functional magnetic resonance imaging (fMRI) to catch promise-breakers in the act.
The team set up a game of trust between an investor and a trustee. In the game, an investor is given real money, which they can choose to invest in a trustee. Giving the money to the trustee increases the amount of money fivefold, but the investor runs the risk that the trustee might not share the winnings but keep all the money for themselves.
Baumgartner's team ran the game twice. The first time, investors simply had to guess whether trustees would share the winnings and then made their decision accordingly. The second, trustees could promise to share the winnings with the investor, if they wanted – although the promise was non-binding.
Almost all the trustees promised to always share their winnings, thereby securing investment. While some of them remained true to their word, others consistently broke their promise, keeping the hoard for themselves.
The trustees had their brains scanned using fMRI during both runs; they spoke to the investor from inside the scanner via an intercom.
The fMRI data revealed that certain brain areas became more active when trustees were breaking a promise. These regions – the dorsolateral prefrontal cortex, anterior cingulate cortex and amygdala – are known to be involved in emotion. They could reveal an emotional conflict in a person who knows they are doing something wrong, or feels guilty, says Baumgartner.
Most interestingly, similar areas were active in people who were making promises that they later broke, but not in people making promises that they ended up honouring. This suggests that the former group fully intended to cheat the investor out of their money with a phoney promise, says Baumgartner.
While others have claimed to spot liars and cheats from brain scans, Baumgartner reckons that people who intend to break a promise are different.
"Even though people are aware that they are doing something wrong, they haven't actually done anything – they still have a chance to remedy the situation and do the right thing," he says.
Baumgartner envisions a future in which brain scanners might help psychiatrists decide whether or not to release on parole criminals who promise they won't reoffend.
He admits, however, that a scan probably wouldn't be able to predict whether someone who doesn't intend to break a promise will end up doing so. It might also fail to pick up the false promises made by people who don't feel any emotional conflict when they do so, such as pathological liars.
"We might still see some conflict, though," says Baumgartner. "Those areas of the brain might register a conflict between saying you will do something and knowing that you won't, although this area needs more research."