The United States Transportation Security Administration (TSA) has spent roughly $1 billion training thousands of “behaviour detection officers” as part of the Screening of Passengers by Observation Techniques (SPOT) programme.

The purpose of SPOT is to identify facial and body expressions that signals terrorist activity. Psychologists – and the GAO – question the effectiveness of the programme. “The common-sense notion that liars betray themselves through body language appears to be little more than a cultural fiction,” says one psychologist.

The results have not been impressive: fewer than one per cent of the more than 30,000 passengers a year who are identified as suspicious end up being arrested, and the offenses have not been linked to terrorism.

A November 2013 report by the Government Accountability Office (GAO) recommended that the TSA should reduce future funding for the agency’s behavioural detection programme because there is little evidence of the programme’s effectiveness. According to the GAO, “available evidence does not support whether behavioural indicators, which are used in the SPOT, can be used to identify persons who may pose a risk to aviation security.”

The recommendation was supported by a survey in which psychologists Charles Bond and Bella DePaulo analysed more than 200 studies in which participants correctly identified 47 per cent of lies as deceptive and 61 per cent of truths as nondeceptive, resulting in an average of 54 per cent — only 4 per cent better than chance. Accuracy rates were lower in experiments when judgement had to be made relying solely on body language.

Researchers have found that the best clues to recognising liars are verbal clues. Dr Nicholas Epley, a professor of behavioural science at the University of Chicago, has found that people over-rely on reading facial expressions. “Reading people’s expressions can give you a little information, but you get so much more just by talking to them,” he says. “The mind comes through the mouth.”