Jurors who swear a religious oath may be biased against defendants who make a secular affirmation

Most of us are familiar with the image of someone in court with their hands on the Bible swearing an oath to tell the truth. Even if we haven't been in court ourselves, we will have seen defendants being sworn in like this on TV or in movies. For those who prefer a secular approach, however, swearing an oath to God isn't necessary: in countries like the UK, USA and Australia, a non-religious affirmation is available, allowing people to "solemnly, sincerely and truly declare and affirm" their truthfulness.

Secular versions of such declarations no doubt appeal to non-religious defendants and witnesses. But how do jurors perceive those who choose an affirmation over an oath – and could these perceptions affect trial outcomes? These are some of the questions posed by a team writing in the British Journal of Psychology. They find that while hypothetical defendants were not generally considered guiltier when giving a secular affirmation, mock jurors who themselves swore a religious oath did discriminate against those who affirmed.

In the first study, 443 British participants were shown the declaration options available to witnesses and defendants in the UK: namely, an oath or affirmation. Participants indicated which they would choose (or which they had chosen, if they'd given evidence in court before), and gave a reason for their selection.

After this, participants were shown information about two witnesses, one of whom chose to take an oath, the other an affirmation; in each case, participants indicated how religious they felt the witness was. Finally, participants gave demographic data including their own religious affiliation and belief in God.

Participants believed that the witness who chose the oath was much more religious than the witness who chose the affirmation. And this belief seemed to reflect reality: participants who themselves said they would choose the oath when in court were more religious. The majority of participants who chose the affirmation cited their lack of religious belief as their reason for doing so, while around a third of those who chose the oath gave their religious beliefs as a reason. Interestingly, 20% of those who chose the oath did so because they felt it was more credible, suggesting people link the oath with credibility.

The next study looked more closely at the relationship between people's choice of declaration and their perceived credibility. A new set of 913 British participants read a vignette about a defendant in a murder trial; half read that he swore an oath, while the other half read that he made an affirmation. They then indicated on a scale from 0 to 100 how guilty they felt he was, and also indicated their own religious affiliation and belief in God.

Religious participants saw the defendant who had chosen to affirm as slightly more likely to be guilty than the defendant who had sworn an oath. However, non-believers' judgements were not affected by the defendant's choice.

The final study looked at whether the type of declaration would influence jurors' perceptions in a more realistic scenario, and one in which they had to make an actual "guilty" or "not guilty" verdict. Participants acted as jurors for an animated version of a mock trial involving a robbery. The video included statements from barristers, and testimony from a police officer, a witness, a victim, and the defendant. Again, one group of participants saw the defendant taking an oath, the other an affirmation. Participants were also asked to select between an oath or an affirmation to take themselves.

Finally, after watching the animation, participants decided whether or not they felt the defendant was guilty. They also rated how confident they were in their verdict, and completed measures of religious affiliation, belief in God, and authoritarianism.

Overall, the defendant's choice of declaration didn't affect the verdict returned by participants. This was true for both non-religious and religious participants.

However, among participants who themselves swore an oath there was an effect: this group was more likely to return a "guilty" verdict for defendants who affirmed than those who swore an oath. Further analysis suggested that this effect was driven by the jurors in this group who scored highly in authoritarianism. These people may see the oath as traditional and therefore 'correct', the team suggests – making them more likely to choose the oath themselves, and more likely to punish those who don't choose it.

Clearly, these findings could have significant ramifications: if those who choose to swear an oath are more likely to discriminate against those who affirm, this could have an impact on whether a jury returns a guilty or not guilty verdict. Future research could look at real world trials, exploring whether defendants who affirm rather than swear an oath are convicted at higher rates. It would also be interesting to see if another form of declaration, that is neither the traditional oath nor an affirmation, would change things, moving away from existing traditions towards something more neutral.