Is this a face you can rely on?

Europeans have become more trustworthy over the past 500 years, and this is probably linked to an increase in living standards. That’s the take-home from intriguing research that moved art appreciation up a notch courtesy of that ubiquitous modern accessory, the algorithm.

A French team led by Nicolas Baumard from PSL Research University in Paris analysed more than 6000 portraits from two major collections looking for facial cues and clues to changes in social trust.

They say social trust is known to be associated with positive societal outcomes, but its origins are unclear, partly because changes are difficult to quantitatively document over time.

“Quite obviously, we cannot go back in time and ask people to fill out questionnaires or play economic games…” they note in their paper in the journal Nature Communications.

We do, however, “have access to what their minds produced”, including books, songs and paintings. And in the case of portraits, we have access to what their faces give away – thanks to the ability to analyse facial muscle contractions.

“Experimental work [has] revealed that specific facial features, such as a smiling mouth or wider eyes, are consistently recognised as cues of trustworthiness across individuals and cultures,” the authors write.

“In this paper, we capitalise on this large empirical literature to build an algorithm that estimates trustworthiness based on a pre-identified set of facial characteristics.”

That algorithm was first tested using photographs of faces where trustworthiness had been rated by humans. The researchers also checked that it was susceptible to the same biases as humans; that is, tending to rate younger, feminine and happy faces as more trustworthy.

When it was then used to analyse 1962 English portraits painted between 1506 and 2016 and held by the National Portrait Gallery, London, Baumard and colleagues found a significant increase in trustworthiness displays over time.

They were able to replicate their findings with 4106 portraits from 1360-1918 – including works from 19 western European countries – featured in the online Web Gallery of Art.

To test the assumption that trustworthiness in portraits does reflect actual changes in social trust, rather than just how people pose for portraits, the algorithm was applied to 2277 selfies from six cities on social media.

Increased displays of trustworthiness were observed, the researchers say, in images from cities where interpersonal trust and cooperation had been assessed as being higher in the European and World Value surveys.

They note that the historical increase in trustworthiness observed in their datasets parallels the rise of liberal values such as religious tolerance, political freedom and democracy. But more significant, they suggest, is that it is linked to increases in living standards.

“We found that changes in GDP per capita predicted future changes in trustworthiness displays in the National Portraits Gallery two decades later, while changes in political institutions did not,” they write.

And, they add, this “is consistent with other works emphasising the importance of economic growth and psychological changes in history”.