A Twitter image-cropping algorithm that had went viral last year when users discovered it preferred white people to Black people was also coded with implicit bias against a number of other groups, researchers have found.

The "saliency algorithm" decided how images would be cropped in Twitter previews, before being clicked on to open at full size.

But when two faces were in the same image, users discovered, the preview crop appeared to favour white faces, hiding the black faces until users clicked through.

The pattern held true for images of former US President Barack Obama and senator Mitch McConnnell - and for stock images of businessmen belonging to different ethnicities.

The findings were part of a first-of-its-kind contest hosted by Twitter over the weekend at the Def Con hacker conference in Las Vegas. The company invited researchers to unearth new ways to prove that an image-cropping algorithm was inadvertently coded to be biased against particular groups of people. Twitter gave out cash prizes including $3,500 to the winner Bogdan Kulynyc and small amounts for runners up.