The number of happy faces on Lego toy mini-figures has been decreasing since the 1990s, and the number of angry faces has increased, giving rise to concerns that children could be affected by the negativity of the toys.
In a study of 3,655 figures produced between 1975 and 2010, Dr Christoph Bartneck, a robot expert at the University of Canterbury in New Zealand, said the manufacturer appeared to be moving towards more conflict-based themes in its toys.
Roar Rude Trangbæk, communications manager for Lego, said "The conflict between good and evil is nothing new" and "The characters always have classic Lego humour – the good guys always win in the end."
Trangbæk would not comment on Bartneck's research directly but added that there was a solution for parents worried about the impact of angry Lego figure faces. "Of course, they can always just switch heads with another figure," he said.