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Dr Christoph Bartneck has studied all 6000 Lego mini-figures and said the number of happy faces had decreased while the number of angry faces had grown.
He said children who grew up with Lego today would not only remember smiley faces on mini-figures, but also anger and fear, as the toymaker introduced more conflict-based themes.
The move away from positive faces could have an impact on how children play, Dr Bartneck said.
"It is important to study how to create appropriate expressions and how these expressions are perceived by the users. Children's toys and how they are perceived can have a significant impact on children."
Dr Bartneck said Lego began producing a greater variety of faces in the 1990s, and the number of new faces is increasing steadily.
Happiness and anger were the most common facial expression.
"We considered the distribution of faces across emotional categories in the context of the Lego themes. Most mini-figures are released in sets that belong to a certain theme, such as Pirates or Harry Potter," he said.
"It is our impression that the themes have been increasingly based on conflicts. Often a good force is struggling with a bad one.
"But the facial expressions are not directly matched to good and evil. Even the good characters suffer in their struggle and the villains can have a smug expression. In any case, the variety of faces has increased considerably."
He said while Lego did not specifically produce military themes in its toys, other products were available that provided Lego-compatible weapons.
Designers of toy faces should take care with expressions and test their effect since toys play an important role in the development of children.
"A comic style expression is sufficient to convey a full spectrum of emotions and intensities."
Dr Bartneck, The head of UC's HIT lab, which focuses on human interface technology, will present his findings at the First International Conference on Human-Agent Interaction in Sapporo, Japan on August 7.
The popular construction toy Lego was founded in Denmark in 1932.