Lego toys are much angrier now than they were during the 1970s. According to a new study, the number of happy-face toys has decreased in the last few years, while the number of angry-face toys has increased.

The study was conducted by Dr. Christoph Bartneck at the University of Canterbury. "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.

According to the study, Lego's mini-figures have evolved from simple happy faces to faces that have expressions of anger, confusion or fear.

The toys' themes have changed along with themes that are based mostly on conflict now, study authors said.

For the study, researchers photographed over 3,600 Lego characters that were released between 1975 and 2010. They then asked 264 American adults to categorize the facial expression of the figurines into "happy", "sad" or "angry" etc., according to The Washington Post.

According to the researchers, children growing up today will remember angry-faced Lego toys along with the happy-faced ones. Toy designers must carefully choose the facial expressions that they want to put on the toys, as they can have a great impact on the child's mental development.

"The example of the mini-figures show that to appeal to users it is necessary to offer a wide range of emotional expressions for today's users. Instead of focusing on realistic expressions, it may be worthwhile to increase the variability of expressions. A comic style expression is sufficient to convey a full spectrum of emotions and intensities," Dr. Bartneck said in a news release.

The study will be presented at the First International Conference on Human-Agent Interaction in Sapporo, Japan, starting Aug. 7. Read the entire study here.