Originally it was thought that children go through three big developmental stages. Young kids (2 to 4 years old) are purely selfish, while older children (5 or 6) will follow a strict equality rule. When children become older (7 or 8) they start to consider the individual contributions of others. The thought being older children are more aware of their surroundings and less impulse driven.
A recent study finds that children will start to evaluate these contributions earlier than expected. Patricia Kanngiesser from the University of Bristol led and published a study about fairness starting in children at the age of three. She said:
“It seems to be intuitive, People have found that even by 18 months of age, children have expectations about how things should be shared fairly.”