Lead author Robert Hepach of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, said the study involved 48 children ages 36-39 months.
The researchers recorded reactions of each child as he or she witnessed an adult acting upset in one of three contexts: when the distress was justified, when it was unjustified and when the cause of the distress was unknown.
The study, published in Developmental Psychology, found when a child witnessed an adult in a justifiably distressing incident, the child's face showed concern, whereas the child's expression indicated he or she was "checking" when the incident did not warrant distress or the adult was out of sight but could be heard.
In subsequent tests, one adult was given one helium balloon and the child was given two. When the adult "accidentally" let go of his helium balloon and became distressed, the child would offer a balloon more quickly to the adult if the child had previously seen him upset due to true harm rather than an inconvenience, Hepach said.
"These very young children really considered what was happening in a given situation rather than automatically responding with sympathy to another person apparently in distress," Hepach said in a statement.