Why people cry even when they are happy

"Tears flow when people are overwhelmed with strong positive emotions and people who do this seem to recover better from those strong emotions," said psychologist Oriana Aragon at the Yale University in the US.

"People restore emotional equilibrium with these expressions," she added.

Aragon and her colleagues at Yale ran participants through some emotional scenarios such as cute babies or a crying spouse who is reunited with her soldier husband returning from war, and measured their responses.

They found that individuals who express negative reactions to positive news were able to moderate intense emotions more quickly.

They also found people, who are most likely to cry at their child's graduation are most likely to want to pinch a cute baby's cheeks.

There is also some evidence that strong negative feelings may provoke positive expressions.

For example, nervous laughter appears when people are confronted with a difficult or frightening situations, and we also smile during extreme sadness.

"The new discoveries begin to explain common things that many people do but do not even understand themselves," Aragon said.

"These insights advance our understanding of how people express and control their emotions, which is importantly related to mental and physical health, the quality of relationships with others, and even how well people work together," she concluded.

The paper was published in the journal Psychological Science.