Why do humans cry tears?

Humans are arguably the only animals that cry (some animals do howl or make other sorrowful noises which may or may not rise to the level of crying) and we are the only mammals ones that cry tears. 

For the most part, tears are necessary to maintain healthy eyes and are usually excreted to keep our eyes moist (this happens constantly) or upon injury or introduction of a foreign object. Other mammals have tear ducts and they also excrete tears for these eye health maintenance reasons. Humans are the only animal that excretes tears for emotional reasons. Emotionally produced tears are called psychic tears.

Interestingly, the chemical makeup of crying tears vs. the other types of tears is different.  Psychic tears contain more types and amount of various hormones than the other types of tears.  This initially led scientists to think that the purpose of crying tears was to expel hormones that build up when we get emotional.  Like an emotional release.  However, the amount of hormones excreted during even an intense, very tearful crying session were determined to be relatively slight and would not have much or any impact on overall hormone levels within the body. That being said, a good crying session typically relieves stress.

From a natural selection perspective, the development of specific traits that are different from other mammals must have given humans a survival advantage or such traits (which arise as the result of genetic mutation) would not proliferate.  What possible survival advantage could psychic tears have?

While sometimes we produce tears in response to great joy (like laughing so hard you cry tears), typically psychic tears signal real emotional distress.  Children cry from birth but only have the ability to cry tears sometime between 6 and 12 months of age.  Tears help parents differentiate between real distress and less urgent (and sometimes fake) crying.  When you see your child cry tears you know they are really upset and as parents we react differently.

As with children, tears in adults also signify real emotional upset and help us determine the emotions of others.  In a study researchers took hundreds of pictures of people crying tears and digitally edited out the tears from the pictures.  They showed the original pictures to one group of people and the edited pictures to another group.  In the group with original pictures with tears volunteers were able to correctly determine that the subject in the picture was displaying emotional sadness nearly 100% of the time.  In the other group – with the edited pictures – the volunteers had a much wider varying response as to what the emotional state of the subject was – many times the response was that the subject was angry or scared or confused rather than upset and crying.

Anthropologists think this ability to cry tears helps create closer relationships between humans.  It makes the family, tribe and community closer because knowing and reacting to each others feelings binds us together.  Early humans who had the mutation of crying tears had more successful groups, tribes and communities.  This led to greater survival and thus the proliferation of tears.