People cry when they experience intense emotions. Crying seems to occur most frequently in intense episodes of sadness and fear, but sometimes also happens when people are very happy or angry. What is the purpose of crying and which role has it played during human evolution? Which theories explain how crying may have given humans an evolutionary advantage?
The two main folks in crying research (of whom I'm aware) are Ad Vingerhoets and Jonathan Rottenberg. They've (together and separately) published reviews of adult crying and crying across the lifespan, as well as empirical articles. The general impression they give is that we know very little about the neuropsychobiology of crying, given that crying has been the focus of very little scientific investigation (despite its prominence in our understanding of emotions like sadness and grief).
There are a bunch of interpersonal, intrapersonal, and evolutionary theories about crying, but none have been particularly well tested (to my knowledge). Vingerhoets, Bylsma, and Rottenberg (2009) have put forth a model of adult crying that may help to explain some things about why we cry.
In general, this model suggests that:
- Crying can attract support from other people, and those people can help reduce the distress that elicited the crying. (For a more general theory of interpersonal emotion regulation, see Zaki & Williams, 2013.)
- Crying might lead to reduction of distress via some (poorly defined) psychobiological mechanisms. These mechanisms might include activation of the parasympathetic nervous system, opioid release, oxytocin release, beliefs about the functions of crying, the calming effects of rhythmic behavior, and more (see Gracanin, Bylsma, Vingerhoets, 2014).
If crying attracts social support and can reduce distress, then this may improve overall evolutionary fitness. This is especially true in vulnerable infants, who use crying as a means to attract the attention of their parents in distressing situations (Rottenberg & Vingerhoets, 2012). Crying may be less important in adulthood, but may have persisted either due to (1) general intra- and interpersonal emotion regulation benefits and/or (2) neoteny (see Rottenberg & Vingerhoets, 2012, p. 225 for an explanation).
However, it's not true that crying is always cathartic. This is very much context- and person-dependent (Bylsma, Vingerhoets, & Rottenberg, 2008; Bylsma et al., 2011). Nonetheless, crying may have stuck around due to its importance in infancy as a loud vocal signal to the parents.
Regarding why we cry during positive experiences--well, there's not a lot of good evidence here. However, it's quite possible that positive experiences do not elicit crying (Vingerhoets & Rottenberg, 2012). Instead, it might be our tendency to reflect on negative experiences during positive moments that elicits crying (e.g., the positive experience of college graduation might elicit reflection on the many college struggles we had or the fact that we're not going to see our friends as often).
Overall, there's a lot more research to be done on crying, so stick around. The hypotheses/theories presented above are probably pretty sound, but there's still a lot we don't understand.
In addition to @mrt's great answer. I feel that the following excerpt from the 'crying' section from the "The Newborn Infant" chapter in my Developmental Psychology classes' textbook would shed light on your question. This is quoted directly from "How Children Develop, Third Edition" by Robert Siegler, Judy DeLoache and Nancy Eisenberg":
How do you feel when you hear a baby cry? We imagine that, like most people, you find the sound of a crying infant extremely unpleasant. Why is an infant's crying so aversive?
From an evolutionary point of view, adults' aversion to infant's crying could have have adaptive value. Infants cry for many reasons - including illness, pain, and hunger - that require the attention of caregivers. Adults' high level of motivation to stop an infant's crying prompts them to take care of the infant's needs and hence promotes the infant's survival. This fact has led some researchers to suggest that in times of hardship, such as famine, cranky babies are more likely to survive than are placed ones, possibly because their distress elicits adult attention and they consequently get more than their share of scarce food resources (DeVries, 1984).
Parents, especially first-timers, are often puzzled and anxious about why their baby is crying. Indeed, one of the most frequent complaints pediatricians hear concerns what parents think is excessive crying (Barr, 1998; Harkness et al.,1996). With experience, parents become better at interpreting their infant's crying, identifying characteristics of the cry itself (a sharp, piercing cry usually signals pain, for example) and considering the context (such as when the infant's last feeding was) (Green, Jones, & Gustafson, 1987).
Crying increases from around 2 weeks of age to a peak at 6 weeks, but then declines to about an hour a day for the rest of the first year (St. James-Roberts & Halil, 1991). On a daily basis, the peak time for crying is late afternoon or evening. The phenomenon of "evening crying", which can be quite disappointing to parents looking forward to interacting with their baby at the end of the workday, may be due to an accumulation of excess stimulation over the course of the day.
The nature of crying and the reason for it change with development. Early on, crying reflects discomfort from pain,hunger, cold or overstimulation, although, from the beginning, infants also cry from frustration (Lewis, Alessandri, & Sullivan, 1990; Sternberg, Campos, & Emde, 1983). Crying gradually becomes more of a communicative act; the crying of older babies often seems geared to "tell" caregivers something to get them to respond (Gustafson & Green, 1988).
Sources cited by textbook:
DeVries, M.W. (1984). Temperament and infant mortality among the Masai of East Africa. American Journal of Psychiatry, 141, 1189-1194.
Barr, R. G. (1998). Colic and crying syndromes in infants. In J. G. Warhol and S. P. Shelov (Eds.), New Perspectives in Early Emotional Development (pp. 147-157) Calverton, New York; Johnson & Johnson Pediatric Institute, LLC.
Harkness, S., Super, C., Keefer, C. H., Raghavan, C. S., & Campbell, E. K. (1996). Ask the doctor: The negotiation of cultural models in American parent-pediatrician discourse. In S. Harkness & C.M. Super (Eds.), Parent's cultural belief systems: Their origins, expressions and consequences. New York: Guildford.
Green, J.A., Jones, L.E., & Gustafson,G.E. (1987) Perception of cries by parents and non parents: Relation to cry acoustics. Developmental Psychology, 23, 370-382.
St. James-Roberts, I., & Halil, T. (1991). Infant crying patterns in the first year: Normal community and clinical findings. Journal of Child Psychology and Psychiatry, 32, 951-968.
Lewis, M. Alessandri, S. M., & Sullivan, M.W. (1990). Violation of expectancy, loss of control, and anger expressions in young infants. Developmental Psychology, 26, 745-751.
Sternberg, C., Campos, J., & Emde, R. (1983). The facial expression of anger in seven-month-old infants. Child Development, 54, 178-184
Gustafson, G. E., & Green, J. A. (1988, April). A role of crying in the development of prelinguistic communicative competence. Paper presented at the International Conference on Infant Studies, Washington, DC.
I recall a study from maybe 30 years ago that got people to cry by chopping onions and others cry because of watching a sad movie. The content of the tears were tested, and the sad tears had a mood-elevating chemical element. I recall the study because I had seen the movie they used: All Mine To Give, about orphaned children, where the eldest had to parcel out the younger children. If you ever need a good cry . . . . Maybe tears of joy are connected with relief and also tied to mood-elevating hormones.
@Cemre,@MariaAnt-Humans' crying is very natural,because of the following fact: These are tears produced in response to that strong emotion that they experience from stress,pleasure,anger,sadness and suffering to indeed,physical pain (and often due to jealousy felt at others' success),Psychic tears even have a natural painkiller,called leucine enkephalin-perhaps the part of the reason why they might feel better after a good cry,These tears also play an useful role in preventing the eyes from drying out.