Why do we cry - science behind tears

Do not apologize for crying. Without this emotion, we are only robots,' says Elizabeth Gilbert in her book 'Eat, Pray, Love '.
Anything from moist eyes to full-scale sobbing is crying. Women seem to be more susceptible to tears than men, with statistics revealing that women cry 5.3 times a month, while men cry about 1.3 times a month on an average. [1] Why? Biologically speaking, men have more testosterone, which inhibits tears. On the other hand, women have more prolactin, which stimulates tears. But it's not just these two hormones that decide whether you cry or not, since there are three types of crying, all involving different mechanisms and chemicals.
Types of crying
A protein-rich antibacterial liquid is constantly secreted by the lacrimal gland present in the outer edge of the eyeball. When we blink, the liquid is released, which then lubricates the entire eye surface. These are called basal tears and their function is to lubricate and protect the eye at all times.
Notice how you 'cry' when chopping an onion? These are reflex tears. Reflex tears protect the eyes from irritants such as wind, smoke, and chemicals. They also help flush out random speck of dirt or any object that gets into the eye.
Another type of crying is emotional crying, in which tears are produced in such large quantity that they overflow, overwhelm the nasal canal of the tear duct and fall down our cheeks. This type of crying occurs in response to stress, frustration, sadness, and happiness, and any other emotion that evokes tears.
Hormones / neurotransmitters at play while crying
Apart from prolactin and testosterone modulating your crying, other hormones and neurotransmitters play a role as well.
Love makes you cry
Serotonin influences a variety of physical and psychological functions. It has a big role in mood balancing, bowel function including suppression of appetite, aids formation of blood clots in wounds, reduces libido, and decreases bone density. That's why persistent high levels of serotonin increase the chance of osteoporosis. So, what does it have to do with crying? Studies have shown tryptophan, which is the precursor of Serotonin (5-HT), is reduced in women in the postpartum period, which is characterised by emotional instability and proneness to high-crying. Being in love is also accompanied by low 5-HT levels, which makes you more prone to crying. Researchers believe that 5-HT levels might increase the threshold for crying. [2]
Flush out the stress
ACTH (adrenocorticotropic hormone) is released from the pituitary gland and it signals the adrenal glands to produce cortisol, the fight-or-flight-response hormone. Cortisol, as we know, increases blood pressure and blood sugar, making us hyperventilate, and triggers other physiological changes to charge us up for action. This causes stress. One way to alleviate stress is through crying. We have all experienced the feeling of calm and being spent after a good cry. This is because excess ACTH is flushed out through the tears and reduces cortisol, the stress hormone.
Another natural stress buster in the body is the chemical enkephalin, an endorphin that reduces pain and improves mood. It is very similar to opiate drugs that we take as painkillers. The neurochemical enkephalin works by blocking the pain signal in the spinal cord.
Crying at different ages
All babies are born with basal tears. New born infants don't have tears when they cry, the fluid comes later when they are slightly older. They cry for their basic needs to be fulfilled - hunger, sleep, feeling pain, irritated or simply uncomfortable.
As they grow and develop, say about 10 months, their crying changes to more manipulative ones to get attention. These are called 'fake' tears.
Testosterone and prolactin kick in during the adolescent years. This is when girls become more weepy and boys crying comes down drastically.
Men tend to cry only when they suffer major losses. Women, however, cry even when frustrated. They cry more loudly and with copious tears. Scientists believe the size of the tear glands to be one of the causes. Men have smaller tear glands than women, so they can't tear up volumes at one go. The other view is tear ducts in men are larger than those of women, so overflowing of fluid in women is faster than in men.
Crying pattern changes as we reach our middle ages because of changing testosterone and progesterone levels. Men begin to cry more and get less angry. Women, on the other hand, cry less.