Crying is a pivotal act in emotional health. It is a biological function, a deep self-expression, a call to attunement, and a surrender. Crying makes it possible for suffering over past events to become past suffering.
Crying is an act of release, usually of sadness, but sometimes also of joy. The most superficial type of crying involves tearing of the eyes. Few adults these days are able to cry anymore deeply than this. The next level is with tremor in the jaw and cheeks, and involuntary . The deepest level involves the entire torso shaking and convulsing including the diaphragm. This is sometimes expressed "wracked with sobs."
The deeper the crying the greater the release. It is known that shaking and quivering behavior limits the effects of trauma. Everyone feels better after a "good cry." Watching sad movies or plays has always been used by wise people as an opportunity to catch up on crying. This crying is not false or spurious, it comes from inner sadness that is evoked by what is being portrayed.
Muscular tension inhibits crying just as it inhibits all emotional expression. Young children are the most able to cry because they have less muscular development. Women in general cry more easily than men, just as they are closer to other emotional expression, because of less muscle mass. However, in our culture there is a message that everyone should cry as little as possible. This is an unfortunate message. In Shakespeare, heroes cry frequently and deeply. Now that is not a 'scientific' source of actual 16th century behavior, but it does indicate the social norm of the time.
Babies cry frequently, because they are frequently helpless. The crying usually gets help and that is erroneously believed to be its only function. Children are encouraged not to cry in part because it is feared that they will manipulate others that way. This groundless fear arises because of today's obsession with power. Fake crying is possible, but this is easily discerned.
Other people are moved to help when they see or hear someone crying. But crying is primarily about releasing sadness, or coping with a bad circumstance in which nothing can be done. People that resist crying are always made uncomfortable by the presence of someone crying, and that is perhaps the greater part of our cultural disdain for crying. One person crying can start capable others crying, and there is no harm to this, just the opposite. Difficulty crying is difficulty grieving. An accumulation of ungrieved losses leads to deadness.
Besides facial stiffness, crying is impeded by a reflex closing of of the throat (choking in the narrows). Alexander Lowen's bioenergetic stool is a tool that in bodywork is used to help open the trachea and lungs and support deep crying.
Laughing: Physically, laughing is completely the same below the chin as crying. The difference is in the face: with laughing the corners of the mouth are upturned, the same as smiling. With crying, the corners of the mouth are downturned. Someone crying can be stopped by asking them to smile (although there is never any real justification for doing so). Likewise, some one laughing heartily at adversity can convert to crying by the turning the mouth corners down (and this may well be justified). Laughing can be a lesser surrender than crying but still a release.