In the same 2000 study, Vingerhoet’s team also discovered that, in adults, crying is most likely to follow a few specific antecedents. When asked to choose from a wide range of reasons for recent spells of crying, participants in the study chose “separation” or “rejection” far more often than other options, which included things like “pain and injury” and “criticism.” Also of note is that, of those who answered “rejection,” the most common subcategory selected was “loneliness.”

In both adults and in children, “the prototypical situations that induce crying are always related to loss or separation, and/or powerlessness,” Vingerhoets says—our experiences of these things just change with age. While, as adults, crying no longer functions as a means of keeping our caregivers close, it still retains a vestigial element of attachment—we keep crying past the point of its practical usefulness because of a deep, lingering lack.

Even when adults cry out of happiness—at, say, a reunion or the birth of a healthy baby—it’s still connected to this same idea of attachment. We cry happily when we recognize, deep down, that every connection we make in life could end up severed.

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The similarity between “happy” tears and “sad” tears also exists in the biochemical and physiological processes that underpin all types of emotion-based crying.

Years ago, the chemist and tear expert Dr. William Frey published a number of highly cited studies showing that when we go through any emotion-based crying (happy and sad), our tears contain stress hormones and other chemical toxins. Frey took this as proof that crying is about the removal of stress hormones from our bodies in order to maintain emotional homeostasis. The actual health benefits of this process have been debated—Vingerhoets suggests that instead of asking, “does crying bring relief?” we should be asking, “for whom and in what conditions does crying bring relief?”

Either way, it’s useful to consider tears as the “outward signs of abrupt shifts in neurophysiology,” write researchers Jay Efran and Mitchell Greene. More specifically, crying, whether happy or sad, is the external result of an internal body shift, when the sympathetic nervous system — the “fight or flight” mechanism— gives up control of the body to the parasympathetic nervous system —a.k.a. “rest-and-digest.”

Think about the last few times you’ve cried. Regardless of what it was that ultimately caused you to tear up, most likely, you didn’t cry during the height of the experience.

For example, say you’re having a terrible day at work—just the most awful eight hours you could imagine. No matter how stressful it gets, it’s unlikely that you’ll cry, because your body won’t let you. In moments of intense stimulation, your sympathetic nervous system kicks into gear. And during this time, your body has no use for crying; it is stressed and aroused, focused and functional. It says “deal with it,” so you do; you go pick up the lunch that was never delivered, you re-write the report that you lost when your computer shut down, and you walk the extra 15 blocks when your train stopped running.