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There are many reasons for PCD, caused either by the special demands brought on by conventions, or the immediate events after a convention.
There is a biological theory of why this may happen. If a person has so much fun at a con that they use up all their happy hormone (serotonin) they may feel depressed until their body has time to make more.
Despite being a mental state, the physical status of a person can often point to many causes for their behavior as well, all of which can be traced back to the convention itself. Many of these causes only arise after the convention, since any given problem may only appear once the fur has calmed back down and attempted to settle back into their normal daily demands.
- Physical exhaustion - The stress of walking around with items for several hours on end may be physically exhausting; even if this stress is not acknowledged during the convention, it can catch up with an individual eventually.
- Poor Convention diet - Being away from home (thus hindering the preparation of homemade meals) and trying to save money for other resources like merchandise, commissions, and lodging, some people may try to cut back on how much they spend on food.
- As cheap food usually isn't the most healthy, this poor diet and the malnutrition that goes with it often contributes to a bad mood. In addition to poor diet, the party atmosphere may encourage people to drink or use other drugs they wouldn't normally consume, contributing to hangovers and other undesirable side effects.
- Lack of sleep - A desire to experience as much of the convention as possible can cause people to reject their normal sleep patterns, staying up late for dances or mature-themed panels. This disruption can reduce the amount of energy people have, making them more irritable. In addition, there is some evidence to suggest disrupting sleep can induce depression. 
- Exposure to foreign illness - All sorts of people come from several parts of the world to attend conventions, and many of them may carry some form of illness (recovering, current, or only recently infected). As a result, conventions are a major infection vector. This is also a primary source of Con Crud.
- Alcohol Consuption - Alcohol is classified as a Central Nervious System depressant. Binge drinking specifically, can contribute to feelings of unhappiness.
Unlike the transition from a fur's normal life to a convention, the adjustment back may be difficult for several reasons.
- Unhappiness with real life - Conventions are seen as stressful yet fun, while normal life may merely be stressful. As a result, there is less enjoyment to be found in normal tasks, and this re-adjustment to normal behavior can cause problems.
- Separation from friends and family - Furs may be more social at conventions than they are in real life, and this separation merely reminds them that the time spent at convention is only temporary and fleeting.
- Adjustment of social status - Individuals that are popular at conventions may not be so in the 'real world' and so the check to their ego may cause old issues in real life to flare up.
- News events - The world does not stop for a convention, and so shocking news of events that people missed while away may jar them, especially if the event is a serious one like the death of a family member.
- Money shock - If someone has overspent at a convention, they may become highly sensitive to future spending efforts, at least for a little while, to compensate for the added costs.
- Over Stimulation - It has been speculated that increased social stimulation may use up available serotonin in the pre-frontal cortex. In addition norepinephrine and doplamine systems may also be involved and contribute to a "con hangover" 
Post-Con Depression, like most holiday-related stress, typically goes away on its own after a few days as the individual settles back into their daily lifestyle. In spite of this, the intensity of the depression may be difficult, so there are a few things that can be done to mitigate this stress.
- Take care of your body - Healthy behaviors will usually be enough to counteract most physical stressors. Showering, sleeping, a full night's rest, and eating properly will help your body to cleanse itself. If you believe you are becoming ill as well, take vitamins and whatever other medicines are needed.
- Analyze your convention experience - Posting to an online journal or on a forum about the great time you had at the convention will help encourage others to do the same and to talk with you about it, providing social support and allowing you to see that many others are having the same issues with returning to normal life. Also, reliving the good parts of the convention may help you feel better.
- If you work at conventions and/or this convention didn't go as well as you hoped it would, this is also a good time to consider how you can improve on things for next time to make your next convention a better one.
- Look forward to other conventions - There's always next year, or the next con as the case may be, and that means you have plenty of time to prepare for it now! Take advantage of this and get ready!
- ↑ http://www.eurekalert.org/pub_releases/2005-06/uorm-imp062105.php
- ↑ http://www.medicalnewstoday.com/articles/60184.php
- ↑ http://www.ncbi.nlm.nih.gov/pubmed/18096212