Being double jointed doesn't mean people have been born with a second set of joints. More accurately, the condition is referred to as joint hypermobility or laxity. What this means is that people can stretch certain joints beyond their normal range of motion, and this condition might result from perfectly ordinary circumstances. Alternately, it can be indicator of presence of serious disease and may prove medically problematic.
Many people who are double jointed are related to other people who are. If parents have this condition, for example, their kids may be more likely to have it too. It may not be present in all joints, but it might make a child more flexible. Children are generally much more flexible than adults and more prone to have looser joints that tighten up as they age.
Some of the genetic factors that could create this condition include the way a person's bones are shaped at joint meetings. Joints are typically sockets, and when one bone is not deep in the socket of the other, it is more flexible. While this may usually be quite benign, it may also make the person more prone to dislocations of the major joints. Some people may be able to self-dislocate joints, popping shoulders or knees out of their sockets and pushing them back in.
The other factors that contribute to joint stability are the muscles and ligaments that surround the joints. One or both of these may be weaker or looser than is ordinary. Loose ligaments mean greater flexibility, and weaker muscles could suggest less resistance to stretching. Again, the presence of this weakness may or may not indicate a serious problem, but some people do report pain with double joint symptoms and might require medical attention to analyze the problem and work on making the joints stronger.
There are a few conditions that have double jointed expression as a symptom. Though not always present, people with Down syndrome have hypermobile joints. Another disease that can affect joint mobility is Marfan syndrome, which may change bone size and growth and affect the body's connective tissue. Any disorder that can affect the actual joints or the areas that protect or control them could result in hypermobility.
Most people who are double jointed don't experience any problems with their joints and aren't suffering from any form of disease. There is a condition called hypermobility syndrome, however, that can be very complicated and requires help. People with this syndrome show hypermobility in many joints, and they are likely to have had joint injuries, like dislocations. One or more joints may also cause a great deal of pain. This illness may be addressed through a variety of tactics, and care can help reduce the continued overstretching of the joints, which is likely to create more pain and greater opportunity for injury.