There are some benefits and drawbacks to joint hypermobility. The upside includes possible advantages for musicians or athletes. Think of a pianist with hypermobile fingers and thumbs, or a gymnast with hypermobile hips or vertebrae. Other advantages include freaking out your family members at the dinner table and winning bets in bars. This ability must be honed in order to maintain it, though, whether you're a concert pianist or just a hypermobile hobbyist. If you have extra range of motion, you must keep your joints limber through regular stretching, or some of that ability may be lost as you age.
On the other hand (the extra-bendy one), hypermobility often comes with a steep price. There is an increased risk of arthritis in hypermobile joints, especially fingers. There may be extreme pain felt in many different joints, especially in younger people who are going through rapid growth spurts. Although some athletes may benefit from hypermobility, other people with hypermobile joints are more vulnerable to injuries. Several different but related conditions that cause pain or discomfort are grouped under the umbrella term hypermobility syndrome (HMS). To be clear, having joints with hypermobility doesn't mean you have HMS -- only if it's the source of chronic pain, which occurs in a minority of people with hypermobile joints. However, if you do suffer from HMS, there is a 1-in-2 chance your offspring will as well [source: Grahame].
Interestingly, if you can do things like put both feet behind your head and walk around on hyperextended arms while swinging your upper body between your elbows, local anesthetics may not be as effective on you as your less flexible compatriots. Research has indicated that local anesthetics seem to have little or no effect on many hypermobile people, something you may want to mention to your doctor if you have a medical procedure or pregnancy approaching.
So, you can't be double-jointed, but only because the term doesn't really mean anything. You can, however, be hypermobile, meaning you put just a little extra in everything you do.
For lots more information on hypermobility, bones and joints, see the next page.