Tom Kurz says it's a misconception that most people are physically incapable of training up to a full side split:
There are simple tests that show you that for the great majority of people the structure of joints and the length of ligaments are not the obstacle for doing splits. You can use these test to determine if you have the potential to do front and side splits before you start your stretching program. (Yes, there are people who cannot do side splits because of hip joint deformity called coxa vara.)
Here is the side split test: Stand beside a chair or table and put your leg on it as shown below. Make sure that both your hips and your raised leg are all in one line. Repeat this test with your other leg.
If you think that the length of your muscles and structure of your hips will not let you do side splits, try this test… The leg resting on the chair is in the position it would have in a split
Now, what have you done? You have done “half side splits” with both your legs!
You have proved to yourself that both your hip joints have all the mobility (range of motion) needed for a full side split!
You have also proved that the muscles of each of your legs are already long enough for a side split. You know that no muscle or ligament runs from one inner thigh to the other (or, if you don’t know it, you can ask your doctor). So, what keeps you from doing the whole side split with both legs spread sideways at the same time? Your nervous system, that’s what!