In a study out last month in the Journal of Orthopedic Surgery and Research, researchers took x-rays of children with scoliosis <25 degrees. They then took a follow-up x-ray about one year later. None of the patients had any treatment of any kind. They found that nearly one-third of the patients saw their curvatures get better, while 26% got worse, and 41% stayed the same.
Incidentally, they also noted that the pattern of the scoliosis changed over time in 46% of the patients. They theorize that this happens due to the spine trying to find a more balanced position to prevent curvature progression.
Here is a link to the full study: