Nature certainly has it's impact, but so does nurture.
Working on straightening your spine structurally isn't too easy, but you can also straighten it energetically.
A doctor could probably tell you if there was something really causing your spine to 'naturally' stay out of line. But your spine is capable of a lot more than you may know. With work you can expand and contract the space between vertebrae, your spine can move more fluidly like a ferret. When your spine starts getting this mobile, you'll see for sure how straight your spine should naturally be.
