Will sucking your thumb cause you to need braces?

Depending on the frequency, duration, intensity, and technique plus the general health of your dental structures and your age (if you're in the growth stages of your jaw and dental structures it will have an increased chance of affect) it may, or may not result in the "need" for braces, a situation that depends on your tolerance for any possible resulting malocclusion as well as the opinion of your dental professional.