I had two younger sisters (12 years younger) and I can say that if they were not spanked for direct defiance (and only direct defiance and situations involving physical danger to them, never for mistakes or reasonable young-child stuff) they would be hideously uncontrollable brats right now. One in particular had a very strong-willed, self-focused personality from the beginning (the other, her twin, was much more laid back) and would look you straight in the eye and do exactly what you told her not to do. She broke her first window at about two years old. She's a sweet 8 year old now, thanks to moderate discipline.