A combination of factors which all came together at the same time. In no particular order - musical ability, a sudden influx of spending power for teenagers, the spread of r 'n' b - based popular music, shrewd management, the personification of a new youth movement, a shrewd sense of media manipulation, the ability to stay together as a group when fame kicked in, a natrual ability to grow and develop as musicians, the ability to take their positions as figureheads of a generation and carry on making new vibrant music throughout their long and unrepeatable career. That set of circumstances is unique - we have all benefited from it.