There are actually some good statistics. If you look at 100 richest people on earth, 37% of them grew up middle-class households, 36% of them grew up poor households (http://fundersandfounders.com/how-much-sweat-it-takes-from-z...).

With the distribution of middle-class and lower-class of around 50-50 in the U.S. (http://en.wikipedia.org/wiki/Social_class_in_the_United_Stat...), it appears that it doesn't matter at all in what environment the 100 richest people today grew up.