My own String theory (Score -1, Offtopic)

Besides the obvious (tight body wild girls wear string more than chubby down-to-earth girls): Women tend to use string a whole lot more during the early stages of dating and a relationship. As the relationship nears a more "safe" stage were the initial heat is over, they start sneaking in the "b-team" underwear: grandmas nickers and worn-in panties, just saving the more s*xy string lingere for "special ocassions". Its an evil and sneaky practice, but thats the string theory for you. Oh the humanity!