Number of generations in free fermionic string models

In string theory there seems to be an intimate connection between spacetime and world-sheet physics. Following this line of thought we investigate the family problem in a particular class of string solutions, namely the free fermionic string models. We find that the number of generations N_g is related to the index of the supersymmetry generator of the underlying N=2 internal superconformal field theory which is always present in any N=1 spacetime supersymmetric string vacuum. We also derive a formula for the index and thus for the number of generations which is sensitive to the boundary condition assignments of the internal fermions and to certain coefficients which determine the weight with which each spin-structure of the model contributes to the one-loop partition function. Finally we apply our formula to several realistic string models in order to derive N_g and we verify our results by constructing explicitly the massless spectrum of these string models.