Various size-consistent approaches to the calculation of molecular hyperpolarizabilities are analyzed based on the double perturbation theory. General equations for the nth-order response property with respect to an external time-independent field are derived on the basis of the Rayleigh-Schrodinger perturbation theory (RSPT) and the coupled-cluster (CC) theory. The corresponding equations for the time-dependent case are also derived by the CC formalism, which is referred to as the time-dependent CC (TDCC) method. In order to clarify the spatial characteristics of polarizability and hyperpolarizability, we present an analysis method using a new concept "the polarizability and hyperpolarizability densities." As an application of the size-consistent methods, the static second hyperpolarizabilities (gamma) of pi-conjugated polymeric systems are calculated by the use of the uncoupled (UCHF), and coupled-Hartree-Fock (CHF) methods combined with the semiempirical INDO approximation. Characteristics of gamma values calculated for regular polyenes, solitonlike polyenes and donor(D) -acceptor(A) disubstituted polyenes are investigated, particularly in relation to the chain-length effect. Further, we employ gamma density analysis method, in which the third. derivatives of the Mulliken charge densities against applied electric fields are plotted for exploring the local contributions of the constituent atoms to gamma values. Results for the finite polyenes are also extrapolated to an infinity of the chain length to predict the intrinsic gamma values per unit carbon-carbon (CC) bond of polymeric chains. Furthermore, the CHF+Moller-Plesset second-order perturbation (MP2) method in the PPP approximation is applied to polymeric systems with larger chain length in order to elucidate the electron correlation effects on the chain-length dependencies of gamma values.