To explain the faint galaxy excess, it is fair to say that there is also an excess in the number of models trying to explain it. When confronted with an excess of faint galaxies, the first step is to decide whether bright, distant galaxies or faint, nearby ones are responsible.
Models can be broadly divided into two scenarios: mergers and luminosity evolution. These scenarios are not mutually exclusive. For example, minor mergers could be triggering bursts of star formation at intermediate redshifts resulting in significant changes to the total luminosity of a galaxy. The two scenarios are simply divided inasmuch as they involve changes in different variables: number density and luminosity. Redshifts surveys have been unable to distinguish between the two since the evolution seen in luminosity functions as a function of redshift can either be fitted by an increase in the Schechter normalization (upward shift of the LF) or an increase in the Schechter luminosity L (leftward shift of the LF). This ambiguity is removed in internal kinematics studies since they are able to measure luminosity evolution in individual galaxies. Mergers and luminosity evolution are discussed below to provide a framework for the interpretation of internal kinematics results.