In a world characterized by perfect and complete capital markets, the success (or failure) of a merger is judged by the merger's impact on stockholder wealth. With completeness, the merger's impact on the probability distribution generating stockholder returns is unimportant. The perfect market assumption guarantees that the stockholder not satisfied with the consolidated firm's return distribution can frictionlessly sell his shares and reorder his portfolio; hence his only concern is the merger's impact on wealth. However, if we acknowledge the existence of commissions, taxes, and other frictions, or if markets are not complete, the merger's impact on the stockholder return distribution becomes relevant. In this study we will analyze 149 mergers involving large N.Y.S.E. firms. We will examine four different hypotheses related to the impact of merger on attributes of the stockholder return distribution. We focus our analysis on risk-related attributes including beta, total variance, residual variance, and several other risk-related attributes. In a companion paper, merger's impact on wealth is calculated for the same sample but will not be reported here.