It may safely be said that sex is one of the important spheres of human behavior. However, there are few sociologists who study mainly in sexual behavior at least in Japan. Then reexamination of the experiences of research and analysis in sexual behavior may give us suggestions to sociological studies.
    First suggestion is about research method. Research in sexual behavior requires many compromising ways because of the fact that sexual matter belongs topmost human privacy.
    Secondly, introducing a viewpoint of sexual behavior may bring new findings to some research fields. As examples of such fields studies in developmental process, social stratification and gender relationship are examined.

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     The social value theory is one of major social psychological theories dilemmas researches. The primary concern of this theory is to examine the effects of members' social value orientations and perceptions of others' value orientations on their behaviors in social dilemma situations. In this paper, we tried to formalize the basic concepts of the social value theory in terms of game theory and model an prisoner's dilemma game with incomplete informations. Based on this game theoretical model, we specified the conditions on which some propositions obtaind from experimental researches were not sustainable. From this finding we drew the conclusion that the conceptualization of social value orientations by the social value theory need to be revised.

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     Who should be a rational decision-maker in a game-theoretical analysis of complex social phenomena in social sciences? It has been in common practice primarily in the United States to assume that a state is a unitary rational actor and its foreign policy choice is collectively rational at the state level when analyzing inter-state strategic interactions in the theory of international relations. This paper examines an analytical bias of this theoretical approach from the viewpoint of methodological individualism. Building a formal model in which an electoral process shapes the foreign policy decision of a democracy, the paper identifies and analyzes the impact of the income distribution on national defense spending decisions, independent of the aggregate income. An analysis of this model reveals the mechanism in which a more unequal income distribution reduces the level of military expenditure. Furthermore, this paper rationally explains a paradox that an increase in the lower-income population in a democracy induces the central government to pursue a public policy in the interests of the higher-income population.

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     The aim of this paper is to analyze the ability of citizen-voters to affect outcomes of legislative decisions through the medium of their legislator's votes. To measure voter's power, the well-known Shapley-Shubik index (SS index) is used.
     We first review the following known fact. If the voting system is formulated as a traditional voting game, the index gives curious and counter-intuitive outcomes: these include in part the fact that power of a voter in a electoral district with even number of representatives is negligibly small. Then we present a new cooperative game formulation: its characteristic function is given by the number of representatives that each coalition of voters can control. In the new game formulation, the SS index of a voter is given by the ratio of the number of representatives apportioned to his/ her district to the number of voters in the district. In case elections of representatives in blocs, consisiting of several districts, are simultaneously carried out, the SS index of a voter is given by the sum of the ratios in his/ her district and in the bloc that contains the distirict. Finally citizen-voters' power in elections of the Japanese Upper and Lower Houses are measured in terms of the SS index in the modified game formulation.
     The Banzhaf index, another well-known and widely used power index, provides counter-intuitive outcomes not only in the voting game formulation but in the new mage formulation.

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     This essay examines authorized power that has sanction. Generally, it is assumed that formal power contributes to maintenance of organization. However, there is a case that formal power realizes Pareto inferior state. Therefore, contribution to maintenance of organization is not an essential property of formal power. This essay shows this fact by using game theory, and it points out game theory is a useful tool to analyze social phenomena.

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