In the institution of modern marriage, sex, reproduction and marriage are considered inseparable, and the functions of reproduction such as childbirth and child-rearing are expected to be exercised within marriage. Moreover, the parent-child relationship is strongly connected with the marital relationship and a legal father is determined by the presumption of legitimacy. In many Western industrialized countries in recent years, with the increase in non-marital cohabitation, births outside marriage, divorce, single-parent families and reconstituted families, the functions of human reproduction have been performed not only within the legally-married family but also within variant family forms. From the standpoint of respect for the human rights of children and a neutrality of lifestyles, such countries have revised laws relating to families, so that the concepts of legitimacy and illegitimacy have been abolished, along with the presumption or denial of legitimacy. Father-child relationships are formed regardless of the parents' marital relationships and joint custody after divorce is accepted. In Japan, although premarital sex is common, many people strongly advocate the norm of legitimacy, and maintain the pairing of marriage and reproduction. In order to view how legal parent-child relationships evolve from the institution of marriage, I considered different formations of father-child relationships from the standpoint of whether or not the father was married to the child's mother, and examine problems in the denial of the presumption of legitimacy, the nationality of an international child born outside marriage, the notification of birth by an unmarried father, single custody, and the institution of responsibility for the rearing of a child.
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