From my reading today, here are letters between spouses in the prelude to one of the most notorious child custody battles of mid-19th c. America.
Ellen to Gonzalve: Happiness I shall never know again; and all my earthly prospects, which a few years since seemed so fair, are darkened, never to be bright again! You alone can restore me to tranquility, by ceasing to pursue my child. My only care is watching over it -- my life is its happiness.
Gonzalve to Ellen: I do not wish my wife, if she does not return with the feelings of a wife for her husband; but I take this last occasion to make you reflect upon the immense responsibility that you are taking, not only with regard to myself and your child; but before God, and what you call society, in voluntarily deserting your husband, and concealing a child from its father.
Ellen to Gonzalve: The arrow has passed through my heart, and I am so unfortunate as to have no longer, there, a feeling towards you which would induce me, according to my conscience, either before God or the world, to live with you again as your wife.
Gonzalve to Ellen: There is a duty reposing on me as a father, as a Swiss, as a member of society, that leaves me no alternative. He is the natural inheritor of my estate, he is destined therefore to enjoy a position in his country, that he can find no where else, and only if he is brought up there: I am surprised that you wish to deprive him of it.
At trial, mom won custody of the boy. The case heralded a shift from legal paternalism (e.g., in the eyes of the law then, "The husband and wife are one person; that person is the husband") to maternalism -- the presumption that children are best off with their birth mothers.
(Michael Grossberg, A Judgment for Solomon: The D'Hauteville Case and Legal Experience in Antebellum America, 1996.)
Tenuous Japanese connection here: "The image of fathers is gradually changing in Japan as younger men eschew their own dads' hands-off approach in favor of closer involvement..."