Very close childhood friends would also generally not be found to marry. Here the best evidence comes from two sources: Israeli kibbutzim and an old Chinese marriage custom. In kibbutzim, children are reared in crèches with unrelated companions. Lifelong friendships are formed, but marriages between fellow kibbutz children are very rare. In Taiwan some families practice “shimpua marriage” in which an infant daughter is brought up by the family of the man she will marry. She is therefore effectively married to her stepbrother. Such marriages are often infertile, largely because the two partners
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