It seems to me to be a strange design decision for GEDCOM to make the structure based around the nuclear family.
What was the officially stated reason or reasoning for choosing this design?
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The Church of Jesus Christ of Latter Day Saints have an engrained concept of Family.
The Lord has designated the family to be the basic unit of the Church and of society. As used in the scriptures, a family consists of a husband and wife, children, and sometimes other relatives living in the same house or under one family head. A family can also be a single person living alone, a single parent with children, or a husband and wife without children.
They also view Family History as a key element of 'service':
Family history is discovering and learning more about our family members and gathering and preserving information about them. It is also performing saving ordinances for them in temples of The Church of Jesus Christ of Latter-day Saints.
Families are central to Heavenly Father’s plan of happiness. He has prepared a way for family relationships to continue throughout eternity. By participating in temple and family history service, we discover more about ourselves and our ancestors.
It is therefore hardly surprising that, when they developed GEDCOM in 1984 as an aid to genealogical research, they developed it around their concept of the family, which is a nuclear family.