Imagine a world with no sisters, no brothers, nephews or cousins. Nary an aunt or uncle. Just parents, grandparents, and maybe great grandparents.
It is a world in which the only biological relatives for many people - perhaps most of them - will be their ancestors.
That's the picture by the numbers, anyway, for the next century if fertility rates continue falling and population actually declines.
"Throughout the remembered human experience," writes demographer Nicholas Eberstadt in the latest issue of Public Interest, "the family has been the primary and indispensable instrument for socializing a people. In the family, the individual found extended bonds of obligation and reciprocal resources - including emotional resources - upon which he could draw."
A majority of parents in the world will have only one child by the middle of the next century, according to one projection from the United Nations. While that average number would vary across different cultures, it poses significant implications for the family.
"For many people, 'family' would be understood as a unit that does not include any biological contemporaries or peers," notes Mr. Eberstadt. "Who will we play with, learn from, love unthinkingly, and fight with ferociously, knowing all the while that we can do these things because we are linked together by an indissoluble common tie?"
Such a situation is fast approaching in parts of Europe.
Women in the former East Germany are having on average less than one child, a fertility rate of 0.7. In Italy, the fertility rate is 1.2. In Western Europe as a whole, it is 1.4 - well below the 2.1 needed to stabilize a population.
"People thought this was a phenomenon that would fade away," notes demographer Wolfgang Lutz of the International Institute for Applied Systems Analysis in Laxenburg, Austria. But it hasn't so far.
If couples keep up the pace - or lack of it - two generations from now almost three-fifths of Italy's children will have no siblings, cousins, aunts, or uncles, says Eberstadt.
In a world of low fertility, he says, business must adjust to a market with fewer children and many more senior citizens. The real estate market, for example, will require houses that hold fewer people.
In developing countries, fertility rates are expected to fall below replacement levels around 2010 to 2030, according to the UN projection. So it will take longer for the traditional family to change.
* The ratio of population between the third world and developed world, now 4:1, would be 7:1 by 2050.
* By 2050, Africans will outnumber Europeans more than 3:1. The ratio is about 1:1 now.
* Nigeria will edge out the US as the fourth most populous country in about 2050.
* No European state will match the Philippines in population.
Such shifts, notes Eberstadt, "presage a tremendous shift in the balance of global power."
* A median of 42 by 2050. It is 25 now.