Although it is not possible to predict how global demographics will affect families or international migration, Cohen points out that three factors set the stage for major changes in families: fertility falling to very low levels; increasing longevity; and changing mores of marriage, cohabitation and divorce.

In a population with one child per family, no children have siblings, Cohen explains. In the next generation, the children of those children have no cousins, aunts, or uncles.

If people are between ages 20 and 30 on the average when they have children and live to 80 years of age, they will have decades of life after their children have reached adulthood, and their children will have decades of life with elderly parents, Cohen also points out.

Cohen's article kicks off a four-week long series titled "The State of the Planet," which examines key issues of our planet's well-being. Cohen was asked to initiate the series because "population is people and people matter."

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