At some point over the past generation, people around the world entered what you might call the age of possibility. They became intolerant of any arrangement that might close off their personal options.

The transformation has been liberating, and it’s leading to some pretty astounding changes. For example, for centuries, most human societies forcefully guided people into two-parent families. Today that sort of family is increasingly seen as just one option among many.

The number of Americans who are living alone has shot up from 9 percent in 1950 to 28 percent today. In 1990, 65 percent of Americans said that children are very important to a successful marriage. Now, only 41 percent of Americans say they believe that. There are now more American houses with dogs than with children.

This is not a phenomenon particular to the United States. In Scandinavia, 40 percent to 45 percent of the people live alone. The number of marriages in Spain has declined from 270,000 in 1975 to 170,000 today, and the number of total Spanish births per year is now lower than it was in the 18th century.