What does the word “that” in paragraph 2 refer to?

During the past half-century, our species has embarked on a remarkable social experiment. For the first time in human history, great numbers of people - at all ages, in all places, of every political persuasion - have begun settling down as singletons. Until the second half of the last century, most of us married young and parted only at death. If death came early, we remarried quickly; if late, we moved in with family, or they with us. Now we marry later. We divorce, and stay single for years or decades.

The rise of living alone has produced significant social benefits, too. Young and middle-aged solos have helped to revitalise cities, because they are more likely to spend money, socialise and participate in public life. Contemporary solo dwellers in the US are primarily women: about 18 million, compared with 14 million men. The majority, more than 16 million, are middle-aged adults between the ages of 3S and 64. The elderly account for about 11 million of the total. Young adults between 18 and 34 number increased more than 5 million, compared with 500,000 in 1950, making them the fastest-growing segment of the solo-dwelling population.

Despite fears that living alone may be environmentally unsustainable, solos tend to live in apartments rather than in big houses, and in relatively green cities rather than in car-dependent suburbs. There's good reason to believe that people who live alone in cities consume less energy than if they coupled up and decamped to pursue a single-family home.

The best title for this passage could be                     .