My 91-year-old mother-in-law is a wonderful person, with most of her faculties reasonably intact. She lives in an assisted living home, where the care is excellent and the staff are kind. The place reminds me of a Holiday Inn, except that all the patrons are elderly and need help with the functions of everyday life – and it isn't cheap.

Although my mother-in-law gets the best of care, her quality of life is much better because she has children close at hand. My sister-in-law runs small errands, brings her books and knows her medications by heart. When something goes wrong, as it often does at that age, we're often the first to notice and make sure the problem gets dealt with promptly. My husband helps her with the television. We do her banking and buy her wine – she likes a glass with dinner – and remind the staff that she likes her bacon crispy. We know her better than anyone else ever will, and we care more.

My husband and I don't have children. When one or both of us are her age, we will probably depend entirely on the kindness of strangers. We try not to think about that too much.

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Who will take care of all the old people? Until now, the answer has always been their families. Grandma would grow old at home, tended to by a niece or daughter. In many cultures, taking care of your parents is a sacred obligation. That's the reason it has always been so crucial for Chinese parents to have a son. A son is their old-age insurance.

But as traditional extended families pass into history, Confucian values are breaking down. China has attempted to address the crisis in elder care with a law saying that grown children are obliged to visit their parents and provide them with emotional support. (There's widespread skepticism that it can be enforced.)

In the West, the modern welfare state has dramatically reduced the family's role as the original social safety net. As family bonds become less and less vital for survival, people are fleeing from the burdens of family ties at breakneck speed. Marriage, children, elder care, sacrifice, duty – all are giving way to what political economist Nicholas Eberstadt describes as "the seemingly unstoppable quest for convenience by adults demanding ever-greater autonomy."

Writing in The Wall Street Journal, Mr. Eberstadt shows how fast these changes are happening and how astonishingly widespread they are. In Italy, one in four fortysomething women is childless; in Berlin it's almost one in three. Marriage rates are plummeting, too. Across the West, the fastest-growing type of family is the single-person household. In Western Europe, nearly one household in three now consists of a single person. In Norway, it's nearly 40 per cent. In Canada, about 27 per cent of us live alone.

European demographers call this the second demographic transition. The first transition was from an age of high birth and death rates to low ones. Now we are moving to a world of short-term, low-obligation unions and subreplacement fertility. Having children is no longer regarded as an investment that will pay off when you age, but as a very costly discretionary expense that will impose a substantial burden on your income and lifestyle. Nor are these trends confined to the developed world. They are sweeping across lower-income countries, too.

The devastating effect of the family's decline on children is well known. But we've barely begun to understand its effect on the elderly. Sure, we know the demographics are pretty dire – people are living longer than ever and the baby boomers are getting old at the same time the working-age population is dramatically shrinking, and who is going to pay for it all? But we've scarcely thought about the social effects, starting with the question of who will care for these boomers as they become increasingly dependent.

One thing I know for sure: Just like the state can't take care of your kids as well as you can, it can't take care of your parents as well as you can. To begin with, people are willing to spend far more on themselves and their own families than they are on other people's families, which means that state-paid elder care will never come close to the standards my mother-in-law enjoys in her geriatric Holiday Inn. Many (if not most) of us will have to settle for extremely basic services, delivered by low-wage caregivers of varying competence and compassion. Currently, such places range from depressing to downright grim.

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Are there any ways to bend the demographic curve? Not that I can see. Maybe if we had universal daycare, more flexibility for working parents and more dads pitching in around the house, we could persuade people to have more children. Or maybe not. Compared to our new-found quest for personal autonomy and freedom, these ideas seem comically inadequate. Besides, we don't even know how to increase the marriage rate.

Sometimes I ask my childless friends what they plan to do when they get old and frail. Their answers vary. Some have no intention of living that long. Some are hoping that a kindly younger niece or nephew will take pity on them, at least at Christmastime. Some say they plan to go to Switzerland. Quite a few are in denial.

Personally, I'm thinking of adopting. Maybe it's not too late. Or maybe we can teach our cat to drive us to the doctor.