I posted what follows as a reply to a comment, and was told it should have been an answer. So I’m reposting it here as such.

As someone married to an only child, I feel compelled to note that his parents *did* expect to be cared for in their old age. If we had not been married, the burden on him would have been crushing. And when they died, he was the only one able to sort out the vast accumulation of junk they had acquired over their very long lives. So if you plan to have only one child, do that child a favour and keep your possessions to a minimum.

The other brutal truth is best illustrated by the Chocolate-Chip-Cookie story. We were visiting my in-laws out of town with our 2-year-old daughter. We had put her down for the night and were about to go out for a walk around the neighbourhood, when my father-in-law trotted out his home-made chocolate-chip cookies and offered us two each. I ate mine on the spot. My husband put his at his place on the kitchen table and said, ‘We’re going for a walk, and I’ll have mine when we get back.’

This was an opening too good to ignore, so I made the observation that anyone could tell he was an only child. This rankled my mother-in-law, who had taken pains over the years not to spoil him, so she demanded to know why I had said that.‘He’s going to leave those cookies there,’ I replied, ‘and he seriously expects them still to be there when we get back.’Well, she was an only child, too, so she and my husband stared at each other in complete lack of comprehension. My father-in-law, who had an older sister and a stepbrother, roared with laughter.

There’s the other ugly truth. It’s not possible for an only child not to be spoilt. They have absolutely no idea what it is to fight tooth and claw for the right to their own privacy or possessions.