“The sandwich generation” is one of the more frivolous complaints of the baby-boom generation — as if we were the first generation in history to have both children and parents!

The sandwich isn't the problem. The problem is having two dependent generations at the same time. That is unique in the history of the human race. But why is it unique to us?

I'll tell you why. It is one of the unintended consequences of the revolution in contraceptive technology.

This technology, including the pill and legalized abortion, made delaying childbearing easier for larger numbers of people. Popping a pill is a lot simpler than self-restraint. It is hardly surprising that the age of first marriage and the age of first childbearing have both steadily risen since the 1960s. This creates a chronological spread across generations. That spread is responsible for the so-called sandwich generation.

Let me illustrate by telling my story. Like many women of my generation, I postponed childbearing until my career was established. I thought it was smart and responsible to be independent. I was 38 when I had my first child.

Let's do a bit of simple math. My mom was 31 when she had me. (I was the third of six kids, by the way. She wasn't just getting started.) By the time I had my first, my mom was in her 60s. If I had had my first child 10 years sooner, I'd have been 28, not an early age at first childbirth.

But look at the difference those 10 years would have made to my mom. Instead of being in her 60s, she'd be in her 50s. Instead of being infirm when I have school-aged kids, she'd be young enough to enjoy them and, incidentally, to help out.

Likewise, by the time she got old enough to need significant help from me and my siblings, my kids would be teen-agers. I wouldn't be driving them to ball games and dance classes; they could drive themselves places. In fact, they might even be some help with their grandparents.

Multiply my personal decisions by hundreds of women. The age at first childbirth has risen steadily in the last 20 years, even though out-of-wedlock births and teen pregnancy have continued to be a problem. Both economic pressures and the cultural ethos encourage women to delay childbearing and to have only one or two children.

This might be workable for a subset of families. Indeed, there has always been tremendous variation in this kind of very private decision making.

But the society-wide trend is creating a situation that is not sustainable for an entire society. As the process continues, women like me won't have a batch of siblings to help take care of grandma. There will be one or two adult children, taking care of at least two sets of elderly parents, or even more if the grandparents have divorced and remarried. These same adults will also be taking care of young children.

The only possible result of this trend will be some form of institutional care for either the very old or the very young, or both. There simply will not be enough hands on deck inside the family to take care of that many dependent people simultaneously. A family with shorter spacing between the generations would be able to stagger the care of its dependent members across time.

The very youngest generation wouldn't be dependent infants at the same time the oldest generation is most in need of care.

My daughter recently asked me, “Mom, how old should I be when I get married and have kids?”

I said to her, “Well, Honey, I was 38 when you were born. If you are 38 when you have your first child, how old will I be?”

I gave her a minute to do the math. “Seventy-six.”

“Right. If you wait that long, you may have me and your baby in diapers at the same time. But, do whatever you want.”

I sympathize, up to a point, with the pressures of the sandwich generation. Really, I do. I have a couple of school-age kids as well as aging parents and in-laws. My mother-in-law lived with us for the last six months of her life. She had cancer and Alzheimer's.

But I don't think it appropriate for me to complain about doing “double-duty” in the sandwich generation. I was not very realistic. Most of my generation didn't think too closely about the fact that our parents would get old and become legitimately dependent on us.

When Thomas Malthus suggested marrying later in life as a means of controlling population growth, he was in effect suggesting a whole lot of delayed sexual gratification. Not surprisingly, few people took his suggestion seriously enough to actually implement it in their own lives.

But today, delayed childbearing doesn't necessarily represent responsible delayed gratification. Quite often, that late age of first childbearing represents the results of 10, 15 or maybe even 20 years of contracepted sex. We had our fun. Now it's time to pay. We discipline our kids by giving them the “natural consequences” of their actions. We are in this sandwich generation because we didn't do the math. Now we are getting the natural consequences of the choices we made long ago.

Jennifer Roback Morse, a research fellow at the Hoover Institution, wrote Love & Economics: Why the Laissez-Faire Family Doesn't Work.