I customarily killed old women. They all died there by the Big River. I didn't use to wait until they were completely dead to bury them.

The women were customarily afraid of me. No wonder. That's the account of a man from the Ache, an indigenous tribe in Eastern Paraguay, as told to anthropologists Kim Hill and Magdalena Hattado.

He explained that Grandmother's helped with chores and babysitting, and when they got too old to be useful, you couldn't be sentimental. The usual method was an axe to the head.

For the old men, Ache custom dictated a different fate. They were sent away and told never to return.

What obligations do we owe to our elders? It's a question as old as humankind, and the answers have varied widely, at least if surviving traditional societies are any guide.

Jarred Diamond, another anthropologist, says the Ache are hardly outliers, among the qua long in Papua New Guinea, when a woman's husband died it was her son's solemn duty to strangle her.

In the Arctic, the Chukchi encouraged old people to kill themselves, with a promise of rewards in the afterlife.

Yet, many tribes took a very different approach. They were gerontocrysis, in which the young do is the old say. Some even expected adults to pre-chew food for their aged and toothless parents.

What does seem common? Is the expectation that, until your body let you down completely, you'd keep working? That's no longer true.

Many of us expect to reach a certain age, then receive money from the state or our former employers, not in return for work today, but in recognition of our work in the past.

This curious stage of life is called retirement, and the payment? A pension.

Pensions for soldiers date back at least as far as ancient Rome. The word pension comes from the Latin for payment. But only in the 19th century did they spread far beyond the military.

The first universal state pension came in Germany, in 1890. The right to support an old age is still far from global. Nearly a third of the world's older people have no pension, and for many of the rest, the pension isn't enough to live on.

Still, in many countries, generations have grown up assuming they will be well looked after in old age.

It's becoming a challenge to meet that expectation. For years, economic policy wonks have been sounding the alarm about a slow-burned crisis in the pension system.

The problem is demographic. Half a century ago, in the OECD, a club of rich nations, the average 65-year-old woman could expect to live about 15 more years.

Today, she can expect at least 20. Meanwhile, families have shrunk from 2.7 children to 1.7.

A pipeline of future workers is drying up.

All that has many implications, some good and some bad. But for pensions, the situation is stark.

There'll be many more retirees to support and many fewer workers paying taxes to support them.

In the 1960s, the world had nearly 12 workers for every older person. Today, it's under 8. By 2050, it'll be just 4.

Both state and private pension systems now look expensive. Employers have been scrambling to make theirs less generous.

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