Monday, May 05, 2014

Breaking a truly ancient tradition

"Modal age at death" means the age at which the largest number of individual deaths occur. The modal age at death is determined by the interaction of two factors, the increasing risk to each individual with advancing age, and the decreasing number of individuals still alive with advancing age. As a population of adults gets older, each person is at ever increasing risk of death, so at first the number of deaths at each age goes up. But eventually, even though individual risk is still increasing, the number of people experiencing that risk is going down so fast that the number of deaths at each age (what demographers call dx) goes down. So modal age at death is neither at the age with the most individuals or the age at the highest risk, but somewhere in between. And studying the modal age at death, how it varies between populations and over time can tell demographers many useful things that I'm not going to go into because I am more interested in subverting the paradigm.

The paradigm in this case is that modal age at death is some time in early old-age, when death rate is going up but not too many people have died yet. Except that often it hasn't been. If one looks at life-tables for historical populations of humans, the highest dx (by far) is often d0, the number of individuals who die before their first birthday. Put another way, far more people died between birth and their 1st birthday than during any other year of age. The Human Mortality Database (the world's premier source for data on this sort of thing) estimates that of the baby girls born in Sweden between 1751 and 1759, slightly over 20% died in their first year. To put in perspective how the resulting distribution of deaths over age looks, please glance at this graph:
If I asked you to guess at what exact age a random female had died, by far your best guess would be age 0. And surely the people living in such a population must have been affected in all sorts of ways by the frequency that babies die. To ask how this population would have adjusted to such a thing is to ask a misleading question, because adjustment would imply that this is something new. In fact, the modal age at death is almost always 0, and modern humans are highly unusual in having it be much later in life. In fact, even in the cohort of Swedes born in the 1920s, modal age at death was still 0. In some countries it may still be 0, although such places are generally harder to get good demographic data from.

How far back does this go? Well it is 0 for hunter gather populations. It is zero for wild primates. It is zero for other mammals. It is zero for.... As far as I can tell, modal age at death has been zero for almost every population of almost every kind of organism for the entire history of life on earth. Contemporary wealthy humans still suffer much higher mortality in our first year than at any other pre-adult age, but infant mortality has been gradually brought down over the last centuries, so far that somewhere in the 1930s and '40s, many of the world's nations unknowingly broke with hundreds of millions of years of tradition by having a modal age of death that wasn't zero. So the right question to ask may well be not, "how did they adjust to zero being the mode?" but rather, "how are we adjusting to zero not being the mode?" And my impression is that one important way that we have adjusted is by very gladly forgetting that things were ever different than they are now.

Seen another way though, we haven't changed the modal age at death at all. If one is willing to classify the loss of an embryo or fetus as a death, modal age at death has never been zero. It has always been, and remains, -1. Minus one because far more individuals are lost in the year prior to birth (even if only nine months) than the year after it, or any other age. Where before many cultures avoided naming newborns until a week or month or even a year had passed, we still often avoid it for those not yet born, for the same set of emotional (some say superstitious) reasons.

Will science progress to the point that we feel safe naming that recently implanted embryo, knowing that she will almost surely make it? Perhaps. If so, modal age at death will finally, at long last, be in the age range that my demographer friends like to consider.


jte said...

In the graph, what's the first age that is lower than the adult mode?

Dan Levitis said...

Age 6.