Wednesday, September 12, 2007


Life expectancy

The statistic we call “life expectancy” is one of the most misunderstood and, therefore, one of the most misused. In an article about life tenure of Supreme Court justices, the New York Times makes one of the standard mistakes:

Life tenure today, of course, has a dimension that would surprise the Constitution’s framers; since 1900, the average life expectancy, now 77 years, has increased by 30 years.

Before we look at this, we have to note that when we talk about “life expectancy”, we usually mean what’s properly called life expectancy at birth — we can, in fact, statistically calculate life expectancy at any age: how much longer you can, on the average, expect to live after having already survived to the given age. Infant mortality and death from early-childhood diseases are significant factors in reducing the life expectancy at birth.

So here’s what’s true: the U.S. life expectancy at birth was, indeed, about 47 years in 1900, and is about 77 years now. The Times is right about that. But assuming that that tells us that the Constitution’s framers expected Supreme Court justices to live, on average, to be 47 or less — the life expectancy at birth in the late 18th century was even lower — is silly. Consider that Thomas Jefferson was 83 when he died, Benjamin Franklin was 84, James Madison was 85, and John Adams lived to be 90. Those ripe old framers knew how things went.

The problem with looking at it the way the Times does is that in 1900, as in the 18th century, a great many infants and young children died. Reaching one’s 5th birthday was no small accomplishment then, but, having done that, you could expect to continue for a good many years, most likely long past 47.

Life expectancy in ancient RomeFor a good visualization of this effect, look at the graph on the right, which shows the life expectancy at various ages in ancient Rome (click the graph to get to the web page that it comes from). The life expectancy at birth was 25 years, but a child who lived to the age of five could expect to reach 48, and someone who became a senator at 50 had, on average, 17 years of life and service still ahead of him.

Another interesting web site to try this out with is this Austrian one, which lets you plug in your date of birth and gives your life expectancy as of now. According to that site’s program (which I haven’t verified), we can see this:

Birth year    Life expectancy    
at birth
Expected age at death
as of now

While the life expectancy at birth goes steadily down as we go back in time, the adult life expectancy doesn’t change much. In fact, someone born in 1920, having made it to the age of 87, still has about five years left, at least in Austria.

Since Supreme Court justices are generally appointed in their 40s or 50s, they can certainly expect reasonably long service on the court, and that was also the case with the Jay and Elsworth courts at the beginning. Some of the very first Supreme Court justices lived to be 78, 84, 87, and even 92, so one can hardly say that our founders expected them to die relatively young.

Anyway, the Times article is otherwise interesting, discussing a proposal, popular among judicial scholars, for changing the tenure of the justices.

1 comment:

Katie said...

That's right, since life expectancy is the average, one could assume those at the bottom of the socio-economic scale could expect to live even less years than those at the top and, those in dangerous occupations - which was farming (those that actually worked the land, not gentlemen farmers like Tom) and the military in those days - would also expect to live shorter lives. So good ole Tom, Benji and such could expect to live longer lives and would know others (their families, their friends and the justices they nominated for "life terms," their socio-economic group) that would live longer, that was their norm. That's what the framers would expect life terms to mean.