Death Rap 

• SLUG SIGNORINO

Historically, what has been the leading cause of death for humans? Imagine a chart of all causes of death for every human who has ever died. Will it be prehistoric causes of death due to the tens of thousands of years they had to accumulate, or will it be more modern causes due to the population explosion that came with civilization? —Mr. Kobayashi

This is going to involve some guesswork, Mr. K. OK, a lot of guesswork. But it needn't be just a shot in the dark. What we need is a defensible method. Let's see what we can figure out.

As you rightly intuit, the answer depends on how many people were alive during the successive epochs of history, since the leading causes of death have shifted over time.

Here we have the benefit of work done by others. In the 1970s, when fears about the population explosion were at their peak, the story arose that 75 percent (or some other large fraction) of all people who'd ever been born were then alive—the idea being that we'd reached the hockey-stick inflection point on the growth curve and population was increasing exponentially.

Though durable, this tale had no basis in reality. We know this thanks to the Population Reference Bureau, a D.C.-based nonprofit that's been tracking global population statistics since 1929. The PRB came up with estimates for the number of people born per era, as summarized below (I've tweaked the numbers to bring things up to date):

• 50,000 B.C. to 8,000 B.C.—about 1 billion;

• 8,000 B.C. to 1 A.D.—about 46 billion;

• 1 to 1850 A.D.—about 47 billion;

• 1850 to present—about 14 billion.

Total: 108 billion. In other words, nearly 90 percent of people who have ever lived were born prior to 1850. The world's population today, roughly 7.5 billion, accounts for about 7 percent of all people who have ever lived.

PRB concedes that plenty of guesswork went into these numbers.

But let's assume they're right. We can reasonably take 1850 as the point at which industrialization and urbanization began to get traction in parts of the world. Prior to then, the vast majority of humanity lived in rural settings without modern sanitation, got by on minimal calories, had no access to health care worthy of the name and died young—in large part because many children died before age 5 and many women died during childbirth. (The latter two problems didn't recede in the U.S. until the 20th century.)

We can thus rule out as candidates for leading cause of death what we might call diseases of modernity: heart disease, cancer, stroke, Alzheimer's, etc. These are what you succumb to if you survive the scourges of antiquity.

Let's talk about those scourges. Disease, famine and war are obvious candidates; mother/child mortality must also be included. Famine and war are episodic and in terms of quantity, year over year, surely trail disease. We've already excluded non-communicable diseases common in the developed world; what remains might be broadly categorized as infectious disease, which overlaps to some extent with mother/child deaths.

Some would be content to let it go at that, but surely we can fine things down a bit. Let's push on.

Our task is complicated by the fact that mortality statistics prior to 1900 are hopelessly inadequate. On the internet we find lots of theories based on fragmentary or anecdotal data. Malaria is popular—some claim it's caused half of all human deaths. Tuberculosis has its adherents. Limit the time period or geographical range and you can make a case for smallpox or the Black Death.

We can do better than that. We don't have good data for most of history, but we have OK data for most of the world now. One of the best-known collectors is the World Health Organization, which publishes Top 10 causes-of-death lists for different "economy income groups."

These lists differ sharply. For high-income economies, the leading cause of death is heart disease, followed by stroke and Alzheimer's.

For the other end of the scale, here's a quote from WHO: "More than half (52 percent) of all deaths in low-income countries in 2015 were caused by the so-called 'Group I' conditions, which include communicable diseases, maternal causes, conditions arising during pregnancy and childbirth and nutritional diseases." Sounds to me like what you'd have expected worldwide prior to 1850.

The leading CoD? Lower-respiratory infection (chiefly pneumonia, bronchitis and influenza), followed by diarrheal disease (dysentery, cholera, etc.), stroke and heart disease. Tuberculosis and malaria are No. 6 and No. 7, preterm birth complications and birth asphyxiation are 8 and 9.

The list doesn't precisely replicate what you'd have found in antiquity. The No. 5 cause of death in poor countries now is HIV/AIDS, and No. 10 is road injury, both modern problems. Even in low-income economies, modern medicine has likely pushed TB and malaria lower on the list than they'd have been in centuries past. But I'd say the WHO ranking is pretty close.

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