How long will you live? Longevity and Life Expectancy Demystified (Part I)

It is only fitting that an article on longevity would be too lengthy for a single post. This is Part I. Part II will be published January 15th.

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It seems like such simple question when you think of it, "How long will you live?" Of course, for most people there is no simple answer. Besides the number of unknowns, variables, and conflicting data, the fact is that we still really don't know much about why people age and die. In fact we don't even understand the reasons for the difference between male and female life expectancies (some argue that women are biologically superior, others argue it's men's hazardous occupations, and still others say that it's because men generally drive, smoke and drink more than women and are even more often murdered. The fact is that we just don't know.)

When someone dies from a disease that we know is fatal, it's easy to study, track and tabulate. But when people's bodies just grow old, scientifically we're pretty much in the dark why it happens.

Furthermore, what we think we know about healthy aging keeps changing--sometimes dramatically. The generation who are now senior citizens grew-up at a time that cigarettes were promoted as healthy. No doubt that some of the things we do and consume daily will one day be discovered to be extremely harmful.

Of course science continues to move forward conquering diseases. Alzheimer's disease is the 7th largest killer of people and the amount that has been learned about that disease in the past 10 years is staggering. It is quite possible that we will see a cure for it in our lifetime eliminating it as a major killer. Since 50% of people over 85 get the disease this alone will have a great impact on how long many of us will live. Ditto with heart disease, breast cancer, AIDS, etc.

It's easy to imagine that the combination of stem cell research and DNA-specific drugs could one day result in drugs that add years to your life, or these predictions could be as wrong as the prediction of flying cars by the year 2000.

In response to all of this complexity, when discussing how long people live we tend to look at mean life expectancies. This works because, as statisticians know, with large enough populations, the extremes get eliminated and we can deduce meaningful trends. (Please note that both "life expectancy" and "longevity" are distinct from "life span", which refers to the number of years that humans could live under ideal conditions.) By looking at life expectancies, we can hypothesize the underlying causes for changes, for example improved health care, reductions in poverty, etc. However, as the saying goes, "the devil is in the details." If you read the average life expectancy is 77 years you need to know what "population" this is for. Is this data for babies born today or is it an average of all living people alive today? Is it for a certain racial group or sex or a mix of all. These are important questions because there are huge differentials between different groups. For example, according to the CDC, the life expectancy of individuals born in 1950 is 68.2 years. However within this group white women have a life expectancy of 72.2 while African American males is 59.1--a significant difference.

Another complicating factor is that life expectancy changes as one gets older. By the time a child reaches their first year, their chances of living longer increase (since they have made it past the child-killing maladies of the first 12 months). By the time an individual reaches late adulthood, the chances of living to a very old age end up being quite good. If you are a 65 year old living today, you have (on average) about 18 years left to live (83) even though the life expectancy or the population is under 78. Confused? You should be.

One thing for certain is that on the whole people are living longer. For Americans born in 1900 the average life expectancy was 47.3 years (for blacks it was just 33 years). By 1950, the average life expectancy had increased to 68.2 years and babies born in 2003 have a life expectancy of 77.5 years (for blacks its 72.7). The 1900 numbers might seem frightfully low by today's standards but comparing them to ancient Rome (with a approximate life expectancy of 22 to 25 years), Americans living at that time must have felt like Methuselah. (Again readers need to keep in mind that life-expectancy-at-birth is a mean, not a median; high infant mortality conceals the substantial number of people who will live well past this age.)

Will the increase in life expectancy continue? With all of the advancements in science one would certainly hope so--at least in developed countries. In sub-Sahara Africa, life expectancy is dropping due to the AIDS epidemic. In developed countries there is a different epidemic threatening life expectancy, the "lifestyle" diseases, such as obesity, diabetes, hypertension, and heart disease, which are slowing and may even reduce national life expectancies.

According to Wikipedia:

"The mainstream view on the future of longevity, such as the US Census Bureau, is that life expectancy in the USA will be in the mid-80s by 2050 (up from 77.85 in 2006) and will top out eventually in the low 90s, barring major scientific advances that can change the rate of human aging itself, as opposed to merely treating the effects of aging (as is done today). The Census Bureau also predicted that the USA would have 5.3 million people aged over 100 in 2100.

Globally, micro-states Andorra, San Marino, and Singapore along with Japan have the world's highest life expectancies (83.5, 82.1, 81.6 and 81.15, respectively). On the other end of the scale is Africa with the world's lowest life expectancies including Swaziland (33.2 years), Botswana (33.9 years) and Lesotho (34.5 years).

Back to our original question, "how long will you live"? Since we're individuals not averages, population-wide statistics will take us only so far. For example, Jeanne Louise Calment who became the world's longest living individual making it to 122 years was born at a time that life expectancy in France was well under 50.

Isn't there a way to take in the available data such as our living habits, our current health, our genes, and tell us how long we'll be around? We'll have to wait for Part II of this article for that answer.