 Hello, I'm Philip Cohen here to answer the question, what is life expectancy? Life expectancy, as we usually use the term, is the average lifespan for members of a hypothetical cohort aging through the measured mortality rates of a given period, usually more or less the present. Life expectancy is a statistical projection of those current conditions. It's a useful way of summarizing the mortality situation for a population in terms that the average person can understand years of life lived. It is also the source of a lot of misunderstanding. What life expectancy is not is the age someone can expect to live starting now. It is not how long someone born today will live, we have no way of knowing that. It is not how long some members of a previous generation have lived. We might be able to know that if we wait long enough to find out, but that's usually not what we want to do either. And life expectancy is not a prediction or a forecast of what will happen in the future. Remember, life expectancy is the average lifespan for members of a hypothetical cohort aging through the measured mortality rates of a given period. The example I'll use today is the average lifespan of U.S. residents living their whole lives under the mortality rates observed in 2017. They're born in 2017, they live their first year in 2017, they live their second year in 2017, they live their whole life in 2017 exposed to those risks of dying that we have already measured. And we'll see how long the average member of that hypothetical cohort lives. In the United States for 2017, the answer was 78.6 years. Formerly, demographers call what I'm talking about period life expectancy. It is the life expectancy of a hypothetical cohort living through one period in time. For our example, we'll start with the mortality rates for every age in the United States in 2017. I've converted the death rates to a log scale here so we can see the fine differences at very low rates of mortality, which we have at young ages. You can see that age zero is the most dangerous year in the early part of life when a little more than one half of 1% of American babies died in their first year of life in the year 2017. Those who made it out of that year experience much lower death rates through their early years until in their teen years, the death rates start rising. People experience accidents of violence and so on. And then later in life, other kinds of diseases, diseases of old age start to come into play like diabetes and hypertension and cancers and heart disease until by the older ages, the death rate has risen and risen and risen until the finally at the very end, everybody will be dead. So we'll take our hypothetical cohort, 100,000 individuals and we'll subject them to those observed mortality rates that we saw in the year 2017 in the United States. We lose about a half of 1% in that first year of life. Then we don't lose very many more for the next 10 or 15 years. And eventually you see them start to drop off as mortality rates eventually rise. You don't get below 90,000 of that 100,000 member cohort until almost age 60. And then you see the mortality rates as they rise. We start to lose more and more until you get eventually all the way down to zero members of the cohort left after age 100. So just looking at the experience of those 100,000 people, we can calculate how long the average member of that hypothetical cohort lived. In this case, United States 2017, the average member of that hypothetical cohort lived 78.6 years. That was the life expectancy in the United States, that top line number that we hear so much, life expectancy 78.6. Technically it's life expectancy at age zero, but we usually just call it life expectancy. One thing you can see is that as soon as you make it past that first year of life, you can start to expect to live longer than the life expectancy at age zero. For example, people who live to age 20 in our figure have a life expectancy of 59.4 years left. That's the average years left for people who reach age 20, which will get them up to age 79.4 on average. If people live all the way up to age 70 in this hypothetical cohort, you see on average they will have 15.7 years left to live, which will get them to age 85.7. In fact, some people live past age 78.6 altogether, and those people obviously are expected to live beyond age 78.6. Using our data from 2017, if you make it to age 100 in the hypothetical cohort, you have an average of 2.2 years left to live. This is why it's not correct to say, I'm 75 years old. I live in a society where the life expectancy is 78.6. Therefore, I have 3.6 years left to live. Once you've made it to age 75, your life expectancy is considerably higher than the life expectancy at age zero. So we return to what life expectancy is. The average lifespan for members of a hypothetical cohort aging through the measured mortality rates of a given period, that statistical projection of current conditions, gives us a good picture of what the mortality experience of that population is without giving us a prediction or a forecast of the future. It's just a way of summarizing the mortality situation for that population in terms the average person can understand years of life lived. Now, if you want to know the actual life expectancy in the United States, today, I always recommend checking the National Center for Health Statistics page, and I've provided the information there.