Smoking takes 10 years off your life - but is this a sufficient reason to give up smoking? Why is a long life a better life?
The United Nations Human Development Index uses life expectancy as a measure of life quality because:
a long life is valuable in itself and... various indirect benefits (such as adequate nutrition and good health) are closely associated with higher life expectancy
That longevity is a proxy for the things that make life enjoyable is a reasonable argument, especially in the context that the UNDP is making it. The elimination of polio, for example, not only increased life expectancy, but also improved life quality, by preventing people from experiencing years of disability. But to the extent that smoking shortens life by taking low quality years off at the end, is that such a bad thing?
And the statement that "a long life is valuable in itself" takes us back to the original question: why?
This diagram shows the evolution of well-being over time for a person with a terminal disease. Palliative care provides a higher quality of life for a shorter time; aggressive treatment extends life, but at the cost of life quality. Contemplating the problem, one can imagine that sometimes one might choose palliative care, and sometimes one might choose aggressive treatment, depending upon the precise well-being - longevity trade-off.
It's a bit like economics. Once the problem is expressed in a diagrammatic form, the answer is fairly straightforward - it's drawing the diagram in the first place that's the challenge. Also, as in economics, the predictions of the model depend critically upon the underlying assumptions - in this case, the relative value given to longevity versus life quality. I didn't read Broome's book carefully, but I suspect he's too smart to even try to address that question.
What I like about Broome's framework is that it shows that trade-offs must be made, and it provides a way for planning, rational people to think about choices. Yet when life and death decisions have to be made, planning goes out the window. The primitive part of our brain, which has been shaped by millennia of fighting for survival, takes over, and votes to hang on at all costs. Death, yes - but not yet.
So how can we think about longevity? How can we abstract from the immediacies of life and death, and discover what our planning selves would want?
I've been thinking about this question lately, because my dog has just reached the grand old age of 14. As his puppyhood friends gradually disappear, and he himself increasingly suffers from bladder stones, cataracts, arthritis, deafness, flatulence, toothlessness, and (I suspect, though it's hard to tell) doggy dementia, I find myself wondering: is longevity all it's cracked up to be?
When a person selects a pet, she reveals her preferences for life expectancy. Using Broome's framework, one can compare the life quality of different breeds. In this case, I've compared a standard poodle, which has an average life expectancy, and a miniature poodle, which has an exceptionally long life expectancy. I've drawn a diagram showing two dogs with identical patterns of life-quality over time, but with one life compressed, and the other extended.
I don't think it's obvious that, in the diagram above, the miniature poodle's life is any objective sense better. Both dogs experience identical average life quality, it's just that one, by virtue of its genetic make-up, experiences that life quality over a longer period of time.
Indeed, when a person selects a pet, life expectancy is one of the last things considered (see, for example, this pet selection guide, or this one or this one). Instead, "experts" recommend choosing a pet who will be a good match for his or her owner in terms of activity level, sociability, and so on. Good health matters - sensible owners avoid breeds prone to health problems. But not life expectancy per se.
With my dog, I try to let him live his life to his full potential - but the fact that his maximum possible life span is 17 or 20 years, and not 10 years or 40 years, is of little consequence.
In the same way, a good human life is one that is lived to its full potential.
But, as a practical matter, what does that mean?
Addendum: some people have argued in the comments that smoking leads to a reduction in average life quality, as well as life length. On average Canadian smokers are less satisfied with life than non-smokers - around 40 percent of non-smokers report being very satisfied with life, as compared to around 30 percent of smokers. There are two big caveats to this graph however. First, it excludes institutional residents, including people in long-term care facilities - that is, the people who are experiencing low-quality late-life years. Second, it's not clear that smoking causes lower life satisfaction - people may smoke because they're unhappy, rather than the other way around.