Let no man boast himself that he has got through the perils of winter till at least the seventh of May.
-Anthony Trollope, Doctor Thorpe 1858
I was recently asked to help settle a debate as to whether or not death is a seasonal event. In other words, is the mortality rate higher during some months of the year?
Sometimes I wonder…Why do I get asked this question? After all, Legacy.com is really more about life than it is about death. We help “bring obituaries to life” for our many newspaper partners and the funeral homes and families they serve; we help people remember; and we strive to make things just a bit easier for people at a very difficult time in their lives when they’ve experienced the loss of a friend or loved one.
In the process however, we do learn a lot about death trends and statistics, and I actually do know the answer to the question, so I will share it here for those who are interested.
Mr. Trollope was an Englishman writing in the 19th century, but his quote is relevant to modern day U.S. Indeed, there are seasonal fluctuations in U.S. deaths. One’s chances of dying in the winter months are significantly greater than in the summer. This is a statistical fact. It is generally true regardless of where one lives in the U.S., and it is borne out year after year.
Number of Monthly Deaths in the U.S. from Jan. 1985 – August 2008
The above chart shows how consistent this pattern is from 1985 – 2008. The blue line plots monthly deaths as measured by the y-axis, starting with about 200,000 in January 1985. They show an unbroken pattern of deaths peaking in the winter and bottoming in the summer. The green and red lines are simple and weighted averages that smooth out the lines to show the change in annual deaths over time.
From a rich assortment of government data, one can also build a predictive model showing relative daily deaths by month in the U.S. The following exhibit “indexes” daily deaths to equal “1” in January, which is the month in which daily death rates are the highest. The average daily deaths (the blue bar) for all other months are then shown as a fraction of 1. So for instance, in month 7, which is July, daily deaths are .8499 of January, or about 15% lower. The red and green bars represent a confidence interval, which in this case means the best prediction is the blue bar, but 90% of the time the actual number of deaths in a month will fall between the red and green bars.
Daily Deaths Indexed by Month to January
Why is this important? Because those who serve the needs of people experiencing the death of a friend or loved one should be aware that their services will typically be in greater demand in December, January, and February than in June, July, and August, and they must be prepared accordingly. For example, if a newspaper picks one month to run public service announcements telling their readers about the obituary services they provide, they probably want that month to be sometime in late fall or winter when deaths are on their seasonal rise, so people will hear their message in advance of most needing to avail themselves of their services.
Is this seasonality experienced in every region of the U.S.?
Are different regions of the country, with milder seasonal weather fluctuations, immune to the overall seasonal death trend? One look at California (below) tells us probably not. In fact, this seasonal pattern of deaths by month is the same in California, Florida, Illinois, New York, Pennsylvania, and Texas.
It is important to note that this does not mean that every single locale across the country will experience similar monthly patterns. In any given market, the smaller the population size, the greater the variability it might see, and the greater the likelihood that the seasonal pattern in any given year could deviate from the nationwide or statewide trend. But overall, this seasonality is a well entrenched phenomenon.
What causes such seasonality?
Once people become aware that death has a bias toward winter, some speculate they know one reason why – suicide. After all, it is the dark, cold nights of winter that push people “over the edge,” or perhaps holiday season blues. It turns out this is a myth. Suicide is actually one cause of death that has a slight bias toward summer. (And, it turns out, Mondays.) Facts are powerful things. Understanding such underlying drivers helps one appreciate the predictability of overall seasonal death trends.
If you analyze the monthly trends of the leading causes of death in the U.S. they can be divided into three groups, each with common characteristics:
Causes of Death in U.S.
Our analysis revealed that many causes of death in the U.S. exhibit seasonality in a “U”-shaped pattern with the most deaths in the winter and the fewest in the summer (Group 1). I won’t attempt to provide the medical explanation for the seasonal patterns of each cause of death, but we all know for example, that Flu strikes hardest in the winter months.
Some causes of death do not exhibit any seasonality. These include Cancer, Assaults, and Infant Deaths, defined as Group 2. Group 3 has seasonality, but in the opposite way – more deaths occur in the summer, and fewer in the winter. Suicide and accidents are included here.
Taken together, death appears to be seasonal with a bias toward winter because more types of death are seasonal with a bias toward winter, than are neutral or with a bias toward summer. We could spend more time investigating why certain conditions result in higher winter mortality, but this feels like a topic for a different blog, at another time, and by a different blogger than me. My goal here is to help all those out there who have debated whether or not more people die in January – they in fact do – and to raise a glass in the direction of Mr. Anthony Trollope, who implied that seasonal mortality drops measurably by about May 7th of each year. It turns out he had that generally right.