Birthday effect

The birthday effect (sometimes called the birthday blues, especially when referring specifically to suicide) is a statistical phenomenon where an individual's likelihood of death appears to increase on or close to their birthday. The birthday effect has been seen in studies of general populations in England and Wales,Switzerland,Ukraine, and the United States, as well as in smaller populations such as Major League Baseball players. Studies do not consistently show this effect; some studies find that men's and women's mortality rates diverge in the run-up to the birthday, while others find no significant gender effect. Suggested mechanisms for the effect include alcohol consumption, psychological stress relating to the birthday, increased suicide risk, terminally ill patients attempting to hold on until their birthday, an increased mortality salience, or a physiological cycle that causes the body to weaken annually. It has also been suggested that it may be a statistical artifact, perhaps as a result of anomalies in reporting, but the birthday effect has also been seen in studies that control for known reporting anomalies.

With the introduction of statistical software that can process large datasets easily, a number of state- or country-wide studies have been carried out to investigate whether birthdays have any effect on mortality. The first large-scale study used the records of 2,745,149 Californians who died between 1969 and 1990. After correcting for confounding factors such as seasonality in deaths, elective surgery, and people born on February 29, there was a significant increase in deaths in the week before the individual's birthday for men, and in the week after the birthday for women – in both cases, mortality did not peak on the birthday, but close to it. This effect was consistent across age and race cohorts.

A similar study among 12,275,033 Swiss found the highest mortality on the actual birthday (17% greater than the expected value), and the effect was largest for those over 80; another study on Swiss data found a 13.8% excess and was able to link this to specific causes: heart attack and stroke (predominant in women) and suicides and accidents (predominant in men), as well as an increase in cancer deaths. Among 25 million Americans who died between 1998 and 2011, 6.7% more people than expected die on their birthday, and the effect was most pronounced at weekends and among the young – among 20 to 29 year olds, the excess was over 25%. An even greater excess was found in the population of Kiev, where between 1990 and 2000 there were 44.4% more deaths than expected among men on their birthdays and 36.2% more than expected among women. Smaller biographical studies have also shown a birthday effect within subpopulations, such as among Major League Baseball (MLB) players and people with entries in the Encyclopedia of American History.

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