Does playing tennis make you live 9.7 years longer?

Episode Summary

Executive Summary: The episode examines a viral claim that tennis adds 9.7 years of life, tracing it to a Mayo Clinic study and testing whether the result reflects tennis itself or confounding by wealth, education, and generally healthier lifestyles. A statistician says the study is decent but too limited to prove causation, especially because tennis players were few in number and often played other sports too.

Main Topics: Viral longevity claim about tennis (Priority: 5/5): The show opens with the tweet-driven claim that tennis can extend life by 9.7 years and compares it with other sports. Quality and limits of the underlying study (Priority: 5/5): The Mayo Clinic/Danish cohort study is described as strong in follow-up and outcome measurement, but still observational and not causal. Confounding by socioeconomic status (Priority: 5/5): Listeners question whether tennis players live longer because they are wealthier, more educated, and healthier rather than because of tennis itself. Statistical adjustment and hazard models (Priority: 4/5): The episode explains Cox proportional hazards regression as the method used to adjust for confounders and compare life expectancy between groups. Small sample size and random variation (Priority: 5/5): Only a small number of tennis players and deaths were available in the adjusted analysis, weakening confidence in the estimate. Sport mix and lifestyle effects (Priority: 4/5): The analysis suggests tennis players often did multiple sports, so the benefit may reflect overall active behavior or a mix of exercise types.

Key Arguments: The original study is reasonably well designed for an observational cohort: it had over 8,000 participants, 25 years of follow-up, and all-cause mortality as a clear outcome. Socioeconomic confounding is real, but adjusting for education and income only reduced the tennis estimate from 10.4 to 9.7 years, implying confounding may not fully explain the result. Even so, the study cannot show that tennis causes longer life, because observational data cannot fully rule out selection effects and unmeasured differences. The adjusted tennis subgroup was very small, with only 111 players and 16 deaths, making the estimate vulnerable to chance variation. Tennis participants also tended to do other sports, especially cycling, so the longevity signal may reflect overall activity levels or exercise variety rather than tennis alone. Broad categories for education and income are too blunt to perfectly control for socioeconomic differences, limiting the strength of the adjustment.

Data Points: Longevity gain for tennis: 9.7 years - Adjusted estimate reported for people who played tennis versus those who did not Unadjusted longevity gain for tennis: 10.4 years - Estimate before adjusting for education, income, and other confounders Babington longevity gain: 6.2 years - One of the eight sports compared in the original study Soccer longevity gain: 4.7 years - One of the eight sports compared in the original study Cycling longevity gain: 3.7 years - One of the eight sports compared in the original study Swimming longevity gain: 3.4 years - One of the eight sports compared in the original study Jogging longevity gain: 3.2 years - One of the eight sports compared in the original study Calisthenics longevity gain: 3.1 years - One of the eight sports compared in the original study Health club activities longevity gain: 1.5 years - One of the eight sports compared in the original study Study size: Over 8,000 participants - Large Danish cohort used in the Mayo Clinic analysis Follow-up period: 25 years - Length of time participants were tracked for mortality outcomes Tennis participants in initial analysis: 167 - Number of people in the tennis group before adjustment Tennis participants in adjusted analysis: 111 - Number of tennis players retained after adjustment Deaths among adjusted tennis participants: 16 - Number of deaths in the tennis subgroup after adjustment Other sports participation among tennis players: About 80% also cycled - Illustrates that tennis players often had broader exercise habits

Pivotal Quotes: "they found that the people who played tennis got 10.4 more years of life expectancy than the people who didn't play tennis" — Adam Jacobs: Explaining the unadjusted result from the study "can we say that playing tennis makes you live 9.7 years longer? No, we cannot say that." — Adam Jacobs: Direct answer on causation versus association "there are only 167 participants who played tennis, and then in their adjusted analysis: They only had 111 people who played tennis, and only 16 of those died." — Tim Harford: Highlighting the small sample that weakens certainty

Implications: The episode warns listeners not to treat observational fitness claims as proof of cause and effect. Tennis may be associated with longer life, but the real driver could be broader health, wealth, or multi-sport activity patterns.