Swimmer's Body Illusion
The swimmer's body illusion occurs when people mistake selection for an effect of an activity. Elite swimmers may have physical traits that helped them reach that level, so their appearance alone does not show what swimming will do for everyone. A related selection effect can make long-term gamblers remember unusually lucky beginnings.
THE IDEA
The Evolution of the Swimmer’s Body
There is a belief among gamblers that beginners are almost always lucky. “It gets worse later, but gamblers are always lucky when they start out,” you hear. The pattern can arise when we look only at people who kept gambling, rather than everyone who tried it. Does this mean that each one of us should become a gambler for a while, take advantage of lady luck’s friendliness to beginners, then stop?
The answer is no. The same optical illusion prevails: those who start gambling will be either lucky or unlucky (given that the casino has the advantage, a slightly greater number will be unlucky). Lucky beginners may feel selected by destiny and continue gambling; discouraged beginners may stop and disappear from the sample. They will probably take up, depending on their temperaments, bird-watching, Scrabble, piracy, or other pastimes. Those who continue gambling will remember having been lucky as beginners. The dropouts, by definition, will no longer be part of the surviving gamblers’ community. This explains beginner’s luck.
This same logical error underlies what is often called the “swimmer’s body illusion” – our tendency to confuse selection factors with results. When asking around about the comparative physical elegance of athletes, I was often told that runners looked anorexic, cyclists bottom-heavy, and weight lifters insecure and a little primitive. I inferred that I should spend time inhaling chlorine to get those “elongated muscles.” Now suspend the causality. Assume that a person’s genetic variance allows for a certain type of body shape. Those born with a natural tendency to develop a swimmer’s body become better swimmers. These are the ones you see splashing up and down at the pools. Their appearance therefore reflects selection as well as training. Some of what we perceive as the result of swimming may have been an entry criterion: physical characteristics such as long arms can help people succeed in the sport.
The same question arises outside athletics. How much of a university’s graduate success reflects its teaching, and how much reflects the students it admits? How much of an advertisement’s effect comes from the product, and how much from choosing an attractive model? Observing the selected group does not separate those effects.
An advertisement showing a successful or attractive person using a product leaves the selection process out of view. To assess its claim, ask how the person was chosen and what happened to other users. The same applies to training and education: selection matters, but its presence does not establish that effort or instruction makes no difference.
When to use it
Evaluating an advertisement by asking how its models or endorsers were selected and what evidence supports the product claim.
Interpreting sports statistics: Understand that professional athletes often have genetic advantages, so their training routines may not yield similar results for everyone.
Judging educational outcomes: When comparing schools or programs, consider the selection of students rather than solely the quality of instruction.
Evaluating whether a specific training regimen (e.g., swimming) will change your body shape as it appears to for elite athletes.
Interpreting stock market success: a few lucky beginners may continue trading, creating the myth that new investors are especially lucky.
Assessing the effectiveness of a new teaching method: if only the best students persist with it, their success may be due to their initial ability, not the method.
Understanding why certain historical figures are remembered as 'lucky' or 'destined' – survivorship bias in historical narratives.
FROM THEORY TO EVERYDAY LIFE
In practice
Swimmer's Physique
A person expects swimming to give them an elite swimmer's physique. Long arms and other advantageous proportions may have helped those athletes reach the top, so their appearance does not isolate the effect of training.
University Rankings
A university admits students who are already strong candidates. Their later success may reflect both that selection and their education; rankings alone do not separate the contributions.
Beginner's Luck in Gambling
Gamblers who won early may be more likely to keep playing and tell stories about their lucky beginnings. Beginners who lost and quit are missing from that group, which can make beginner's luck appear more common than it was.
Swimmer's Body
A coach compares swimmers with other athletes before and after a training program. Looking at starting proportions and subsequent changes helps distinguish selection from changes associated with training.
A MOMENT TO REFLECT
Put it to work.
Think of a success story you admire. How much of that success was due to innate traits or selection versus hard work?
When you see an advertisement showing a beautiful model using a product, what hidden selection factors might explain the model's appearance?
Can you identify a situation in your own life where you confused selection factors with results?
Can you think of a situation where you assumed a certain activity led to a positive outcome, but the outcome was actually due to pre-existing traits?
How might the Swimmer's Body Illusion affect your interpretation of success stories in business or self-improvement?
What steps can you take to avoid confusing correlation with causation when evaluating the effectiveness of a strategy or habit?
CONTINUE THE CONVERSATION
Source texts
- The 25 Cognitive Biases Uncovering The Myth Of Rational Thinking (Charles Holm)
- The Black Swan The Impact of the Highly Improbable With a new section On Robustness and Fragility (Nassim Nicholas Taleb)