Goodhart’s Law and the Nocebo Trap
Why Some Fitness Metrics Do More Harm Than Good
Goodhart’s Law states that “when a measure becomes a target, it ceases to be a good measure.” Many people new to fitness and nutrition focus on popular metrics, believing that tracking them will improve their desired outcomes. The problem? Over time, people fixate only on the numbers and forget what they were actually trying to measure in the first place.
For a metric to be useful, it needs to positively influence behavior—otherwise, you’re just collecting data for the sake of collecting it. Worse, some metrics don’t just waste time; they can create a nocebo effect—where tracking a number leads to negative effects even when the data itself isn’t inherently harmful.
For example, monitoring cortisol, blood sugar (if you’re not diabetic), or heart rate variability (for the average Joe) can often cause more harm than good because these numbers can be volatile and/or have a tenuous relationship with the actual outcomes people are trying to achieve.
What Makes a Good Fitness or Nutrition Metric?
A good metric should:
Have a tight relationship with an outcome
Be something you can directly influence
Be beneficial to track, without causing unnecessary stress
Measuring behaviors is better than measuring outcomes. A well-chosen behavior, when improved consistently, naturally leads to better results without the psychological downsides of fixating on specific numbers or outcomes.
Some of the best metrics include:
Step count – Easy to track, influences behavior, and reflects natural human activity
Calorie & protein tracking – Can influence body composition and performance and simple enough to avoid overwhelm
Workout adherence – Tracks consistency, which drives results
Weightlifting numbers – Shows progress in strength and performance
Running time & duration – Simple and directly tied to endurance improvements
These metrics are actionable, behavior-driven, and low-risk for creating nocebo effects.
The explosion of health tracking has made it easier than ever to collect data, but more data isn’t always better. I wrote about how we’re inundated with health metrics and why too much tracking can actually hurt progress. You can check it out here: Health Data Overload.
Metrics That Might Do More Harm Than Good
On the other hand, some metrics create more stress than results:
Blood sugar monitoring (for non-diabetics) – Small fluctuations are normal, but tracking them can lead to unnecessary dietary paranoia and pareidolia (what a fun word pairing!).
Cortisol tracking – Stress levels fluctuate constantly; seeing a “high cortisol” reading can ironically cause more stress.
Precise body fat percentage – Often inaccurate, hard to influence directly, and can lead to body image issues.
Movement scores from screenings – May label someone as “dysfunctional” without real-world relevance.
Heart rate variability (HRV) – Can fluctuate wildly due to sleep, hydration, and stress; fixating on it can limit training efforts.
These metrics either have a weak relationship with real progress, are difficult to control, or increase negative psychological effects without a clear benefit.
The Bottom Line
Tracking data isn’t enough—you need to track the right data. Good metrics should encourage better behaviors, drive progress, and avoid unnecessary stress. Instead of obsessing over numbers that don’t truly impact your goals, focus on the behaviors that actually move the needle.
Thanks for reading. As always, feel free to drop any comments below.