Smoking is the best-known risk factor for chronic obstructive pulmonary disease (COPD), a debilitating lung condition that can severely limit a person’s day-to-day activities. But curiously, only a minority of lifelong smokers develops the disease, while non-smokers represent more than 25% of all COPD cases. A new study published today in The Journal of the American Medical Association suggests that a developmental mismatch between airway and lung size—a condition called dysanapsis—could answer why.