NEFL and dementia: This observed non‐multiplicative, positive additive interaction suggests that individuals with TBI may be more susceptible to neuronal degeneration (i.e., elevated NfL) in terms of dementia risk, not because NfL increases the relative risk more (i.e., non‐multiplicative interaction), but because individuals with TBI have higher baseline risk of dementia (i.e., positive additive interaction) – this means that the same relative risk translates into a larger absolute increase in risk of dementia associated with NfL among individuals with versus without TBI.