Low Birth Weight Paradox
Updated 2026-08-05
INTRODUCTION
English translation pending.
CORE DEFINITION
Observed in birth statistics, the paradox is that within the low-birth-weight group, infants of smoking mothers have lower mortality than infants of non-smoking mothers. The pattern does not mean smoking protects. Smoking mothers' low-weight babies tend to be small only because of malnutrition, while non-smoking mothers' low-weight babies are more often small because of severe genetic or physiological defects. Conditioning on the outcome, being low birth weight, opens a collider path that reverses the association, an advanced form of survivorship bias.
SCAFFOLDING EFFECT
Reduce cognitive load
- Catch reversed signals: treat any harmful factor that looks protective inside a subgroup as a suspect pattern. - Find the collider: check whether the analysis conditions on an outcome or its consequence. - Draw the causal graph: map the paths before accepting any stratified association as causal.
Anchor fast decisions
Conditioning on a collider, here the low-birth-weight outcome, induces a spurious association between its causes. Among low-weight infants, non-smoking mothers' babies are small for graver reasons, so their mortality is higher, which makes smoking appear protective. The harm of smoking is real in the full population; selection into the subgroup reverses the sign.
MINIMUM ACTION
In progress 0/1Practice this model in one real situation:
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Source support: Explicit
- en.wikipedia.orghttps://en.wikipedia.org/wiki/Low_birth-weight_paradoxverified
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